How can define Default method in interfaces (Java 8) ?

Default method in interface

Java 8 allows to define default method in interface, to add default implementation by using "default " keyword

package com.genius;

interface MyInterface{ 

  abstract void add();

  default void display(){

    System.out.println("default method of interface");

  }

}

BONUS QUESTIONS

Q1. Can we have a default method with body in the interface without specifying the keyword "default" ?

No, Compilation error will occur.

It will treat it as an abstract method so it shouldn't have the body.

Q2. Can a class implement two Interfaces having default method with same signature ?

public interface Interface1 {

  default public void show() { … }

}

public interface Interface2

  default public void show() { … }

public class MyClass implements Interface1, Interface2

No, Compilation error "Duplicate Default Methods"

Q3. What will happen using above example, if we make method as abstract in second interface ?

public interface Interface1 {

  default public void show() { … }

}

public interface Interface2

  public void show() { … }

public class MyClass implements Interface1, Interface2

Same problem, Compilation error "Duplicate Default Methods"

Q4. What will happen using above example, if we override the conflicting method in the Class ?

public interface Interface1 {

  default public void show() { … }

}

public interface Interface2

  default public void show() { … }

public class MyClass implements Interface1, Interface2 {

  public void show() { … }

}

No compilation error, and class method will be executed.

Q5. What will happen using above example, if there is a default method conflict as mentioned above and we have same signature method in the base class instead of overriding in the existing class ?

public interface Interface1 {

  default public void show() { … }

}

public interface Interface2

  default public void show() { … }

public class BaseClass {

  public void show() { … }

}

public class MyClass extends BaseClass implements Interface1, Interface2

No compilation error, as base class method have precedence over the Interface Default methods.

Q6. If there is a conflict between Base Class Method definition and Interface Default method definition, which definition is used ?

Method from Base class

Q7. If there is a conflict between Base Class Method definition, Interface Default method definition and Derived class Method definition, which definition is used ?

Method from Derived class