Java Language Part 2

The return method only returns from the lambda, not the outer method. Beware that this is different from Scala and Kotlin! void threeTimes(IntConsumer r) { for (int i = 0; i < 3; i++) { r.accept(i); } } void demo() { threeTimes(i -> { System.out.println(i); return; // Return from lambda to threeTimes only! }); } This can lead to unexpected behavior when attempting to write own language constructs, as in builtin constructs such as for loops return behaves differently: void demo2() { for (int i = 0; i < 3; i++) { System.out.println(i); return; // Return from 'demo2' entirely } } In Scala and Kotlin, demo and demo2 would both only print 0. But this is not more consistent. The Java approach is consistent with refactoring and the use of classes - the return in the code at the top, and the code below behaves the same: void demo3() { threeTimes(new MyIntConsumer()); } class MyIntConsumer implements IntConsumer { public void accept(int i) { System.out.println(i); return; } } Therefore, the Java return is more consistent with class methods and refactoring, but less with the for and while builtins, these remain special. Because of this, the following two are equivalent in Java: IntStream.range(1, 4) .map(x -> x * x) .forEach(System.out::println); IntStream.range(1, 4) .map(x -> { return x * x; }) .forEach(System.out::println); https://riptutorial.com/ 675

Furthermore, the use of try-with-resources is safe in Java: class Resource implements AutoCloseable { public void close() { System.out.println(\"close()\"); } } void executeAround(Consumer<Resource> f) { try (Resource r = new Resource()) { System.out.print(\"before \"); f.accept(r); System.out.print(\"after \"); } } void demo4() { executeAround(r -> { System.out.print(\"accept() \"); return; // Does not return from demo4, but frees the resource. }); } will print before accept() after close(). In the Scala and Kotlin semantics, the try-with-resources would not be closed, but it would print before accept() only. Java Closures with lambda expressions. A lambda closure is created when a lambda expression references the variables of an enclosing scope (global or local). The rules for doing this are the same as those for inline methods and anonymous classes. Local variables from an enclosing scope that are used within a lambda have to be final. With Java 8 (the earliest version that supports lambdas), they don't need to be declared final in the outside context, but must be treated that way. For example: int n = 0; // With Java 8 there is no need to explicit final Runnable r = () -> { // Using lambda int i = n; // do something }; This is legal as long as the value of the n variable is not changed. If you try to change the variable, inside or outside the lambda, you will get the following compilation error: \"local variables referenced from a lambda expression must be final or effectively final\". For example: int n = 0; Runnable r = () -> { // Using lambda int i = n; // do something }; n++; // Will generate an error. https://riptutorial.com/ 676

If it is necessary to use a changing variable within a lambda, the normal approach is to declare a final copy of the variable and use the copy. For example int n = 0; final int k = n; // With Java 8 there is no need to explicit final Runnable r = () -> { // Using lambda int i = k; // do something }; n++; // Now will not generate an error r.run(); // Will run with i = 0 because k was 0 when the lambda was created Naturally, the body of the lambda does not see the changes to the original variable. Note that Java does not support true closures. A Java lambda cannot be created in a way that allows it to see changes in the environment in which it was instantiated. If you want to implement a closure that observes or makes changes to its environment, you should simulate it using a regular class. For example: // Does not compile ... public IntUnaryOperator createAccumulator() { int value = 0; IntUnaryOperator accumulate = (x) -> { value += x; return value; }; return accumulate; } The above example will not compile for reasons discussed previously. We can work around the compilation error as follows: // Compiles, but is incorrect ... public class AccumulatorGenerator { private int value = 0; public IntUnaryOperator createAccumulator() { IntUnaryOperator accumulate = (x) -> { value += x; return value; }; return accumulate; } } The problem is that this breaks the design contract for the IntUnaryOperator interface which states that instances should be functional and stateless. If such a closure is passed to built-in functions that accept functional objects, it is liable to cause crashes or erroneous behavior. Closures that encapsulate mutable state should be implemented as regular classes. For example. // Correct ... public class Accumulator { private int value = 0; public int accumulate(int x) { value += x; return value; } } https://riptutorial.com/ 677

Lambda - Listener Example Anonymous class listener Before Java 8, it’s very common that an anonymous class is used to handle click event of a JButton, as shown in the following code. This example shows how to implement an anonymous listener within the scope of btn.addActionListener. JButton btn = new JButton(\"My Button\"); btn.addActionListener(new ActionListener() { @Override public void actionPerformed(ActionEvent e) { System.out.println(\"Button was pressed\"); } }); Lambda listener Because the ActionListener interface defines only one method actionPerformed(), it is a functional interface which means there’s a place to use Lambda expressions to replace the boilerplate code. The above example can be re-written using Lambda expressions as follows: JButton btn = new JButton(\"My Button\"); btn.addActionListener(e -> { System.out.println(\"Button was pressed\"); }); Traditional style to Lambda style Traditional way interface MathOperation{ boolean unaryOperation(int num); } public class LambdaTry { public static void main(String[] args) { MathOperation isEven = new MathOperation() { @Override public boolean unaryOperation(int num) { return num%2 == 0; } }; System.out.println(isEven.unaryOperation(25)); System.out.println(isEven.unaryOperation(20)); } } Lambda style 1. Remove class name and functional interface body. https://riptutorial.com/ 678

public class LambdaTry { public static void main(String[] args) { MathOperation isEven = (int num) -> { return num%2 == 0; }; System.out.println(isEven.unaryOperation(25)); System.out.println(isEven.unaryOperation(20)); } } 2. Optional type declaration MathOperation isEven = (num) -> { return num%2 == 0; }; 3. Optional parenthesis around parameter, if it is single parameter MathOperation isEven = num -> { return num%2 == 0; }; 4. Optional curly braces, if there is only one line in function body 5. Optional return keyword, if there is only one line in function body MathOperation isEven = num -> num%2 == 0; Lambdas and memory utilization Since Java lambdas are closures, they can \"capture\" the values of variables in the enclosing lexical scope. While not all lambdas capture anything -- simple lambdas like s -> s.length() capture nothing and are called stateless -- capturing lambdas require a temporary object to hold the captured variables. In this code snippet, the lambda () -> j is a capturing lambda, and may cause an object to be allocated when it is evaluated: public static void main(String[] args) throws Exception { for (int i = 0; i < 1000000000; i++) { int j = i; doSomethingWithLambda(() -> j); } } Although it might not be immediately obvious since the new keyword doesn't appear anywhere in the snippet, this code is liable to create 1,000,000,000 separate objects to represent the instances of the () -> j lambda expression. However, it should also be noted that future versions of Java1 may be able to optimize this so that at runtime the lambda instances were reused, or were represented in some other way. 1 - For instance, Java 9 introduces an optional \"link\" phase to the Java build sequence which will provide the https://riptutorial.com/ 679

opportunity for doing global optimizations like this. Using lambda expressions & predicates to get a certain value(s) from a list Starting with Java 8, you can use lambda expressions & predicates. Example: Use a lambda expressions & a predicate to get a certain value from a list. In this example every person will be printed out with the fact if they are 18 and older or not. Person Class: public class Person { private String name; private int age; public Person(String name, int age) { this.name = name; this.age = age; } public int getAge() { return age; } public String getName() { return name; } } The built-in interface Predicate from the java.util.function.Predicate packages is a functional interface with a boolean test(T t) method. Example Usage: import java.util.ArrayList; import java.util.List; import java.util.function.Predicate; public class LambdaExample { public static void main(String[] args) { List<Person> personList = new ArrayList<Person>(); personList.add(new Person(\"Jeroen\", 20)); personList.add(new Person(\"Jack\", 5)); personList.add(new Person(\"Lisa\", 19)); print(personList, p -> p.getAge() >= 18); } private static void print(List<Person> personList, Predicate<Person> checker) { for (Person person : personList) { if (checker.test(person)) { System.out.print(person + \" matches your expression.\"); } else { System.out.println(person + \" doesn't match your expression.\"); } } } } The print(personList, p -> p.getAge() >= 18); method takes a lambda expression (because the Predicate is used a parameter) where you can define the expression that is needed. The checker's https://riptutorial.com/ 680

test method checks if this expression is correct or not: checker.test(person). You can easily change this to something else, for example to print(personList, p -> p.getName().startsWith(\"J\"));. This will check if the person's name starts with a \"J\". Read Lambda Expressions online: https://riptutorial.com/java/topic/91/lambda-expressions https://riptutorial.com/ 681

Chapter 105: LinkedHashMap Introduction LinkedHashMap class is Hash table and Linked list implementation of the Map interface, with predictable iteration order. It inherits HashMap class and implements the Map interface. The important points about Java LinkedHashMap class are: A LinkedHashMap contains values based on the key. It contains only unique elements. It may have one null key and multiple null values. It is same as HashMap instead maintains insertion order. Examples Java LinkedHashMap class Key Points:- • Is Hash table and Linked list implementation of the Map interface, with predictable iteration order. • inherits HashMap class and implements the Map interface. • contains values based on the key. • only unique elements. • may have one null key and multiple null values. • same as HashMap instead maintains insertion order. Methods :- • void clear(). • boolean containsKey(Object key). • Object get(Object key). • protected boolean removeEldestEntry(Map.Entry eldest) Example :- public static void main(String arg[]) { LinkedHashMap<String, String> lhm = new LinkedHashMap<String, String>(); lhm.put(\"Ramesh\", \"Intermediate\"); lhm.put(\"Shiva\", \"B-Tech\"); lhm.put(\"Santosh\", \"B-Com\"); lhm.put(\"Asha\", \"Msc\"); lhm.put(\"Raghu\", \"M-Tech\"); Set set = lhm.entrySet(); https://riptutorial.com/ 682

Iterator i = set.iterator(); while (i.hasNext()) { Map.Entry me = (Map.Entry) i.next(); System.out.println(me.getKey() + \" : \" + me.getValue()); } System.out.println(\"The Key Contains : \" + lhm.containsKey(\"Shiva\")); System.out.println(\"The value to the corresponding to key : \" + lhm.get(\"Asha\")); } Read LinkedHashMap online: https://riptutorial.com/java/topic/10750/linkedhashmap https://riptutorial.com/ 683

Chapter 106: List vs SET Introduction What are differences between List and Set collection at the top level and How to choose when to use List in java and when to use Set in Java Examples List vs Set import java.util.ArrayList; import java.util.HashSet; import java.util.List; import java.util.Set; public class SetAndListExample { public static void main( String[] args ) { System.out.println(\"List example .....\"); List list = new ArrayList(); list.add(\"1\"); list.add(\"2\"); list.add(\"3\"); list.add(\"4\"); list.add(\"1\"); for (String temp : list){ System.out.println(temp); } System.out.println(\"Set example .....\"); Set<String> set = new HashSet<String>(); set.add(\"1\"); set.add(\"2\"); set.add(\"3\"); set.add(\"4\"); set.add(\"1\"); set.add(\"2\"); set.add(\"5\"); for (String temp : set){ System.out.println(temp); } } } Output List example ..... 1 2 3 4 1 Set example ..... 3 2 10 5 4 Read List vs SET online: https://riptutorial.com/java/topic/10125/list-vs-set https://riptutorial.com/ 684

Chapter 107: Lists Introduction A list is an ordered collection of values. In Java, lists are part of the Java Collections Framework. Lists implement the java.util.List interface, which extends java.util.Collection. Syntax • ls.add(E element); //Adds an element • ls.remove(E element); //Removes an element • for(E element : ls){} //Iterates over each element • ls.toArray(new String[ls.length]); //Converts a List of Strings to an array of Strings • ls.get(int index); //Returns the element at the specified index. • ls.set(int index, E element); //Replaces the element at a specified position . • ls.isEmpty(); //Returns true if the array contains no elements, otherwise it returns false. • ls.indexOf(Object o); //Returns the index of the first location of the specified element o, or, if it is not present, returns -1. • ls.lastIndexOf(Object o); //Returns the index of the last location of the specified element o, or, if it is not present, returns -1. • ls.size(); //Returns the number of elements in the List. Remarks A list is an object which stores a an ordered collection of values. \"Ordered\" means the values are stored in a particular order--one item comes first, one comes second, and so on. The individual values are commonly called \"elements\". Java lists typically provide these features: • Lists may contain zero or more elements. • Lists may contain duplicate values. In other words, an element can be inserted into a list more than once. • Lists store their elements in a particular order, meaning one element comes first, one comes next, and so on. • Each element has an index indicating its position within the list. The first element has index 0, the next has index 1, and so on. • Lists permit inserting elements at the beginning, at the end, or at any index within the list. • Testing whether a list contains a particular value generally means examining each element in the list. This means that the time to perform this check is O(n), proportional to the size of the list. Adding a value to a list at some point other than the end will move all of the following elements \"down\" or \"to the right\". In other words, adding an element at index n moves the element which used to be at index n to index n+1, and so on. For example: https://riptutorial.com/ 685

List<String> list = new ArrayList<>(); list.add(\"world\"); System.out.println(list.indexOf(\"world\")); // Prints \"0\" // Inserting a new value at index 0 moves \"world\" to index 1 list.add(0, \"Hello\"); System.out.println(list.indexOf(\"world\")); // Prints \"1\" System.out.println(list.indexOf(\"Hello\")); // Prints \"0\" Examples Sorting a generic list The Collections class offers two standard static methods to sort a list: • sort(List<T> list) applicable to lists where T extends Comparable<? super T>, and • sort(List<T> list, Comparator<? super T> c) applicable to lists of any type. Applying the former requires amending the class of list elements being sorted, which is not always possible. It might also be undesirable as although it provides the default sorting, other sorting orders may be required in different circumstances, or sorting is just a one off task. Consider we have a task of sorting objects that are instances of the following class: public class User { public final Long id; public final String username; public User(Long id, String username) { this.id = id; this.username = username; } @Override public String toString() { return String.format(\"%s:%d\", username, id); } } In order to use Collections.sort(List<User> list) we need to modify the User class to implement the Comparable interface. For example public class User implements Comparable<User> { public final Long id; public final String username; public User(Long id, String username) { this.id = id; this.username = username; } @Override public String toString() { return String.format(\"%s:%d\", username, id); } https://riptutorial.com/ 686

@Override /** The natural ordering for 'User' objects is by the 'id' field. */ public int compareTo(User o) { return id.compareTo(o.id); } } (Aside: many standard Java classes such as String, Long, Integer implement the Comparable interface. This makes lists of those elements sortable by default, and simplifies implementation of compare or compareTo in other classes.) With the modification above, the we can easily sort a list of User objects based on the classes natural ordering. (In this case, we have defined that to be ordering based on id values). For example: List<User> users = Lists.newArrayList( new User(33L, \"A\"), new User(25L, \"B\"), new User(28L, \"\")); Collections.sort(users); System.out.print(users); // [B:25, C:28, A:33] However, suppose that we wanted to sort User objects by name rather than by id. Alternatively, suppose that we had not been able to change the class to make it implement Comparable. This is where the sort method with the Comparator argument is useful: Collections.sort(users, new Comparator<User>() { @Override /* Order two 'User' objects based on their names. */ public int compare(User left, User right) { return left.username.compareTo(right.username); } }); System.out.print(users); // [A:33, B:25, C:28] Java SE 8 In Java 8 you can use a lambda instead of an anonymous class. The latter reduces to a one-liner: Collections.sort(users, (l, r) -> l.username.compareTo(r.username)); Further, there Java 8 adds a default sort method on the List interface, which simplifies sorting even more. users.sort((l, r) -> l.username.compareTo(r.username)) Creating a List https://riptutorial.com/ 687

Giving your list a type To create a list you need a type (any class, e.g. String). This is the type of your List. The List will only store objects of the specified type. For example: List<String> strings; Can store \"string1\", \"hello world!\", \"goodbye\", etc, but it can't store 9.2, however: List<Double> doubles; Can store 9.2, but not \"hello world!\". Initialising your list If you try to add something to the lists above you will get a NullPointerException, because strings and doubles both equal null! There are two ways to initialise a list: Option 1: Use a class that implements List List is an interface, which means that does not have a constructor, rather methods that a class must override. ArrayList is the most commonly used List, though LinkedList is also common. So we initialise our list like this: List<String> strings = new ArrayList<String>(); or List<String> strings = new LinkedList<String>(); Java SE 7 Starting from Java SE 7, you can use a diamond operator: List<String> strings = new ArrayList<>(); or List<String> strings = new LinkedList<>(); Option 2: Use the Collections class The Collections class provides two useful methods for creating Lists without a List variable: • emptyList(): returns an empty list. • singletonList(T): creates a list of type T and adds the element specified. https://riptutorial.com/ 688

And a method which uses an existing List to fill data in: • addAll(L, T...): adds all the specified elements to the list passed as the first parameter. Examples: import java.util.List; import java.util.Collections; List<Integer> l = Collections.emptyList(); List<Integer> l1 = Collections.singletonList(42); Collections.addAll(l1, 1, 2, 3); Positional Access Operations The List API has eight methods for positional access operations: • add(T type) • add(int index, T type) • remove(Object o) • remove(int index) • get(int index) • set(int index, E element) • int indexOf(Object o) • int lastIndexOf(Object o) So, if we have a List: List<String> strings = new ArrayList<String>(); And we wanted to add the strings \"Hello world!\" and \"Goodbye world!\" to it, we would do it as such: strings.add(\"Hello world!\"); strings.add(\"Goodbye world!\"); And our list would contain the two elements. Now lets say we wanted to add \"Program starting!\" at the front of the list. We would do this like this: strings.add(0, \"Program starting!\"); NOTE: The first element is 0. Now, if we wanted to remove the \"Goodbye world!\" line, we could do it like this: strings.remove(\"Goodbye world!\"); And if we wanted to remove the first line (which in this case would be \"Program starting!\", we could do it like this: https://riptutorial.com/ 689

strings.remove(0); Note: 1. Adding and removing list elements modify the list, and this can lead to a ConcurrentModificationException if the list is being iterated concurrently. 2. Adding and removing elements can be O(1) or O(N) depending on the list class, the method used, and whether you are adding / removing an element at the start, the end, or in the middle of the list. In order to retrieve an element of the list at a specified position you can use the E get(int index); method of the List API. For example: strings.get(0); will return the first element of the list. You can replace any element at a specified position by using the set(int index, E element);. For example: strings.set(0,\"This is a replacement\"); This will set the String \"This is a replacement\" as the first element of the list. Note: The set method will overwrite the element at the position 0. It will not add the new String at the position 0 and push the old one to the position 1. The int indexOf(Object o); returns the position of the first occurrence of the object passed as argument. If there are no occurrences of the object in the list then the -1 value is returned. In continuation of the previous example if you call: strings.indexOf(\"This is a replacement\") the 0 is expected to be returned as we set the String \"This is a replacement\" in the position 0 of our list. In case where there are more than one occurrence in the list when int indexOf(Object o); is called then as mentioned the index of the first occurrence will be returned. By calling the int lastIndexOf(Object o) you can retrieve the index of the last occurrence in the list. So if we add another \"This is a replacement\": strings.add(\"This is a replacement\"); strings.lastIndexOf(\"This is a replacement\"); This time the 1 will be returned and not the 0; Iterating over elements in a list For the example, lets say that we have a List of type String that contains four elements: \"hello, \", https://riptutorial.com/ 690

\"how \", \"are \", \"you?\" The best way to iterate over each element is by using a for-each loop: public void printEachElement(List<String> list){ for(String s : list){ System.out.println(s); } } Which would print: hello, how are you? To print them all in the same line, you can use a StringBuilder: public void printAsLine(List<String> list){ StringBuilder builder = new StringBuilder(); for(String s : list){ builder.append(s); } System.out.println(builder.toString()); } Will print: hello, how are you? Alternatively, you can use element indexing ( as described in Accessing element at ith Index from ArrayList ) to iterate a list. Warning: this approach is inefficient for linked lists. Removing elements from list B that are present in the list A Lets suppose you have 2 Lists A and B, and you want to remove from B all the elements that you have in A the method in this case is List.removeAll(Collection c); #Example: public static void main(String[] args) { List<Integer> numbersA = new ArrayList<>(); List<Integer> numbersB = new ArrayList<>(); numbersA.addAll(Arrays.asList(new Integer[] { 1, 3, 4, 7, 5, 2 })); numbersB.addAll(Arrays.asList(new Integer[] { 13, 32, 533, 3, 4, 2 })); System.out.println(\"A: \" + numbersA); System.out.println(\"B: \" + numbersB); numbersB.removeAll(numbersA); https://riptutorial.com/ 691

System.out.println(\"B cleared: \" + numbersB); } this will print A: [1, 3, 4, 7, 5, 2] B: [13, 32, 533, 3, 4, 2] B cleared: [13, 32, 533] Finding common elements between 2 lists Suppose you have two lists: A and B, and you need to find the elements that exist in both lists. You can do it by just invoking the method List.retainAll(). Example: public static void main(String[] args) { List<Integer> numbersA = new ArrayList<>(); List<Integer> numbersB = new ArrayList<>(); numbersA.addAll(Arrays.asList(new Integer[] { 1, 3, 4, 7, 5, 2 })); numbersB.addAll(Arrays.asList(new Integer[] { 13, 32, 533, 3, 4, 2 })); System.out.println(\"A: \" + numbersA); System.out.println(\"B: \" + numbersB); List<Integer> numbersC = new ArrayList<>(); numbersC.addAll(numbersA); numbersC.retainAll(numbersB); System.out.println(\"List A : \" + numbersA); System.out.println(\"List B : \" + numbersB); System.out.println(\"Common elements between A and B: \" + numbersC); } Convert a list of integers to a list of strings List<Integer> nums = Arrays.asList(1, 2, 3); List<String> strings = nums.stream() .map(Object::toString) .collect(Collectors.toList()); That is: 1. Create a stream from the list 2. Map each element using Object::toString 3. Collect the String values into a List using Collectors.toList() Creating, Adding and Removing element from an ArrayList ArrayList https://riptutorial.com/ 692

is one of the inbuilt data structures in Java. It is a dynamic array (where the size of the data structure not needed to be declared first) for storing elements (Objects). It extends AbstractList class and implements List interface. An ArrayList can contain duplicate elements where it maintains insertion order. It should be noted that the class ArrayList is non- synchronized, so care should be taken when handling concurrency with ArrayList. ArrayList allows random access because array works at the index basis. Manipulation is slow in ArrayList because of shifting that often occurs when an element is removed from the array list. An ArrayList can be created as follows: List<T> myArrayList = new ArrayList<>(); Where T ( Generics ) is the type that will be stored inside ArrayList. The type of the ArrayList can be any Object. The type can't be a primitive type (use their wrapper classes instead). To add an element to the ArrayList, use add() method: myArrayList.add(element); Or to add item to a certain index: myArrayList.add(index, element); //index of the element should be an int (starting from 0) To remove an item from the ArrayList, use the remove() method: myArrayList.remove(element); Or to remove an item from a certain index: myArrayList.remove(index); //index of the element should be an int (starting from 0) In-place replacement of a List element This example is about replacing a List element while ensuring that the replacement element is at the same position as the element that is replaced. This can be done using these methods: • set(int index, T type) • int indexOf(T type) Consider an ArrayList containing the elements \"Program starting!\", \"Hello world!\" and \"Goodbye world!\" List<String> strings = new ArrayList<String>(); https://riptutorial.com/ 693

strings.add(\"Program starting!\"); strings.add(\"Hello world!\"); strings.add(\"Goodbye world!\"); If we know the index of the element we want to replace, we can simply use set as follows: strings.set(1, \"Hi world\"); If we don't know the index, we can search for it first. For example: int pos = strings.indexOf(\"Goodbye world!\"); if (pos >= 0) { strings.set(pos, \"Goodbye cruel world!\"); } Notes: 1. The set operation will not cause a ConcurrentModificationException. 2. The set operation is fast ( O(1) ) for ArrayList but slow ( O(N) ) for a LinkedList. 3. An indexOf search on an ArrayList or LinkedList is slow ( O(N) ). Making a list unmodifiable The Collections class provides a way to make a list unmodifiable: List<String> ls = new ArrayList<String>(); List<String> unmodifiableList = Collections.unmodifiableList(ls); If you want an unmodifiable list with one item you can use: List<String> unmodifiableList = Collections.singletonList(\"Only string in the list\"); Moving objects around in the list The Collections class allows for you to move objects around in the list using various methods (ls is the List): Reversing a list: Collections.reverse(ls); Rotating positions of elements in a list The rotate method requires an integer argument. This is how many spots to move it along the line by. An example of this is below: List<String> ls = new ArrayList<String>(); ls.add(\" how\"); ls.add(\" are\"); https://riptutorial.com/ 694

ls.add(\" you?\"); ls.add(\"hello,\"); Collections.rotate(ls, 1); for(String line : ls) System.out.print(line); System.out.println(); This will print \"hello, how are you?\" Shuffling elements around in a list Using the same list above, we can shuffle the elements in a list: Collections.shuffle(ls); We can also give it a java.util.Random object that it uses to randomly place objects in spots: Random random = new Random(12); Collections.shuffle(ls, random); Classes implementing List - Pros and Cons The List interface is implemented by different classes. Each of them has its own way for implementing it with different strategies and providing different pros and cons. Classes implementing List These are all of the public classes in Java SE 8 that implement the java.util.List interface: 1. Abstract Classes: • AbstractList • AbstractSequentialList 2. Concrete Classes: • ArrayList • AttributeList • CopyOnWriteArrayList • LinkedList • RoleList • RoleUnresolvedList • Stack • Vector Pros and Cons of each implementation in term of time complexity https://riptutorial.com/ 695

ArrayList public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable ArrayList is a resizable-array implementation of the List interface. Storing the list into an array, ArrayList provides methods (in addition to the methods implementing the List interface) for manipulating the size of the array. Initialize ArrayList of Integer with size 100 List<Integer> myList = new ArrayList<Integer>(100); // Constructs an empty list with the specified initial capacity. - PROS: The size, isEmpty, get, set, iterator, and listIterator operations run in constant time. So getting and setting each element of the List has the same time cost: int e1 = myList.get(0); // \\ | => All the same constant cost => O(1) int e2 = myList.get(10); // / myList.set(2,10); // - CONS: Being implemented with an array (static structure) adding elements over the size of the array has a big cost due to the fact that a new allocation need to be done for all the array. However, from documentation: The add operation runs in amortized constant time, that is, adding n elements requires O(n) time Removing an element requires O(n) time. AttributeList On coming CopyOnWriteArrayList On coming LinkedList https://riptutorial.com/ 696

public class LinkedList<E> extends AbstractSequentialList<E> implements List<E>, Deque<E>, Cloneable, Serializable LinkedList is implemented by a doubly-linked list a linked data structure that consists of a set of sequentially linked records called nodes. Iitialize LinkedList of Integer List<Integer> myList = new LinkedList<Integer>(); // Constructs an empty list. - PROS: Adding or removing an element to the front of the list or to the end has constant time. myList.add(10); // \\ myList.add(0,2); // | => constant time => O(1) myList.remove(); // / - CONS: From documentation: Operations that index into the list will traverse the list from the beginning or the end, whichever is closer to the specified index. Operations such as: myList.get(10); // \\ myList.add(11,25); // | => worst case done in O(n/2) myList.set(15,35); // / RoleList On coming RoleUnresolvedList On coming Stack On coming Vector https://riptutorial.com/ 697

On coming Read Lists online: https://riptutorial.com/java/topic/2989/lists https://riptutorial.com/ 698

Chapter 108: Literals Introduction A Java literal is a syntactic element (i.e. something you find in the source code of a Java program) that represents a value. Examples are 1, 0.333F, false, 'X' and \"Hello world\\n\". Examples Hexadecimal, Octal and Binary literals A hexadecimal number is a value in base-16. There are 16 digits, 0-9 and the letters A-F (case does not matter). A-F represent 10-16. An octal number is a value in base-8, and uses the digits 0-7. A binary number is a value in base-2, and uses the digits 0 and 1. All of these numbers result in the same value, 110: int dec = 110; // no prefix --> decimal literal int bin = 0b1101110; // '0b' prefix --> binary literal int oct = 0156; // '0' prefix --> octal literal int hex = 0x6E; // '0x' prefix --> hexadecimal literal Note that binary literal syntax was introduced in Java 7. The octal literal can easily be a trap for semantic errors. If you define a leading '0' to your decimal literals you will get the wrong value: int a = 0100; // Instead of 100, a == 64 Using underscore to improve readability Since Java 7 it has been possible to use one or more underscores (_) for separating groups of digits in a primitive number literal to improve their readability. For instance, these two declarations are equivalent: Java SE 7 int i1 = 123456; int i2 = 123_456; System.out.println(i1 == i2); // true This can be applied to all primitive number literals as shown below: 699 Java SE 7 https://riptutorial.com/

byte color = 1_2_3; short yearsAnnoDomini= 2_016; int socialSecurtyNumber = 999_99_9999; long creditCardNumber = 1234_5678_9012_3456L; float piFourDecimals = 3.14_15F; double piTenDecimals = 3.14_15_92_65_35; This also works using prefixes for binary, octal and hexadecimal bases: Java SE 7 short binary= 0b0_1_0_1; int octal = 07_7_7_7_7_7_7_7_0; long hexBytes = 0xFF_EC_DE_5E; There are a few rules about underscores which forbid their placement in the following places: • At the beginning or end of a number (e.g. _123 or 123_ are not valid) • Adjacent to a decimal point in a floating point literal (e.g. 1._23 or 1_.23 are not valid) • Prior to an F or L suffix (e.g. 1.23_F or 9999999_L are not valid) • In positions where a string of digits is expected (e.g. 0_xFFFF is not valid) Escape sequences in literals String and character literals provide an escape mechanism that allows express character codes that would otherwise not be allowed in the literal. An escape sequence consists of a backslash character (\\) followed by one ore more other characters. The same sequences are valid in both character an string literals. The complete set of escape sequences is as follows: Escape sequence Meaning \\\\ Denotes an backslash (\\) character \\' Denotes a single-quote (') character \\\" Denotes a double-quote (\") character \\n Denotes a line feed (LF) character \\r Denotes a carriage return (CR) character \\t Denotes a horizontal tab (HT) character \\f Denotes a form feed (FF) character \\b Denotes a backspace (BS) character \\<octal> Denotes a character code in the range 0 to 255. https://riptutorial.com/ 700

The <octal> in the above consists of one, two or three octal digits ('0' through '7') which represent a number between 0 and 255 (decimal). Note that a backslash followed by any other character is an invalid escape sequence. Invalid escape sequences are treated as compilation errors by the JLS. Reference: • JLS 3.10.6. Escape Sequences for Character and String Literals Unicode escapes In addition to the string and character escape sequences described above, Java has a more general Unicode escaping mechanism, as defined in JLS 3.3. Unicode Escapes. A Unicode escape has the following syntax: '\\' 'u' <hex-digit> <hex-digit> <hex-digit> <hex-digit> where <hex-digit> is one of '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f', 'A', 'B', 'C', 'D', 'E', 'F'. A Unicode escape is mapped by the Java compiler to a character (strictly speaking a 16-bit Unicode code unit), and can be used anywhere in the source code where the mapped character is valid. It is commonly used in character and string literals when you need to represent a non-ASCII character in a literal. Escaping in regexes TBD Decimal Integer literals Integer literals provide values that can be used where you need a byte, short, int, long or char instance. (This example focuses on the simple decimal forms. Other examples explain how to literals in octal, hexadecimal and binary, and the use of underscores to improve readability.) Ordinary integer literals The simplest and most common form of integer literal is a decimal integer literal. For example: 0 // The decimal number zero (type 'int') 1 // The decimal number one (type 'int') 42 // The decimal number forty two (type 'int') You need to be careful with leading zeros. A leading zero causes an integer literal to be interpreted as octal not decimal. https://riptutorial.com/ 701

077 // This literal actually means 7 x 8 + 7 ... or 63 decimal! Integer literals are unsigned. If you see something like -10 or +10, these are actually expressions using the unary - and unary + operators. The range of integer literals of this form have an intrinsic type of int, and must fall in the range zero to 231 or 2,147,483,648. Note that 231 is 1 greater than Integer.MAX_VALUE. Literals from 0 through to 2147483647 can be used anywhere, but it is a compilation error to use 2147483648 without a preceding unary - operator. (In other words, it is reserved for expressing the value of Integer.MIN_VALUE.) int max = 2147483647; // OK int min = -2147483648; // OK int tooBig = 2147483648; // ERROR Long integer literals Literals of type long are expressed by adding an L suffix. For example: 0L // The decimal number zero (type 'long') 1L // The decimal number one (type 'long') 2147483648L // The value of Integer.MAX_VALUE + 1 long big = 2147483648; // ERROR long big2 = 2147483648L; // OK Note that the distinction between int and long literals is significant in other places. For example int i = 2147483647; // Produces a negative value because the operation is long l = i + 1; // performed using 32 bit arithmetic, and the // addition overflows long l2 = i + 1L; // Produces the (intuitively) correct value. Reference: JLS 3.10.1 - Integer Literals Boolean literals Boolean literals are the simplest of the literals in the Java programming language. The two possible boolean values are represented by the literals true and false. These are case-sensitive. For example: boolean flag = true; // using the 'true' literal flag = false; // using the 'false' literal String literals https://riptutorial.com/ 702

String literals provide the most convenient way to represent string values in Java source code. A String literal consists of: • An opening double-quote (\") character. • Zero or more other characters that are neither a double-quote or a line-break character. (A backslash (\\) character alters the meaning of subsequent characters; see Escape sequences in literals.) • A closing double-quote character. For example: \"Hello world\" // A literal denoting an 11 character String \"\" // A literal denoting an empty (zero length) String \"\\\"\" // A literal denoting a String consisting of one // double quote character \"1\\t2\\t3\\n\" // Another literal with escape sequences Note that a single string literal may not span multiple source code lines. It is a compilation error for a line-break (or the end of the source file) to occur before a literal's closing double-quote. For example: \"Jello world // Compilation error (at the end of the line!) Long strings If you need a string that is too long to fit on a line, the conventional way to express it is to split it into multiple literals and use the concatenation operator (+) to join the pieces. For example String typingPractice = \"The quick brown fox \" + \"jumped over \" + \"the lazy dog\" An expression like the above consisting of string literals and + satisfies the requirements to be a Constant Expression. That means that the expression will be evaluated by the compiler and represented at runtime by a single String object. Interning of string literals When class file containing string literals is loaded by the JVM, the corresponding String objects are interned by the runtime system. This means that a string literal used in multiple classes occupies no more space than if it was used in one class. For more information on interning and the string pool, refer to the String pool and heap storage example in the Strings topic. The Null literal The Null literal (written as null) represents the one and only value of the null type. Here are some https://riptutorial.com/ 703

examples MyClass object = null; MyClass[] objects = new MyClass[]{new MyClass(), null, new MyClass()}; myMethod(null); if (objects != null) { // Do something } The null type is rather unusual. It has no name, so you cannot express it in Java source code. (And it has no runtime representation either.) The sole purpose of the null type is to be the type of null. It is assignment compatible with all reference types, and can be type cast to any reference type. (In the latter case, the cast does not entail a runtime type check.) Finally, null has the property that null instanceof <SomeReferenceType> will evaluate to false, no matter what the type is. Floating-point literals Floating point literals provide values that can be used where you need a float or double instance. There are three kinds of floating point literal. • Simple decimal forms • Scaled decimal forms • Hexadecimal forms (The JLS syntax rules combine the two decimal forms into a single form. We treat them separately for ease of explanation.) There are distinct literal types for float and double literals, expressed using suffixes. The various forms use letters to express different things. These letters are case insensitive. Simple decimal forms The simplest form of floating point literal consists of one or more decimal digits and a decimal point (.) and an optional suffix (f, F, d or D). The optional suffix allows you to specify that the literal is a float (f or F) or double (d or D) value. The default (when no suffix is specified) is double. For example 0.0 // this denotes zero .0 // this also denotes zero 0. // this also denotes zero 3.14159 // this denotes Pi, accurate to (approximately!) 5 decimal places. 1.0F // a `float` literal 1.0D // a `double` literal. (`double` is the default if no suffix is given) https://riptutorial.com/ 704

In fact, decimal digits followed by a suffix is also a floating point literal. 1F // means the same thing as 1.0F The meaning of a decimal literal is the IEEE floating point number that is closest to the infinite precision mathematical Real number denoted by the decimal floating point form. This conceptual value is converted to IEEE binary floating point representation using round to nearest. (The precise semantics of decimal conversion are specified in the javadocs for Double.valueOf(String) and Float.valueOf(String), bearing in mind that there are differences in the number syntaxes.) Scaled decimal forms Scaled decimal forms consist of simple decimal with an exponent part introduced by an E or e, and followed by a signed integer. The exponent part is a short hand for multiplying the decimal form by a power of ten, as shown in the examples below. There is also an optional suffix to distinguish float and double literals. Here are some examples: 1.0E1 // this means 1.0 x 10^1 ... or 10.0 (double) 1E-1D // this means 1.0 x 10^(-1) ... or 0.1 (double) 1.0e10f // this means 1.0 x 10^(10) ... or 10000000000.0 (float) The size of a literal is limited by the representation (float or double). It is a compilation error if the scale factor results in a value that is too large or too small. Hexadecimal forms Starting with Java 6, it is possible to express floating point literals in hexadecimal. The hexadecimal form have an analogous syntax to the simple and scaled decimal forms with the following differences: 1. Every hexadecimal floating point literal starts with a zero (0) and then an x or X. 2. The digits of the number (but not the exponent part!) also include the hexadecimal digits a through f and their uppercase equivalents. 3. The exponent is mandatory, and is introduced by the letter p (or P) instead of an e or E. The exponent represents a scaling factor that is a power of 2 instead of a power of 10. Here are some examples: 0x0.0p0f // this is zero expressed in hexadecimal form (`float`) 0xff.0p19 // this is 255.0 x 2^19 (`double`) Advice: since hexadecimal floating-point forms are unfamiliar to most Java programmers, it is advisable to use them sparingly. Underscores https://riptutorial.com/ 705

Starting with Java 7, underscores are permitted within the digit strings in all three forms of floating point literal. This applies to the \"exponent\" parts as well. See Using underscores to improve readability. Special cases It is a compilation error if a floating point literal denotes a number that is too large or too small to represent in the selected representation; i.e. if the number would overflow to +INF or -INF, or underflow to 0.0. However, it is legal for a literal to represent a non-zero denormalized number. The floating point literal syntax does not provide literal representations for IEEE 754 special values such as the INF and NaN values. If you need to express them in source code, the recommended way is to use the constants defined by the java.lang.Float and java.lang.Double; e.g. Float.NaN, Float.NEGATIVE_INFINITY and Float.POSITIVE_INFINITY. Character literals Character literals provide the most convenient way to express char values in Java source code. A character literal consists of: • An opening single-quote (') character. • A representation of a character. This representation cannot be a single-quote or a line-break character, but it can be an escape sequence introduced by a backslash (\\) character; see Escape sequences in literals. • A closing single-quote (') character. For example: char a = 'a'; char doubleQuote = '\"'; char singleQuote = '\\''; A line-break in a character literal is a compilation error: char newline = ' // Compilation error in previous line char newLine = '\\n'; // Correct Read Literals online: https://riptutorial.com/java/topic/8250/literals https://riptutorial.com/ 706

Chapter 109: Local Inner Class Introduction A class i.e. created inside a method is called local inner class in java. If you want to invoke the methods of local inner class, you must instantiate this class inside the method. Examples Local Inner Class public class localInner1{ private int data=30;//instance variable void display(){ class Local{ void msg(){System.out.println(data);} } Local l=new Local(); l.msg(); } public static void main(String args[]){ localInner1 obj=new localInner1(); obj.display(); } } Read Local Inner Class online: https://riptutorial.com/java/topic/10160/local-inner-class https://riptutorial.com/ 707

Chapter 110: Localization and Internationalization Remarks Java comes with a powerful and flexible mechanism for localizing your applications, but it's also easy to misuse and wind up with a program that disregards or mangles the user's locale, and therefore how they expect your program to behave. Your users will expect to see data localized to the formats they're used to, and attempting to support this manually is a fools errand. Here is just a small example of the different ways users expect to see content you might assume is \"always\" displayed a certain way: Dates Numbers Local Currency Foreign Currency Distances Brazil China Egypt Mexico 20/3/16 1.234,56 $1,000.50 1,000.50 USD UK 20/3/16 1,234.56 £1,000.50 100 km USA 3/20/16 1,234.56 $1,000.50 1,000.50 MXN 60 mi General Resources • Wikipedia: Internationalization and Localization Java Resources • Java Tutorial: Internationalization • Oracle: Internationalization: Understanding Locale in the Java Platform • JavaDoc: Locale Examples Automatically formatted Dates using \"locale\" SimpleDateFormatter is great in a pinch, but like the name suggests it doesn't scale well. https://riptutorial.com/ 708

If you hard-code \"MM/dd/yyyy\" all over your application your international users won't be happy. Let Java do the work for you Use the static methods in DateFormat to retrieve the right formatting for your user. For a desktop application (where you'll rely on the default locale), simply call: String localizedDate = DateFormat.getDateInstance(style).format(date); Where style is one of the formatting constants (FULL, LONG, MEDIUM, SHORT, etc.) specified in DateFormat. For a server-side application where the user specifies their locale as part of the request, you should pass it explicitly to getDateInstance() instead: String localizedDate = DateFormat.getDateInstance(style, request.getLocale()).format(date); String Comparison Compare two Strings ignoring case: \"School\".equalsIgnoreCase(\"school\"); // true Don't use text1.toLowerCase().equals(text2.toLowerCase()); Languages have different rules for converting upper and lower case. A 'I' would be converted to 'i' in English. But in Turkish a 'I' becomes a 'ı'. If you have to use toLowerCase() use the overload which expects a Locale: String.toLowerCase(Locale). Comparing two Strings ignoring minor differences: Collator collator = Collator.getInstance(Locale.GERMAN); collator.setStrength(Collator.PRIMARY); collator.equals(\"Gärten\", \"gaerten\"); // returns true Sort Strings respecting natural language order, ignoring case (use collation key to: String[] texts = new String[] {\"Birne\", \"äther\", \"Apfel\"}; Collator collator = Collator.getInstance(Locale.GERMAN); collator.setStrength(Collator.SECONDARY); // ignore case Arrays.sort(texts, collator::compare); // will return {\"Apfel\", \"äther\", \"Birne\"} Locale https://riptutorial.com/ 709

The java.util.Locale class is used to represent a \"geographical, political or cultural\" region to localize a given text, number, date or operation to. A Locale object may thus contain a country, region, language, and also a variant of a language, for instance a dialect spoken in a certain region of a country, or spoken in a different country than the country from which the language originates. The Locale instance is handed to components that need to localize their actions, whether it is converting the input, output, or just need it for internal operations. The Locale class cannot do any internationalization or localization by itself Language The language must be an ISO 639 2 or 3 character language code, or a registered language subtag of up to 8 characters. In case a language has both a 2 and 3 character language code, use the 2 character code. A full list of language codes can be found in the IANA Language Subtag Registry. Language codes are case insensitive, but the Locale class always use lowercase versions of the language codes Creating a Locale Creating a java.util.Locale instance can be done in four different ways: Locale constants Locale constructors Locale.Builder class Locale.forLanguageTag factory method Java ResourceBundle You create a ResourceBundle instance like this: Locale locale = new Locale(\"en\", \"US\"); ResourceBundle labels = ResourceBundle.getBundle(\"i18n.properties\"); System.out.println(labels.getString(\"message\")); Consider I have a property file i18n.properties: message=This is locale Output: This is locale https://riptutorial.com/ 710

Setting Locale If you want to reproduce the state using other languages, you can use setDefault() method. Its usage: setDefault(Locale.JAPANESE); //Set Japanese Read Localization and Internationalization online: https://riptutorial.com/java/topic/4086/localization-and-internationalization https://riptutorial.com/ 711

Chapter 111: LocalTime Syntax • LocalTime time = LocalTime.now(); // Initializes with current system clock • LocalTime time = LocalTime.MIDNIGHT; // 00:00 • LocalTime time = LocalTime.NOON; // 12:00 • LocalTime time = LocalTime.of(12, 12, 45); // 12:12:45 Parameters Method Output LocalTime.of(13, 12, 11) 13:12:11 LocalTime.MIDNIGHT 00:00 LocalTime.NOON 12:00 LocalTime.now() Current time from system clock LocalTime.MAX The maximum supported local time 23:59:59.999999999 LocalTime.MIN The minimum supported local time 00:00 LocalTime.ofSecondOfDay(84399) 23:59:59 , Obtains Time from second-of-day value LocalTime.ofNanoOfDay(2000000000) 00:00:02 , Obtains Time from nanos-of-day value Remarks As class name denotes, LocalTime represents a time without a time-zone. It doesn't represent a date. It's a simple label for a given time. The class is value-based and the equals method should be used when doing comparisons. This class is from the package java.time. Examples Time Modification You can add hours, minutes, seconds and nanoseconds: LocalTime time = LocalTime.now(); https://riptutorial.com/ 712

LocalTime addHours = time.plusHours(5); // Add 5 hours LocaLTime addMinutes = time.plusMinutes(15) // Add 15 minutes LocalTime addSeconds = time.plusSeconds(30) // Add 30 seconds LocalTime addNanoseconds = time.plusNanos(150_000_000) // Add 150.000.000ns (150ms) Time Zones and their time difference import java.time.LocalTime; import java.time.ZoneId; import java.time.temporal.ChronoUnit; public class Test { public static void main(String[] args) { ZoneId zone1 = ZoneId.of(\"Europe/Berlin\"); ZoneId zone2 = ZoneId.of(\"Brazil/East\"); LocalTime now = LocalTime.now(); LocalTime now1 = LocalTime.now(zone1); LocalTime now2 = LocalTime.now(zone2); System.out.println(\"Current Time : \" + now); System.out.println(\"Berlin Time : \" + now1); System.out.println(\"Brazil Time : \" + now2); long minutesBetween = ChronoUnit.MINUTES.between(now2, now1); System.out.println(\"Minutes Between Berlin and Brazil : \" + minutesBetween +\"mins\"); } } Amount of time between two LocalTime There are two equivalent ways to calculate the amount of time unit between two LocalTime: (1) through until(Temporal, TemporalUnit) method and through (2) TemporalUnit.between(Temporal, Temporal). import java.time.LocalTime; import java.time.temporal.ChronoUnit; public class AmountOfTime { public static void main(String[] args) { LocalTime start = LocalTime.of(1, 0, 0); // hour, minute, second LocalTime end = LocalTime.of(2, 10, 20); // hour, minute, second long halfDays1 = start.until(end, ChronoUnit.HALF_DAYS); // 0 long halfDays2 = ChronoUnit.HALF_DAYS.between(start, end); // 0 long hours1 = start.until(end, ChronoUnit.HOURS); // 1 long hours2 = ChronoUnit.HOURS.between(start, end); // 1 long minutes1 = start.until(end, ChronoUnit.MINUTES); // 70 long minutes2 = ChronoUnit.MINUTES.between(start, end); // 70 https://riptutorial.com/ 713

long seconds1 = start.until(end, ChronoUnit.SECONDS); // 4220 long seconds2 = ChronoUnit.SECONDS.between(start, end); // 4220 long millisecs1 = start.until(end, ChronoUnit.MILLIS); // 4220000 long millisecs2 = ChronoUnit.MILLIS.between(start, end); // 4220000 long microsecs1 = start.until(end, ChronoUnit.MICROS); // 4220000000 long microsecs2 = ChronoUnit.MICROS.between(start, end); // 4220000000 long nanosecs1 = start.until(end, ChronoUnit.NANOS); // 4220000000000 long nanosecs2 = ChronoUnit.NANOS.between(start, end); // 4220000000000 // Using others ChronoUnit will be thrown UnsupportedTemporalTypeException. // The following methods are examples thereof. long days1 = start.until(end, ChronoUnit.DAYS); long days2 = ChronoUnit.DAYS.between(start, end); } } Intro LocalTime is an immutable class and thread-safe, used to represent time, often viewed as hour- min-sec. Time is represented to nanosecond precision. For example, the value \"13:45.30.123456789\" can be stored in a LocalTime. This class does not store or represent a date or time-zone. Instead, it is a description of the local time as seen on a wall clock. It cannot represent an instant on the time-line without additional information such as an offset or time-zone. This is a value based class, equals method should be used for comparisons. Fields MAX - The maximum supported LocalTime, '23:59:59.999999999'. MIDNIGHT, MIN, NOON Important Static Methods now(), now(Clock clock), now(ZoneId zone), parse(CharSequence text) Important Instance Methods isAfter(LocalTime other), isBefore(LocalTime other), minus(TemporalAmount amountToSubtract), minus(long amountToSubtract, TemporalUnit unit), plus(TemporalAmount amountToAdd), plus(long amountToAdd, TemporalUnit unit) ZoneId zone = ZoneId.of(\"Asia/Kolkata\"); LocalTime now = LocalTime.now(); LocalTime now1 = LocalTime.now(zone); LocalTime then = LocalTime.parse(\"04:16:40\"); Difference in time can be calculated in any of following ways long timeDiff = Duration.between(now, now1).toMinutes(); long timeDiff1 = java.time.temporal.ChronoUnit.MINUTES.between(now2, now1); https://riptutorial.com/ 714

You can also add/subtract hours, minutes or seconds from any object of LocalTime. minusHours(long hoursToSubtract), minusMinutes(long hoursToMinutes), minusNanos(long nanosToSubtract), minusSeconds(long secondsToSubtract), plusHours(long hoursToSubtract), plusMinutes(long hoursToMinutes), plusNanos(long nanosToSubtract), plusSeconds(long secondsToSubtract) now.plusHours(1L); now1.minusMinutes(20L); Read LocalTime online: https://riptutorial.com/java/topic/3065/localtime https://riptutorial.com/ 715

Chapter 112: log4j / log4j2 Introduction Apache Log4j is a Java-based logging utility, it is one of several Java logging frameworks. This topic is to show how to setup and configure Log4j in Java with detailed examples on all of its possible aspects of usage. Syntax • Logger.debug(\"text to log\"); // Logging debugging info • Logger.info(\"text to log\"); // Logging common info • Logger.error(\"text to log\"); // Logging error info • Logger.warn(\"text to log\"); // Logging warnings • Logger.trace(\"text to log\"); // Logging trace info • Logger.fatal(\"text to log\"); // Logging fatal errors • Log4j2 usage with parameter logging: • Logger.debug(\"Debug params {} {} {}\", param1, param2, param3); // Logging debug with parameters • Logger.info(\"Info params {} {} {}\", param1, param2, param3); // Logging info with parameters • Logger.error(\"Error params {} {} {}\", param1, param2, param3); // Logging error with parameters • Logger.warn(\"Warn params {} {} {}\", param1, param2, param3); // Logging warnings with parameters • Logger.trace(\"Trace params {} {} {}\", param1, param2, param3); // Logging trace with parameters • Logger.fatal(\"Fatal params {} {} {}\", param1, param2, param3); // Logging fatal with parameters • Logger.error(\"Caught Exception: \", ex ); // Logging exception with message and stacktrace (will automatically be appended) Remarks End of Life for Log4j 1 reached On August 5, 2015 the Logging Services Project Management Committee announced that Log4j 1.x had reached end of life. For complete text of the announcement please see the Apache Blog. Users of Log4j 1 are recommended to upgrade to Apache Log4j 2. From: http://logging.apache.org/log4j/1.2/ https://riptutorial.com/ 716

Examples 717 How to get Log4j Current version (log4j2) Using Maven: Add the following dependency to your POM.xml file: <dependencies> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-api</artifactId> <version>2.6.2</version> </dependency> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-core</artifactId> <version>2.6.2</version> </dependency> </dependencies> Using Ivy: <dependencies> <dependency org=\"org.apache.logging.log4j\" name=\"log4j-api\" rev=\"2.6.2\" /> <dependency org=\"org.apache.logging.log4j\" name=\"log4j-core\" rev=\"2.6.2\" /> </dependencies> Using Gradle: dependencies { compile group: 'org.apache.logging.log4j', name: 'log4j-api', version: '2.6.2' compile group: 'org.apache.logging.log4j', name: 'log4j-core', version: '2.6.2' } Getting log4j 1.x Note: Log4j 1.x has reached End-of-Life (EOL) (see Remarks). Using Maven: Declare this dependency in the POM.xml file: <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.17</version> https://riptutorial.com/

</dependency> Using Ivy: <dependency org=\"log4j\" name=\"log4j\" rev=\"1.2.17\"/> Usign Gradle: compile group: 'log4j', name: 'log4j', version: '1.2.17' Using Buildr: 'log4j:log4j:jar:1.2.17' Adding manually in path build: Download from Log4j website project How to use Log4j in Java code First need to create a final static logger object: final static Logger logger = Logger.getLogger(classname.class); Then, call logging methods: //logs an error message logger.info(\"Information about some param: \" + parameter); // Note that this line could throw a NullPointerException! //in order to improve performance, it is advised to use the `isXXXEnabled()` Methods if( logger.isInfoEnabled() ){ logger.info(\"Information about some param: \" + parameter); } // In log4j2 parameter substitution is preferable due to readability and performance // The parameter substitution only takes place if info level is active which obsoletes the use of isXXXEnabled(). logger.info(\"Information about some param: {}\" , parameter); //logs an exception logger.error(\"Information about some error: \", exception); Setting up property file Log4j gives you posibility to log data into console and file at same time. Create a log4j.properties file and put inside this basic configuration: # Root logger option log4j.rootLogger=DEBUG, stdout, file https://riptutorial.com/ 718

# Redirect log messages to console log4j.appender.stdout=org.apache.log4j.ConsoleAppender log4j.appender.stdout.Target=System.out log4j.appender.stdout.layout=org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n # Redirect log messages to a log file, support file rolling. log4j.appender.file=org.apache.log4j.RollingFileAppender log4j.appender.file.File=C:\\\\log4j-application.log log4j.appender.file.MaxFileSize=5MB log4j.appender.file.MaxBackupIndex=10 log4j.appender.file.layout=org.apache.log4j.PatternLayout log4j.appender.file.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n If you are using maven, put this propertie file in path: /ProjectFolder/src/java/resources Basic log4j2.xml configuration file <?xml version=\"1.0\" encoding=\"UTF-8\"?> <Configuration> <Appenders> <Console name=\"STDOUT\" target=\"SYSTEM_OUT\"> <PatternLayout pattern=\"%d %-5p [%t] %C{2} %m%n\"/> </Console> </Appenders> <Loggers> <Root level=\"debug\"> <AppenderRef ref=\"STDOUT\"/> </Root> </Loggers> </Configuration> This is a basic log4j2.xml configuration which has a console appender and a root logger. The pattern layout specifies which pattern should be used for logging the statements. In order to debug the loading of log4j2.xml you can add the attribute status = <WARN | DEBUG | ERROR | FATAL | TRACE | INFO> in the configuration tag of your log4j2.xml. You can also add a monitor interval so that it loads the configuration again after the specified interval period. The monitor interval can be added to the configuration tag as follows: monitorInterval = 30. It means that the config will be loaded every 30 seconds. Migrating from log4j 1.x to 2.x If you want to migrate from existing log4j 1.x in your project to log4j 2.x then remove all existing log4j 1.x dependencies and add the following dependency: Log4j 1.x API Bridge Maven Build <dependencies> https://riptutorial.com/ 719

<dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-1.2-api</artifactId> <version>2.6.2</version> </dependency> </dependencies> Ivy Build <dependencies> <dependency org=\"org.apache.logging.log4j\" name=\"log4j-1.2-api\" rev=\"2.6.2\" /> </dependencies> Gradle Build dependencies { compile group: 'org.apache.logging.log4j', name: 'log4j-1.2-api', version: '2.6.2' } Apache Commons Logging Bridge If your project is using Apache Commons Logging which use log4j 1.x and you want to migrate it to log4j 2.x then add the following dependencies: Maven Build <dependencies> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-jcl</artifactId> <version>2.6.2</version> </dependency> </dependencies> Ivy Build <dependencies> <dependency org=\"org.apache.logging.log4j\" name=\"log4j-jcl\" rev=\"2.6.2\" /> </dependencies> Gradle Build dependencies { compile group: 'org.apache.logging.log4j', name: 'log4j-jcl', version: '2.6.2' } Note: Do not remove any existing dependencies of Apache commons logging Reference: https://logging.apache.org/log4j/2.x/maven-artifacts.html Properties-File to log to DB For this example to work you'll need a JDBC driver compatible to the system the database is https://riptutorial.com/ 720

running on. An opensource one that allows you to connect to DB2 databases on an IBM System i can be found here: JT400 Even though this example is DB2 specific, it works for almost every other system if you exchange the driver and adapt the JDBC URL. # Root logger option log4j.rootLogger= ERROR, DB # Redirect log messages to a DB2 # Define the DB appender log4j.appender.DB=org.apache.log4j.jdbc.JDBCAppender # Set JDBC URL (!!! adapt to your target system !!!) log4j.appender.DB.URL=jdbc:as400://10.10.10.1:446/DATABASENAME;naming=system;errors=full; # Set Database Driver (!!! adapt to your target system !!!) log4j.appender.DB.driver=com.ibm.as400.access.AS400JDBCDriver # Set database user name and password log4j.appender.DB.user=USER log4j.appender.DB.password=PASSWORD # Set the SQL statement to be executed. log4j.appender.DB.sql=INSERT INTO DB.TABLENAME VALUES('%d{yyyy-MM- dd}','%d{HH:mm:ss}','%C','%p','%m') # Define the layout for file appender log4j.appender.DB.layout=org.apache.log4j.PatternLayout Filter Logoutput by level (log4j 1.x) You can use a filter to log only messages \"lower\" than e.g. ERROR level. But the filter is not supported by PropertyConfigurator. So you must change to XML config to use it. See log4j- Wiki about filters. Example \"specific level\" <appender name=\"info-out\" class=\"org.apache.log4j.FileAppender\"> <param name=\"File\" value=\"info.log\"/> <layout class=\"org.apache.log4j.PatternLayout\"> <param name=\"ConversionPattern\" value=\"%m%n\"/> </layout> <filter class=\"org.apache.log4j.varia.LevelMatchFilter\"> <param name=\"LevelToMatch\" value=\"info\" /> <param name=\"AcceptOnMatch\" value=\"true\"/> </filter> <filter class=\"org.apache.log4j.varia.DenyAllFilter\" /> </appender> Or \"Level range\" <appender name=\"info-out\" class=\"org.apache.log4j.FileAppender\"> <param name=\"File\" value=\"info.log\"/> <layout class=\"org.apache.log4j.PatternLayout\"> https://riptutorial.com/ 721

<param name=\"ConversionPattern\" value=\"%m%n\"/> </layout> <filter class=\"org.apache.log4j.varia.LevelRangeFilter\"> <param name=\"LevelMax\" value=\"info\"/> <param name=\"LevelMin\" value=\"info\"/> <param name=\"AcceptOnMatch\" value=\"true\"/> </filter> </appender> Read log4j / log4j2 online: https://riptutorial.com/java/topic/2472/log4j---log4j2 https://riptutorial.com/ 722

Chapter 113: Logging (java.util.logging) Examples Using the default logger This example shows how to use the default logging api. import java.util.logging.Level; import java.util.logging.Logger; public class MyClass { // retrieve the logger for the current class private static final Logger LOG = Logger.getLogger(MyClass.class.getName()); public void foo() { LOG.info(\"A log message\"); LOG.log(Level.INFO, \"Another log message\"); LOG.fine(\"A fine message\"); // logging an exception try { // code might throw an exception } catch (SomeException ex) { // log a warning printing \"Something went wrong\" // together with the exception message and stacktrace LOG.log(Level.WARNING, \"Something went wrong\", ex); } String s = \"Hello World!\"; // logging an object LOG.log(Level.FINER, \"String s: {0}\", s); // logging several objects LOG.log(Level.FINEST, \"String s: {0} has length {1}\", new Object[]{s, s.length()}); } } Logging levels Java Logging Api has 7 levels. The levels in descending order are: • SEVERE (highest value) • WARNING • INFO • CONFIG • FINE • FINER • FINEST (lowest value) https://riptutorial.com/ 723

The default level is INFO (but this depends on the system and used a virtual machine). Note: There are also levels OFF (can be used to turn logging off) and ALL (the oposite of OFF). Code example for this: import java.util.logging.Logger; public class Levels { private static final Logger logger = Logger.getLogger(Levels.class.getName()); public static void main(String[] args) { logger.severe(\"Message logged by SEVERE\"); logger.warning(\"Message logged by WARNING\"); logger.info(\"Message logged by INFO\"); logger.config(\"Message logged by CONFIG\"); logger.fine(\"Message logged by FINE\"); logger.finer(\"Message logged by FINER\"); logger.finest(\"Message logged by FINEST\"); // All of above methods are really just shortcut for // public void log(Level level, String msg): logger.log(Level.FINEST, \"Message logged by FINEST\"); } } By default running this class will output only messages with level higher then CONFIG: Jul 23, 2016 9:16:11 PM LevelsExample main SEVERE: Message logged by SEVERE Jul 23, 2016 9:16:11 PM LevelsExample main WARNING: Message logged by WARNING Jul 23, 2016 9:16:11 PM LevelsExample main INFO: Message logged by INFO Logging complex messages (efficiently) Let's look at a sample of logging which you can see in many programs: public class LoggingComplex { private static final Logger logger = Logger.getLogger(LoggingComplex.class.getName()); private int total = 50, orders = 20; private String username = \"Bob\"; public void takeOrder() { // (...) making some stuff logger.fine(String.format(\"User %s ordered %d things (%d in total)\", username, orders, total)); // (...) some other stuff } // some other methods and calculations https://riptutorial.com/ 724