Java中的类等效

Question

A small hiccup led me to a basic question. I intend to check the class equivalence in the following case (academic curiosity - Please do not direct me to instanceof or isinstance ):

public class staticChild extends staticParent{
    ....
        public static void main(String[] args){
        staticChild thisobj=new staticChild();
        System.out.println(thisobj.getClass()==staticParent.class);
    }

}

This gives compilation error:

error: incomparable types: Class<CAP#1> and Class<staticParent>
                System.out.println(thisobj.getClass()==staticParent.class);
 where CAP#1 is a fresh type-variable: CAP#1 extends staticChild from capture of ? extends staticChild

1. What does this mean?
2. What is the difference between type and class(specifically to this context - what is the type and class of staticchild)?
3. How do I get rid of the compilation error?

Answer 1

Since thisobj is declared to be of type staticChild , the expression:

thisobj.getClass()

returns an object of type Class<? extends staticChild> Class<? extends staticChild> . You are trying to compare that to an object of type Class<staticParent> but the compiler cannot match the generic type parameters. It knows that staticParent cannot possibly extend staticChild so there's nothing it can assign to ? in Class<? extends staticChild> Class<? extends staticChild> .

There are several ways to get rid of the error. For instance, you could declare thisobj to be of type Object (or even of type staticParent ).

Note, however, that the test will always fail.

Answer 2

1. It means that the two objects you are trying to compare can never be equal because their types are not convertible to each other. According to JLS 15.21.3 ,

It is a compile-time error if it is impossible to convert the type of either operand to the type of the other by a casting conversion (§5.5). The run-time values of the two operands would necessarily be unequal.

1. There is no difference between type and class in this context.

2. Do not try to compare two objects that are not convertible to one another. In this case you are comparing Class<CAP#1> with Class<staticParent> . However, if you wanted to "fix" the error, you could use .equals instead, with the understanding that it would always return false .

    System.out.println(thisobj.getClass().equals(staticParent.class)); 

Answer 3

The compiler is already smart enough to say classes are not equivalent. It assumes you are writing a usual program, not solving an academic task, and tries to assist at its best.

If you still need to do this manually "because of Academic curiosities", assign them to the general variable of type Class first:

Class a = staticChild.class;
Class b = staticParent.class;

if (a == b) // false

(will show "raw type" warning but should compile).

Object x = a.newInstance();
Object y = b.newInstance();

Rest is up to you.

Answer 4

It means you have an error in your code.

The error indicates that you have incompatible types. Yes, you can see it from the error message, but what incompatible types. You are comparing two classes via "==".

If you are trying to check if thisobj is an instance of staticParent then you should use:

System.out.println(thisobj instanceof staticParent);

But specifically the error is because you are running a comparison for true/false based on two classes that can never be equal.