java - 泛型方法继承

Question

问题是关于以下代码块:

public class SomeClass {
    public static class A {
        public void f(int x) {
            System.out.println("1");
        }

        public void f(Object x) {
            System.out.println("2");
        }
    }

    public static class B extends A {
        public <T> void f(T x) {   // *
            System.out.println("3");
        }
    }
}

行 * 无法编译，并出现以下错误:

Description Resource Path Location Type Name clash: The method f(T) of type SomeClass.B has the same erasure as f(Object) of type SomeClass.A but does not override it

避免重复:我看过:Type erasure, overriding and generics , Method has the same erasure as another method in type虽然我收到了答案(部分是因为我仍然没有完全理解它)为什么会出现编译错误(以避免由于类型删除而产生歧义？是这样吗？)主要问题是我不'不明白为什么，如果我在 f(Object x) 和 f(T x) 两种方法之间切换，那么代码将如下所示:

public class SomeClass {
    public static class A {
        public void f(int x) {
            System.out.println("1");
        }


        public <T> void f(T x) {
            System.out.println("3");
        }

    }

    public static class B extends A {
        public void f(Object x) {
            System.out.println("2");
        }
    }
}

我没有收到编译错误。为什么会发生这种情况？对于第一个代码，我得到编译错误，而对于第二个代码，我没有得到编译错误，这之间的关键区别是什么？希望对此进行一些解释，谢谢!

Answer 1

One of the answers to one of the questions you linked基本上已经解决了这个问题:

For overriding with instance methods you need the overriding method to be a subsignature of the overridden method.

要进一步深入了解这一点，JLS 8.4.8.1说的是:

An instance method m_C declared in or inherited by class C, overrides from C another method m_A declared in class A, iff all of the following are true:

• [...]

• The signature of m_C is a subsignature (§8.4.2) of the signature of m_A.

子签名在 JLS 8.4.2 中定义:

The signature of a method m₁ is a subsignature of the signature of a method m₂ if either:

• [...]

• the signature of m₁ is the same as the erasure (§4.6) of the signature of m₂.

您正在考虑以下两个签名:

1. void f(Object x)
2. <T> void f(T x)

#2 的删除是 void f(Object x) ，因此 #1 是它的子签名。然而，反之亦然是不正确的。