Use of "instanceof" in Java [duplicate]

What is the 'instanceof' operator used for?

I learned that Java has the instanceof operator. Can you elaborate where it is used and what are its advantages?

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4 Answers

Basically, you check if an object is an instance of a specific class. You normally use it, when you have a reference or parameter to an object that is of a super class or interface type and need to know whether the actual object has some other type (normally more concrete).

Example:

public void doSomething(Number param) { if( param instanceof Double) { System.out.println("param is a Double"); } else if( param instanceof Integer) { System.out.println("param is an Integer"); } if( param instanceof Comparable) { //subclasses of Number like Double etc. implement Comparable //other subclasses might not -> you could pass Number instances that don't implement that interface System.out.println("param is comparable"); }
}

Note that if you have to use that operator very often it is generally a hint that your design has some flaws. So in a well designed application you should have to use that operator as little as possible (of course there are exceptions to that general rule).

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instanceof is used to check if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface.

Read more from the Oracle language definition here.

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instanceof can be used to determine the actual type of an object:

class A { }
class C extends A { }
class D extends A { }
public static void testInstance(){ A c = new C(); A d = new D(); Assert.assertTrue(c instanceof A && d instanceof A); Assert.assertTrue(c instanceof C && d instanceof D); Assert.assertFalse(c instanceof D); Assert.assertFalse(d instanceof C);
}

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instanceof is a keyword that can be used to test if an object is of a specified type.

Example :

public class MainClass { public static void main(String[] a) { String s = "Hello"; int i = 0; String g; if (s instanceof java.lang.String) { // This is going to be printed System.out.println("s is a String"); } if (i instanceof Integer) { // This is going to be printed as autoboxing will happen (int -> Integer) System.out.println("i is an Integer"); } if (g instanceof java.lang.String) { // This case is not going to happen because g is not initialized and // therefore is null and instanceof returns false for null. System.out.println("g is a String"); }
} 

Here is my source.

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