How every object are different from one another

Question

I have read articles about object identity in object oriented context.Which says "Every object you create has its own unique identity". But I got confused by below code.

     String str="Hello";
     String str1="Hello";
     System.out.println(str.hashCode()); //69609650
     System.out.println(str1.hashCode()); //69609650
     System.out.println(System.identityHashCode(str));//19313225
     System.out.println(System.identityHashCode(str1));//19313225

hash code and identityhashcode for both str and str1 are same. Please correct me if I understood wrong.

Also what is difference between hashcode() and system.identityhashcode()

Note that the hash code is not always unique for different objects; the hash code is not a unique identifier for objects. — Jesper, Sep 17 '13 at 10:16

T.J. Crowder · Accepted Answer · 2013-09-17T10:19:11.773

What you're seeing is because you're using String, which has a very special (and nearly unique) behavior: Your two strings are actually one String object, because string literals are automatically intern'd. The JDK and JVM work together to put string literals into a pool of String instances which are reused, rather than creating separate String instances for the same sequence of characters.

Try your experiment with new Object() instead:

 Object a = new Object();
 Object b = new Object();
 System.out.println(a.hashCode());
 System.out.println(b.hashCode());
 System.out.println(System.identityHashCode(a));
 System.out.println(System.identityHashCode(b));

Also what is difference between hashcode() and system.identityhashcode()

The hashCode function can be overridden by a class to return something appropriate for that class. System.identityHashCode returns the same hashCode that Object#hashCode would have returned if the subclass hadn't overridden it.

So for Object, you'd get the same return value from each of them. But for any class that overrides hashCode to return something more appropriate for that class (which includes String), you'd get different values.

Adding to @TJ's answer, even `new String("Hello");` would show the difference in `identityHashCode`! — Rahul, Sep 17 '13 at 10:11
@R.J: True, but I didn't want to introduce even more confusion. :-) — T.J. Crowder, Sep 17 '13 at 10:12
+1 I would also add that `new String()` means `create new object on heap/return new object` and `String a="text"` means `use existing object from heap/string pool.` — pepuch, Sep 17 '13 at 10:19

score 1 · Answer 2 · answered Sep 17 '13 at 10:19

Note that if you declare

String s2 = new String("Hello")

,

System.identityHashCode(s2);

will return different hashcode. This is because when declaration is like

String s = "something"

, jvm checks string pool to find out if there is identical literal. When declaration is like

String s = new String("something");

jvm always creates a new object.

score 0 · Answer 3 · edited May 23 '17 at 12:05

0

In addition to the previous answer, you can read a discussion around your second question here: How do hashCode() and identityHashCode() work at the back end?

edited May 23 '17 at 12:05

Community

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answered Sep 17 '13 at 10:11

Stephan L

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You could have posted this as an comment to @TJ's answer instead of mentioning it here and giving just another link! – Rahul Sep 17 '13 at 10:15

score 0 · Answer 4 · answered Sep 17 '13 at 10:12

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As said in the documentation (http://docs.oracle.com/javase/7/docs/api/java/lang/System.html#identityHashCode(java.lang.Object)), identityHashCode gives the same HashCode, wether or not the hashCode method was overriden.

answered Sep 17 '13 at 10:12

Clément Berthou

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1. This is a comment, not an answer to the question. The question is about why the OP sees the same value when he thought he had two separate objects. 2. It's a duplicate of the same comment made by Sajan several minutes prior. – T.J. Crowder Sep 17 '13 at 10:15
Well, the OP had 2 questions in his post. As the first one was already answered, I answered the second one. – Clément Berthou Sep 17 '13 at 10:17
So, then, this was just a duplicate of Sajan's answer. (But he's deleted his answer now.) – T.J. Crowder Sep 17 '13 at 10:21

score 0 · Answer 5 · answered Sep 17 '13 at 10:47

Both your objects are interned string constants. The JVM ensures the are the same objects.

The difference between hashCode and identityHashCode is hashCode is a hash of the balue of the string object, whereas the identityHashCode is a hash of the ibject's internal identifier.

score 0 · Answer 6 · answered Sep 17 '13 at 11:15

You are not using String objects, you are using String literals.. There is a very big difference between them. In your case, str and str1 both are pointing to the same "Hello" object in String pool. Try doing something like this :

StringBuilder sb1= new StringBuilder("Hello");
StringBuilder sb2= new StringBuilder("Hello");
System.out.println(sb1.hashCode()); 
System.out.println(sb2.hashCode()); 
System.out.println(System.identityHashCode(sb1));
System.out.println(System.identityHashCode(sb2));

output : 7214088 15020296 7214088 15020296

As StringBuilder creates new objects, you can see that the hashCodefor sb1 and sb2 are different. Which means they are not pointing to the same object. But str1 and str2 are pointing to the same String object. Read somthing about String Pools and permgen space for further information.

How every object are different from one another

6 Answers6