Java Programming/Keywords/instanceof
From Wikibooks, open books for an open world
instanceof is a keyword.
It checks if an object reference is an instance of a type, and returns a boolean value;
The <object-reference> instanceof Object will return true for all non-null object references, since all Java objects are inherited from Object . instanceof will always return false if <object-reference> is null .
Syntax:
<object-reference> instanceof TypeName
For example:
Computer code
class Fruit { //... } class AppleextendsFruit { //... } class OrangeextendsFruit { //... } publicclass Test { publicstaticvoidmain(String[]args) { Collection<Object>coll=newArrayList<Object>(); Appleapp1=newApple(); Appleapp2=newApple(); coll.add(app1); coll.add(app2); Orangeor1=newOrange(); Orangeor2=newOrange(); coll.add(or1); coll.add(or2); printColl(coll); } privatestaticStringprintColl(Collection<?>coll) { for(Objectobj:coll) { if(objinstanceofObject) { System.out.print("It is a Java Object and"); } if(objinstanceofFruit) { System.out.print("It is a Fruit and"); } if(objinstanceofApple) { System.out.println("it is an Apple"); } if(objinstanceofOrange) { System.out.println("it is an Orange"); } } } }
Run the program:
java Test
The output:
"It is a Java Object and It is a Fruit and it is an Apple""It is a Java Object and It is a Fruit and it is an Apple""It is a Java Object and It is a Fruit and it is an Orange""It is a Java Object and It is a Fruit and it is an Orange"
Note that the instanceof operator can also be applied to interfaces.
For example, if the example above was enhanced with the interface
Computer code
interface Edible { //... }
and the classes modified such that they implemented this interface
Computer code
class OrangeextendsFruitimplementsEdible { ... }
we could ask if our object were edible.
Computer code
if(objinstanceofEdible) { System.out.println("it is edible"); }