CODEWITHSUNDEEP
javahashcode
Shakeeb Ayaz
Shakeeb Ayaz

Reputation: 6096

How every object are different from one another

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()

Upvotes: 1

Views: 128

Answers (6)

TheLostMind
TheLostMind

Reputation: 36304

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.

Upvotes: 0

Bohemian
Bohemian

Reputation: 425328

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.

Upvotes: 0

Gaurav
Gaurav

Reputation: 1567

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.

Upvotes: 1

T.J. Crowder
T.J. Crowder

Reputation: 1075527

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.

Upvotes: 8

Clément Berthou
Clément Berthou

Reputation: 1948

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.

Upvotes: 0

Stephan L
Stephan L

Reputation: 412

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?

Upvotes: 0

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