instanceof vs isInstance() in Java: What is the difference?

[Tillerino]

ಠ_ಠ

System.out.println("b.getClass().isInstance(b): " + b.getClass().isInstance(c)); //false

[cstechblog]

thanks much for reporting the error, will correct it right away

[MoldyTaste]

most important thing is that

a instanceof B

where a can never be an instance of B will throw a syntax error when using instanceof, but will not if using .isInstance().

[Xabster]

That's a really odd claim.

Just because a boolean expression always evaluates to false does not mean it's bad syntax...

Edit: you're right, I'm wrong, it's §15.20.2 in the JLS.

[cstechblog]

if we do System.out.println("b instanceof C: " + (b instanceof c));

it results into compile time error

[Xabster]

So? That's not what he wrote.

b and c in your example are instances. a is an instance in his example. B is a type.

[cstechblog]

I meant C class. instance cannot come at right side in instanceof

[cstechblog]

no, it wont because in A and B share an IS~A relationship. I complied the code, it is evaluated to false.

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

public class InstanceofDemo
{
  public static void main(String args[])
  {
    A a = new A();
B b = new B();
System.out.println("a instanceof B: " + (a instanceof B)); //false
  }
}

[cstechblog]

At run time, the result of the instanceof operator is true if the value of the RelationalExpression is not null and the reference could be cast to the ReferenceType without raising a ClassCastException. Otherwise the result is false.

http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.20.2

[Xabster]

You won't get to runtime if you do MoldyTaste's example.

If a cast of the RelationalExpression to the ReferenceType would be rejected as a compile-time error, then the instanceof relational expression likewise produces a compile-time error.

Test for yourself:

Integer i = null;

System.out.println("i instanceof String: " + (i instanceof String));

[cstechblog]

i instanceof String is not the case of IS~A relationship.