instanceOf - most useless operator ever?

[Mike Curwen]

So I thought I had found an errata in a most excellent SCJP book. But when I actually typed in the code, of course the author was correct.

class Ticker extends Component {
  
  public static void main (String args[]) {
     Ticker t = new Ticker();
     boolean test = t instanceof String;
   
  }
}

The last line will produce the compiler error:

C:\Ticker.java:8: inconvertible types
found   : Ticker
required: java.lang.String
     boolean test = t instanceof String;

So the only comparisons that get by the compiler are Object or Component or Ticker.

I thought this would be a run-time thing, and that "no 't' is not a String" would produce boolean false.

So if the only objects that I can use on either side of the operator are *already* in the same tree, I already know that they are in an "IS-A" relation (assuming that the LHS operand is lower in the tree than the RHS).

So what good is instanceOf ?

[Michael Matola]

Originally posted by Mike Curwen:
So if the only objects that I can use on either side of the operator are *already* in the same tree, I already know that they are in an "IS-A" relation (assuming that the LHS operand is lower in the tree than the RHS).

I don't have any good response to your question in general, but just wanted to point out that what you're saying is not exactly true if the type you're checking against is an interface.
Checking instanceof Iterator below compiles and evaluates to false.

import java.util.Iterator ;
interface Marker
{
}
interface SubMarker extends Marker
{
}
public class Ticker implements SubMarker
{
    public static void main (String args[])
    {
        Ticker t = new Ticker();
        // boolean test = t instanceof String;
        boolean test1 = t instanceof Ticker ;
        System.out.println( test1 );
        boolean test2 = t instanceof Object ;
        System.out.println( test2 );
        boolean test3 = t instanceof Marker ;
        System.out.println( test3 );
        boolean test4 = t instanceof SubMarker ;
        System.out.println( test4 );
        boolean test5 = t instanceof Iterator ;
        System.out.println( test5 );
    }
}

[Michael Matola]

Ok, I think I'm getting it.
If you upcast your refrence all the way to Object, then the compiler's OK with instanceof String:
Object t7 = new Ticker() ;
boolean test7 = t7 instanceof String ;
System.out.println( test7 );
But if your reference is Ticker (or Component or Marker or SubMarker...), then it knows even at compile time that the check of instanceof is false/not relevant:
Component t8 = new Ticker();
boolean test8 = t8 instanceof String ;
System.out.println( test8 );
=> compiler error
[ February 05, 2003: Message edited by: Michael Matola ]