Java, Object.hashCode() result constant across all JVMs/Systems?

Question

Is the output of Object.hashCode() required to be the same on all JVM implementations for the same Object?

For example if "test".hashCode() returns 1 on 1.4, could it potentially return 2 running on 1.6. Or what if the operating systems were different, or there was a different processor architecture between instances?

Answer 1

No. The output of hashCode is liable to change between JVM implementations and even between different executions of a program on the same JVM.

However, in the specific example you gave, the value of "test".hashCode() will actually be consistent because the implementation of hashCode for String objects is part of the API of String (see the Javadocs for java.lang.String and this other SO post).

Answer 2

From the API

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 hashtables.

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.)