Overriding the Hashcode Method
Problem
You
want to use your
objects in a hash, and
you need to write a hashCode( )
.
Discussion
The hashCode()
method is supposed to return an
int
that should uniquely identify different
objects.
A properly written hashCode( )
method will follow
these rules:
It is repeatable:
hashCode(x)
must return the sameint
when called again unless set methods have been called.It is symmetric: if
x.equals(y)
, thenx.hashCode( )
must ==y.hashCode( )
, i.e., either both return true, or both return false.If
!x.equals(y)
, it is not required thatx.hashCode( )
!=y.hashCode( )
, but doing so may improve performance of hash tables, i.e., hashes may callhashCode( )
beforeequals( )
.
The default hashCode( )
on Sun’s JDK returns
a machine address, which conforms to Rule 1. Conformance to Rules 2
and 3 depends, in part, upon your equals( )
method. Here is a program that
prints the hashcodes of a small
handful of objects:
/** Display hashCodes from some objects */ public class PrintHashCodes { /** Some objects to hashCode( ) on */ protected static Object[] data = { new PrintHashCodes( ), new java.awt.Color(0x44, 0x88, 0xcc), new SomeClass( ) }; public static void main(String[] args) { System.out.println("About to hashCode " + data.length + " objects."); for (int i=0; i<data.length; i++) { System.out.println(data[i].toString( ) + " --> " + data[i].hashCode( )); } System.out.println("All done."); } }
What does it print?
> jikes +E -d . PrintHashCodes.java > java PrintHashCodes About ...
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