Implementing two interfaces with two default methods of the same signature in Java 8

Question

Suppose I have two interfaces:

public interface I1 {     default String getGreeting() {         return "Good Morning!";     } }  public interface I2 {     default String getGreeting() {         return "Good Afternoon!";     } } 

If I want to implement both of them, what implementation will be used?

public class C1 implements I1, I2 {     public static void main(String[] args)     {         System.out.println(new C1().getGreeting());     } } 

Answer 1

This is a compile-time error. You cannot have two implementation from two interfaces.

However, it is correct, if you implement the getGreeting method in C1:

public class C1 implements I1, I2 // this will compile, bacause we have overridden getGreeting() {     public static void main(String[] args)     {         System.out.println(new C1().getGreeting());     }      @Override public String getGreeting()     {         return "Good Evening!";     } } 

I just want to add that even if the method in I1 is abstract, and default in I2, you cannot implement both of them. So this is also a compile-time error:

public interface I1 {     String getGreeting(); }  public interface I2 {     default String getGreeting() {         return "Good afternoon!";     } }  public class C1 implements I1, I2 // won't compile {     public static void main(String[] args)     {         System.out.println(new C1().getGreeting());     } } 

Answer 2

This is not specific to the question. But, I still think that it adds some value to the context. As an addition to @toni77's answer, I would like to add that the default method can be invoked from an implementing class as shown below. In the below code, the default method getGreeting() from interface I1 is invoked from an overridden method:

public interface I1 {      default String getGreeting() {         return "Good Morning!";      } }  public interface I2 {     default String getGreeting() {         return "Good Night!";     } }  public class C1 implements I1, I2 {            @Override     public String getGreeting() {         return I1.super.getGreeting();     } }