Default Methods In Java 8

17 Feb, 2020
Before Java 8, interfaces could have only abstract methods. The implementation of these methods has to be provided in a separate class. So, if a new method is to be added in an interface, then its implementation code has to be provided in the class implementing the same interface. To overcome this issue, Java 8 has introduced the concept of default methods which allow the interfaces to have methods with implementation without affecting the classes that implement the interface.
// A simple program to Test Interface default// methods in javainterface TestInterface{    // abstract method    public void square(int a);      // default method    default void show()    {      System.out.println("Default Method Executed");    }}  class TestClass implements TestInterface{    // implementation of square abstract method    public void square(int a)    {        System.out.println(a*a);    }      public static void main(String args[])    {        TestClass d = new TestClass();        d.square(4);          // default method executed        d.show();    }}
Output:
 16
 Default Method Executed

The default methods were introduced to provide backward compatibility so that existing interfaces can use the lambda expressions without implementing the methods in the implementation class. Default methods are also known as defender methods or virtual extension methods.
Static Methods:The interfaces can have static methods as well which is similar to static method of classes.
// A simple Java program to TestClassnstrate static// methods in javainterface TestInterface{    // abstract method    public void square (int a);      // static method    static void show()    {        System.out.println("Static Method Executed");    }}  class TestClass implements TestInterface{    // Implementation of square abstract method    public void square (int a)    {        System.out.println(a*a);    }      public static void main(String args[])    {        TestClass d = new TestClass();        d.square(4);          // Static method executed        TestInterface.show();    }}
Output:
 16
 Static Method Executed
Default Methods and Multiple InheritanceIn case 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.
// A simple Java program to demonstrate multiple// inheritance through default methods.interface TestInterface1{    // default method    default void show()    {        System.out.println("Default TestInterface1");    }}  interface TestInterface2{    // Default method    default void show()    {        System.out.println("Default TestInterface2");    }}  // Implementation class codeclass TestClass implements TestInterface1, TestInterface2{    // Overriding default show method    public void show()    {        // use super keyword to call the show        // method of TestInterface1 interface        TestInterface1.super.show();          // use super keyword to call the show        // method of TestInterface2 interface        TestInterface2.super.show();    }      public static void main(String args[])    {        TestClass d = new TestClass();        d.show();    }}
Output:
Default TestInterface1
Default TestInterface2
Important Points:
Interfaces can have default methods with implementation in Java 8 on later.Interfaces can have static methods as well, similar to static methods in classes.Default methods were introduced to provide backward compatibility for old interfaces so that they can have new methods without affecting existing code.
Interfaces can have default methods with implementation in Java 8 on later.
Interfaces can have static methods as well, similar to static methods in classes.
Default methods were introduced to provide backward compatibility for old interfaces so that they can have new methods without affecting existing code.
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