是否可以在不创建实例的情况下检查一个 class 是否在 java 中扩展另一个？

Question

I have two Class<? extends SomeClass> Class<? extends SomeClass> variables, one obtained from looping the keys of a HashMap , we'll call it A , and one passed as a parameter, which we'll call B . I'd like to see if B extends A .

So, as I currently understand things, I'd make an instance of B (using createInstance ) and check instanceof . But this is not only pretty slow, but cumbersome as classes need an empty constructor. See here and here , where they discuss isAssignableFrom and getSuperClass , but isAssignableFrom (from what I read there) doesn't work on the same class (SomeClass.isAssignableFrom(SomeClass)?), and getSuperClass doesn't walk up the tree.

Is there a way I can check if B extends A without creating an instance of B ?

For example:

class SomeClass {}
class ExtendingClassA extends SomeClass {}
class ExtendingClassAB extends ExtendingClassA {}
class ExtendingClassB extends SomeClass {}
boolean ClassExtendsClass(Class<? extends SomeClass> A,Class<? extends SomeClass> B) {
    return A.class instanceof B.class;
}
ClassExtendsClass(SomeClass.class, ExtendingClassA.class); //true
ClassExtendsClass(SomeClass.class, ExtendingClassAB.class); //true
ClassExtendsClass(SomeClass.class, ExtendingClassB.class); //true
ClassExtendsClass(ExtendingClassA.class, ExtendingClassB.class); //false

Answer 1

Yup. Generally, check the API (javadoc) of the type you think would be the right place for it. In this case, that'd be Class itself, and, luckily, it's there :

Class<?> a = Integer.class;
Class<?> b = Number.class;
System.out.println(b.isAssignableFrom(a));
System.out.println(b.isAssignableFrom(b));
> true
> true

but isAssignableFrom (from what I read there) doesn't work on the same class

You should test that stuff before making assumptions. It works fine.

Answer 2

you could use the getSuperclass method for a class object recursively until you have no more superclasses:

class A {}
class B extends A {}
class C extends B {}

public class Main {
    public static void main(String[] args) {
        Class clazz = C.class;

        System.out.println(clazz.getClass());
        System.out.println(clazz.getSuperclass());
        System.out.println(clazz.getSuperclass().getSuperclass());
        System.out.println(clazz.getSuperclass().getSuperclass().getSuperclass());
        System.out.println(clazz.getSuperclass().getSuperclass().getSuperclass().getSuperclass());
    }
}

the above example will produce

class java.lang.Class
class B
class A
class java.lang.Object
null