CODEWITHSUNDEEP
javahashcode
Maciek Murawski
Maciek Murawski

Reputation: 444

Long hashCode returns same number for different objects in java

Let's say I have this situation 

    Long id = -1L;
    System.out.println( id.hashCode() );

    id = 0L;
    System.out.println( id.hashCode() );

And guess what? Both outputs give same number (0) ! My questions are:

  1. Why this is happening ?
  2. How can I ommit this and calculate proper hash for 0 and -1 ?

Thanks in advance for reply :)

Upvotes: 1

Views: 2478

Answers (2)

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272497

Why is this happening?

Because the implementation of Long.hashCode is as follows:

The result is the exclusive OR of the two halves of the primitive long value held by this Long object. That is, the hashcode is the value of the expression:

(int)(this.longValue()^(this.longValue()>>>32))

How can I omit this and calculate proper hash for 0 and -1 ?

These are proper hashes. Hashes are not guaranteed to be unique; in fact they're guaranteed not to be unique if there are more than 232 possible input values.

If you want a different behaviour, you'd need to write a MyInteger class that behaves differently (although I suspect there's no real good reason to do so).

Upvotes: 6

Ruchira Gayan Ranaweera
Ruchira Gayan Ranaweera

Reputation: 35557

Depending on the hashing function, 2 different objects can have the same hash code.

From Java doc.

The general contract of hashCode is:

Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application. If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

Upvotes: 0

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