覆盖泛型方法(Override generic method)

 在JLS（Java语言规范）中：子签名的概念被设计用来表示两个方法之间的关系，这两个方法的签名不相同，但其中一个可能会覆盖另一个。 具体来说，它允许一种方法，其签名不使用泛型类型来覆盖该方法的任何泛化版本。 
 
interface CollectionConverter<U> {
    <T> List<T> toList(Collection<T> c);

    void fooMethod(Class<?> c);

    <E>Comparable<E> method3(E e);

    Comparable<U> method4(U u);
}

class Overrider implements CollectionConverter<Integer> {
    @Override
    public List toList(Collection c) {
        return null;
    }

    @Override
    public void fooMethod(Class c) {

    }

    @Override
    public  Comparable method3(Object o) {
        return null;
    }

    @Override
    // compile error, have to change Object to Integer 
    public Comparable method4(Object u) {                       

        return null;
    }
}
 
 根据JLS，我明白为什么前3种方法能够正常工作，但我无法弄清楚为什么method4有一个编译错误，它说类型Overrider的方法method4（Object）必须覆盖或实现一个超类型方法。 

According to the JLS (Java Language Specification):
 
 
 
The notion of subsignature is designed to express a relationship between two methods whose signatures are not identical, but in which one may override the other. Specifically, it allows a method whose signature does not use generic types to override any generified version of that method.
 
 
This code is based on the JLS example:
 
interface CollectionConverter<U> {
    <T> List<T> toList(Collection<T> c);

    void fooMethod(Class<?> c);

    <E>Comparable<E> method3(E e);

    Comparable<U> method4(U u);
}

class Overrider implements CollectionConverter<Integer> {
    @Override
    public List toList(Collection c) {
        return null;
    }

    @Override
    public void fooMethod(Class c) {

    }

    @Override
    public  Comparable method3(Object o) {
        return null;
    }

    @Override
    // compile error, have to change Object to Integer 
    public Comparable method4(Object u) {                       

        return null;
    }
}
 
According to the JLS, I understand why the first three methods work well, but I can't figure out why method4 has this compilation error:
 
 
 
The method method4(Object) of type Overrider must override or implement a supertype method.
 

原文：https://stackoverflow.com/questions/9731574