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

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());
    }
}

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();
    }
}

If a class implements 2 interfaces both of which have a java-8 default method with the same signature (as in your example) the implementing class is obliged to override the method. The class can still access the default method using I1.super.getGreeting();. It can access either, both or neither. So the following would be a valid implementation of C1

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

    @Override //class is obliged to override this method
    public String getGreeting() {
        //can use both default methods
        return I1.super.getGreeting()+I2.super.getGreeting();
    }

    public String useOne() {
        //can use the default method within annother method
        return "One "+I1.super.getGreeting();
    }

    public String useTheOther() {
        //can use the default method within annother method
        return "Two "+I2.super.getGreeting();
    }


}