Java 8默认方法继承 [英] Java 8 default method inheritance

问题描述

假设有以下类型：

public interface Base {

    default void sayHi(){
        System.out.println("hi from base");
    }
}

public interface Foo extends Base {
    @Override
    default void sayHi(){
        System.out.println("hi from foo");
    }
}

public interface Bar extends Base {
}

public class MyClass implements Foo, Bar {
    public static void main(String[] args) {
        MyClass c = new MyClass();
        c.sayHi();
    }
}

在这种情况下，如果执行main ，打印hi from foo。为什么 Foo 的实现优先？不 Bar 从 Base 继承 sayHi（），因为如果 MyClass 只是实现 Bar ，那么 Base 实施会被称为？因此，代码仍然无法编译是有意义的。另外，因为 Bar 应该有 Base 的 sayHi（），为什么我不能在 MyClass 中覆盖它，如：

In this scenario, if main is executed, "hi from foo" is printed. Why does Foo's implementation take precedence? Doesn't Bar inherit sayHi() from Base, since if MyClass was to only implement Bar, the Base implementation would be called? So it would make sense for the code to still not compile. Also, since Bar should have Base's implementation of sayHi(), why can't I override it in MyClass like:

@Override
public void sayHi() {
    Bar.super.sayHi();
}

尝试这样做时出现以下错误：

The following error occurs when trying to do so:

错误类型限定符默认超级调用方法中的条形，在Foo中重写sayHi（）

bad type qualifier Bar in default super call method, sayHi() is overridden in Foo

推荐答案

使用 JLS 9.4.1 ，只是更改了一些名称：

This behavior is specified using almost your exact example in JLS 9.4.1, just with some names changed around:

interface Top {
    default String name() { return "unnamed"; }
}
interface Left extends Top {
    default String name() { return getClass().getName(); }
}
interface Right extends Top {}

interface Bottom extends Left, Right {}

从Top继承name（），但Bottom从Left继承name（），
不是Right。这是因为Left中的name（）会覆盖Top中name（）的声明
。

Right inherits name() from Top, but Bottom inherits name() from Left, not Right. This is because name() from Left overrides the declaration of name() in Top.

JLS不会似乎给出了我能看到的任何特别具体的理由;这就是Java设计者决定继承的方式。

The JLS doesn't seem to give any especially concrete reason that I can see; this is just how the Java designers decided inheritance would work.

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