java - 扩展其他接口(interface)的默认方法和接口(interface)

Question

假设有两个接口(interface)Interface1和Interface2，其中Interface2扩展Interface1。

interface Interface1 {

    default void method() {
        System.out.println("1");
    }

    // Other methods
}

interface Interface2 extends Interface1 {

    @Override
    default void method() {
        System.out.println("2");
    }

    // Other methods
}

假设我想创建一个实现 Interface2 的类，但我希望 method() 成为 Interface1 中的版本。如果我写

class MyClass implements Interface1, Interface2 {

    public void method() {
        Interface1.super.method();
    }
}

我得到编译错误:

bad type qualifier in default super call: redundant interface Interface1 is extended by Interface2

可以通过创建第三个接口(interface)来解决这个问题:

interface Interface3 extends Interface1 {

    default void method() {
        Interface1.super.method();
    }
}

然后:

class MyClass implements Interface1, Interface2, Interface3 {

    public void method() {
        Interface3.super.method();
    }
}

这编译得很好，如果我实例化一个新的 MyClass 并调用 method()，输出是 1 正如预期的那样。

所以我的问题是，鉴于很容易绕过限制，您只能为链中最具体的接口(interface)编写 InterfaceName.super.method()，那么什么是限制的原因？首先禁止您编写 Interface1.super.method() 可以防止哪些问题？

Answer 1

JLS 在 15.12.3. 中完全解决了这个问题“编译时第 3 步:选择的方法是否合适？”.

If the form is TypeName . super . [TypeArguments] Identifier, then:

• […]
• If TypeName denotes an interface, let T be the type declaration immediately enclosing the method invocation. A compile-time error occurs if there exists a method, distinct from the compile-time declaration, that overrides (§9.4.1) the compile-time declaration from a direct superclass or direct superinterface of T.

JLS 继续解释为什么制定该规则:

In the case that a superinterface overrides a method declared in a grandparent interface, this rule prevents the child interface from "skipping" the override by simply adding the grandparent to its list of direct superinterfaces. The appropriate way to access functionality of a grandparent is through the direct superinterface, and only if that interface chooses to expose the desired behavior.

所以它或多或少是专门用来阻止你做你想做的事情的。

但 JLS 似乎也承认您的解决方法:

(Alternately, the developer is free to define his own additional superinterface that exposes the desired behavior with a super method invocation.)