Java Generics - 为什么返回超级 class 实例会产生编译错误

Question

Hi the following code gives compile error but I do not understand why.

class SomeClass <T extends BaseClass> {
    T method(T x) {
        return x;                   // OK
    }
    
    T method() {
        return new BaseClass();    // compile error
    }
}

The error says requires T but provided BaseClass .
And similarly this one:

class SomeClass {
    <T extends BaseClass> T method(T x) {
        return x;                          // OK
    }
    
    <T extends BaseClass> T method() {
        return new BaseClass();           // compile error
    }
}

I know that it can be resolved like this:

class SomeClass <T extends BaseClass> {
    T method(T x) {
        return x;                       // OK
    }
    
    @SuppressWarnings( "unchecked" )
    T method() {
        return (T) new BaseClass();    // OK
    }
}

But may I have some explanation on why this is not allow?

Answer 1

That's just how inheritance works. Say you have a superclass A, and it has subclasses B and C. Now imagine your method accepts a parameter of type A and returns an object of type B. Your method would accept objects of type C as a parameter, but you couldn't return them because they are not of type B, even though both B and C are subclasses of A.