11. Instanceof

The instanceof operator tells you if a variable is an “instance of” (made from a subclass of) a given class. Example: if (x instanceof Cat) { ... }.

Casting looks like this: (Cat) x. Casting can fail (with a “runtime exception”) if x is not really a Cat. This can happen inside functions that do not know where their inputs are coming from.

Frequently you check to see if something is the right class before using a cast to “convert it” to that class. The quotes are around “convert” because casting just reveals a relationship that is already true, it does not create a new relationship or change the object in any way.

Fuller example:

 Object x = new Cat()
 Cat y = x; // fails
 Cat w = (Cat) x; // ok
 if ( x instanceof Cat ) { 
     Cat z = (Cat) x; // ok
     System.out.println("Really a cat"); 
 }

Practice

1. Create an public static boolean isCat(Object x) function that will tell if you an object is a cat.

2. Create a public static void petIt(Object x) function that prints the noise the Cat makes if x is a Cat. Otherwise it should print “No way!”

Notes

Notice that the line marked below is useless. It converts to a Cat and then back to an Object.

 public static void useless(Object x) {
     x = (Cat) x;  // <---
     // still cannot use x.noise();
 }