java - 重写方法中的泛型

Question

遇到一个有趣的问题；下面的类编译:

public class Test {

    public static void main(String[] args) throws Exception {
        A a = new A();
        B b = new B();

        foo(a);
        foo(b);
    }   

    private static void foo(A a) {
        System.out.println("In A");
    }   

    private static void foo(B b) {
        System.out.println("In B");
    }   

    private static class A {}

    private static class B extends A {}

}

但是这个失败了:

public class Test {

    public static void main(String[] args) throws Exception {
        A<String> a = new A<>();
        B b = new B();

        foo(a);
        foo(b);
    }   

    private static void foo(A<String> a) {
        System.out.println("In A");
    }   

    private static void foo(B b) {
        System.out.println("In B");
    }   

    private static class A<T> {}

    private static class B extends A {}

}

出现此错误:

Test.java:8: error: reference to foo is ambiguous, both method foo(A<String>) in Test and method foo(B) in Test match              
        foo(b);                                                                                                                    
        ^                                                                                                                          
Note: Test.java uses unchecked or unsafe operations.                                                                               
Note: Recompile with -Xlint:unchecked for details.                                                                                 
1 error

我原以为，由于类型删除，它们本质上是相同的。有人知道这里发生了什么吗？

Answer 1

原因是您混合了泛型和原始类型(B 应该声明为 class B<T> extends A<T> 或 class B extends A<SomeType>)。

发生这种情况的真正原因隐藏在 the JLS, section #15.12.2.7 中的某处。和以下 - 祝你好运简洁地表达它;-)