在 Java 8 中使用两个具有相同签名的默认方法实现两个接口 [英] Implementing two interfaces with two default methods of the same signature in Java 8

问题描述

假设我有两个接口:

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

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

如果我想同时实现它们，会使用什么实现?

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());
    }
}

推荐答案

这是一个编译时错误.两个接口不能有两个实现.

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

但是，如果你在C1中实现了getGreeting方法是正确的:

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!";
    }
}

我只想补充一点，即使I1中的方法是抽象的，而I2中的默认方法，您也不能同时实现它们.所以这也是一个编译时错误:

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());
    }
}

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