Default Methods in Java 8

Default Methods in Java 8

With the release of Java 8, it is now possible for an interface method to define a default implementation. This new capability is called the default method.

Default method enables us a means by which interfaces could be expanded without breaking pre-existing code.

In simple terms default methods enable us to add new functionalities to interfaces without breaking the classes that implements that interface.

When a non-abstract class implements an interface, it must implement all methods defined by that interface. If a new method is to an existing interface, then the addition of that method would break pre-existing code, because no implementation would be found for that new method in pre-existing classes. The default method solves this problem by supplying an implementation that will be used if no other implementation is explicitly provided. Thus, the addition of a default method will not cause pre-existing code to break. This enables interfaces to be gracefully evolved over time without negative consequences.

Example of Default Method

public interface Account { default void OpenAccount(){       System.out.println("This is the Account Interface . . . ."); } }

public class SavingAccount implements Account{ public void OpenSavingAccount(){       System.out.println("This is the Saving Account Class . . . ."); } }

public class Main { public static void main(String[] args) {       SavingAccount sa=new SavingAccount();       sa.OpenAccount(); // Default method of interface is called       sa.OpenSavingAccount(); } }

Upon executing the Main class, we get the following output.

This is the Account Interface . . . .

This is the Saving Account Class . . . .

Default Methods and Multiple Inheritance
In case of multiple Inheritance, where both the implemented interfaces contain default methods with same method signature, the implementing class should explicitly specify which default method is to be used or it should override the default method.

interface InterfaceOne {       // Default method       default void show()       {             System.out.println("Default InterfaceOne");       } }

interface InterfaceTwo {       // Default method       default void show()       {             System.out.println("Default InterfaceTwo");       } }

public class MainClass implements InterfaceOne, InterfaceTwo {       // Overriding default show method       public void show()       {             // use super keyword to call the show             // method of InterfaceOne interface             InterfaceOne.super.show();             // use super keyword to call the show             // method of InterfaceTwo interface             InterfaceTwo.super.show();       }       public static void main(String args[])       {             MainClass d = new MainClass();             d.show();       } }

Upon executing the MainClass, we get the following output.

Default InterfaceOne

Default InterfaceTwo

Important Points:

1.     Interfaces can have default methods with implementation from java 8 onwards.

2.     Interfaces can have static methods as well similar to static method of classes.

3.     Default methods were introduced to provide backward comparability for old interfaces so that they can have new methods without effecting existing code.

Lambda expressions in Java 8

Lambda expressions are a new and important feature included in Java SE 8. They provide a clear and concise way to represent one method interface using an expression. Lambda expressions also improve the Collection libraries making it easier to iterate through, filter, and extract data from a Collection.

A lambda expression can be understood as a concise representation of an anonymous function that can be passed around: it doesn’t have a name, but it has a list of parameters, a body, a return type, and also possibly a list of exceptions that can be thrown.

o    Anonymous— Anonymous because it doesn’t have an explicit name like a method would normally have: less to write and think about!

o    Function— Function because a lambda isn’t associated with a particular class like a method is. But like a method, a lambda has a list of parameters, a body, a return type, and a possible list of exceptions that can be thrown.

o    Passed around— A lambda expression can be passed as argument to a method or stored in a variable.

o    Concise— You don’t need to write a lot of boilerplate like you do for anonymous classes.

Lambdas technically let you do anything that you could do prior to Java 8. But you no longer have to write clumsy code using anonymous classes.

 The result is that your code will be clearer and more flexible. For example, using a lambda expression you can create a custom Comparator object in a more concise way.

Before:

Comparator<Apple> byWeight = new Comparator<Apple>() { public int compare(Apple a1, Apple a2){ return a1.getWeight().compareTo(a2.getWeight()); } };

After (with lambda expressions):

Comparator<Apple> byWeight = (Apple a1, Apple a2) -> a1.getWeight().compareTo(a2.getWeight());

Lambda Expression Syntax

Lambda expressions address the bulkiness of anonymous inner. A lambda expression is composed of three parts.

A lambda expression is composed of parameters, an arrow,

and a body.

Argument List	Arrow Token	Body
(int x, int y)	->	x + y

The body can be either a single expression or a statement block. In the expression form, the body is simply evaluated and returned. In the block form, the body is evaluated like a method body and a return statement returns control to the caller of the anonymous method. The break and continue keywords are illegal at the top level, but are permitted within loops. If the body produces a result, every control path must return something or throw an exception.

Take a look at these examples:

(int x, int y) -> x + y

() -> 42

(String s) -> { System.out.println(s); }

 

The first expression takes two integer arguments, named x and y, and uses the expression form to return x+y.

The second expression takes no arguments and uses the expression form to return an integer 42.

The third expression takes a string and uses the block form to print the string to the console, and returns nothing.

Lambda Examples

Runnable Lambda

public class RunnableTest {   public static void main(String[] args) {     System.out.println("=== RunnableTest ===");        // Anonymous Runnable     Runnable r1 = new Runnable(){          @Override       public void run(){         System.out.println("Hello world one!");       }     };         // Lambda Runnable     Runnable r2 = () -> System.out.println("Hello world two!");         // Run them     r1.run();     r2.run();       } }

Comparator Lambda

In Java, the Comparator class is used for sorting collections. In the following example, an ArrayList consisting of Person objects is sorted based on surName. The following are the fields included in the Person class.

public class Person { private String givenName; private String surName; private int age; private String eMail; private String phone; private String address; }

The following code applies a Comparator by using an anonymous inner class and a couple lambda expressions.

public class ComparatorTest {  public static void main(String[] args) {        // Create List of Person         List<Person> personList1 = null;           // Sort with Inner Class     Collections.sort(personList1, new Comparator<Person>(){       public int compare(Person p1, Person p2){         return p1.getSurName().compareTo(p2.getSurName());       }     });         System.out.println("=== Sorted Asc SurName ===");     for(Person p:personList1){       System.out.println(                   "Name: " + p.getGivenName() + " " + p.getSurName());     }         // Use Lambda instead         // Print Asc     System.out.println("=== Sorted Asc SurName ===");     Collections.sort(personList1, (Person p1, Person p2) -> p1.getSurName().compareTo(p2.getSurName()));     for(Person p:personList1){ System.out.println(                   "Name: " + p.getGivenName() + " " + p.getSurName());     }         // Print Desc     System.out.println("=== Sorted Desc SurName ===");     Collections.sort(personList1, (p1,  p2) -> p2.getSurName().compareTo(p1.getSurName()));     for(Person p:personList1){ System.out.println(                   "Name: " + p.getGivenName() + " " + p.getSurName());     }       } }