Confusion Related to instanceof operator

[Maneeesh Saxena]

Dear Ranchers,

I came across a confusion while reading instanceof operator. As per my understanding below code must have thrown a compile time error as Map & Collection are not in same class hierarchy. But code compiles fine please go through the code & let me know why is it happening ?

import java.util.Collection;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

class TestInstanceof {

	static Map m = new HashMap();
	
	public void testInstanceOf(){
		System.out.println("m instanceof Map: " + (m instanceof Map));
		System.out.println("m instanceof HashMap: " + (m instanceof HashMap));
		System.out.println("m instanceof Object: " + (m instanceof Object));
		System.out.println("m instanceof Date: " + (m instanceof Collection)); //No Compile Time Error - Why 
		System.out.println("m instanceof Date: " + (m instanceof Date)); //No Compile Time Error - Why 
		System.out.println("m instanceof Date: " + (m instanceof String)); //Compile Time Error
	}
	public static void main(String[] args) {
			new TestInstanceof().testInstanceOf();
	}
}

Thanks & Regards

[Harsha Smith]

String class is final and cannot be extended. using any class that's final , you get the same compile error

[Matthew Brown]

Using instanceof will give a compiler error any time there's absolutely no way the check can be true (because it must be an error - why would you check in that case?).

Now, looking at two of the cases you've highlighted, at first glance it would seem they can't be true. But the point here is that Map is an interface, so it is possible to have an object where the checks succeed. Consider these classes:

class BizarreMapAndCollectionHybrid implements Map, Collection {
    // implement the methods from Map and Collection
}

class EvenMoreBizarreDateAndMap extends Date implements Map {
    // implement the methods from Map
}

The same trick doesn't work with String because, as Harsha says, it's final, so you can't subclass it.

If you change the type of m to HashMap, you'll find the Date one fails as well. You can have an object that's a Date and a Map (see above), but you can't have an object that's a Date and a HashMap, as that would need multiple inheritance.

[Harsha Smith]

The compiler is intelligent enough to know nothing can be instanceOf String except String

[naveen yadav]

instanceof gives ERROR only when the the class of reference is of different hierarchy
which is he case with Line 16.

the map reference checking the instanceof operator on PEER classes , Which always return false because they are not IN hierarchy.
which is the case with Line 14 and 15

[Aniket S. Kulkarni]

This may help you
instanceof
is-a