Re: equals and hascode

Lew <>
Sat, 18 Dec 2010 08:12:09 -0500
mark jason wrote:

The inheritance hierachy is

MyMatrix extends DenseDoubleMatrix2D extends DoubleMatrix2D .......

DenseDoubleMatrix2D does not define equals()
DoubleMatrix2D redefines equals() as below but not hashCode()

public boolean equals(Object obj) {
     if (this == obj) return true;
     if (obj == null) return false;
     if (!(obj instanceof DoubleMatrix2D)) return false;

Note that this code effectively checks twice whether 'obj == null'.

     return cern.colt.matrix.linalg.Property.DEFAULT.equals(this,
(DoubleMatrix2D) obj);

Others have pointed out that the "colt" library itself has the bug. This
means that its matrix classes will behave strangely in hashed collections such
as 'HashSet'.

Read and study /Effective Java/ (2nd ed.) by Joshua Bloch for the rationale
and best practices around 'hashCode' and 'equals'. This book is /de rigueur/
for anyone wishing to describe themselves as a Java programmer.

Ceci n'est pas une pipe.

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