重温 Thinking in Java 7 - instanceof vs Class equivalence

instanceof vs Class equivalence

 

/*
	When you are querying for type information, there's an important different between either form of instanceof(that is, instanceof or
	isInstance(), which produce equivalent results) and the direct comparison of the Class objects. Here's an example that demonstrates
	the difference.
*/

class Base {
}

class Derived extends Base {
}

public class FamilyVsExactType {
	
	static void test(Object x){
		System.out.println("Testing x of type " + x.getClass());
		System.out.println("x instanceof Base " + (x instanceof Base));
		System.out.println("x instanceof Derived " + (x instanceof Derived));
		System.out.println("Base.isInstance(x) " + Base.class.isInstance(x));
		System.out.println("Derived.isInstance(x) " + Derived.class.isInstance(x));
		System.out.println("x.getClass() == Base.class " + (x.getClass() == Base.class));
		System.out.println("x.getClass() == Derived.class " + (x.getClass() == Derived.class));
		System.out.println("x.getClass().equals(Base.class)) " + (x.getClass().equals(Base.class)));
		System.out.println("x.getClass().equals(Derived.class)) " + (x.getClass().equals(Derived.class)));
		System.out.println();
	}
	
	public static void main(String[] args){
		test(new Base());
		test(new Derived());
	}
}
/* Output:
Testing x of type class Base
x instanceof Base true
x instanceof Derived false
Base.isInstance(x) true
Derived.isInstance(x) false
x.getClass() == Base.class true
x.getClass() == Derived.class false
x.getClass().equals(Base.class)) true
x.getClass().equals(Derived.class)) false

Testing x of type class Derived
x instanceof Base true
x instanceof Derived true
Base.isInstance(x) true
Derived.isInstance(x) true
x.getClass() == Base.class false
x.getClass() == Derived.class true
x.getClass().equals(Base.class)) false
x.getClass().equals(Derived.class)) true

Reassuringly, instanceof and isInstance() produce exactly the same results, as do equals() and ==. But the tests themselves draw
different conclusions. In keeping with the concept of type, instanceof says, "Are you this class, or a class derived from this class?"
On the other hand, if you compare the actual Class objects using ==, there is no concern with inheritance - it's either the exact type
or it isn't.
*/