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conditional check code usage in equals & hashCode

Derek Harper
Greenhorn

Joined: Aug 02, 2006
Posts: 25
Hi guys,

I have a question that i would like some feedback on. Take a look at the following code fragments:


Example A:

public boolean equals(Object obj) {
if(this == obj)
return true;

// 1st conditional check

if(obj == null) || (obj.getClass() != this.getClass()))
return false;

Example B:

public boolean equals(Object obj) {
if(this == obj)
return true;

// 2nd conditional check

if(!(obj instanceof Test)) return false;


My question is wouldn't the 1st conditional check be best to use in case the argument is a subclass of the superclass? It is my understanding that the 'instanceof' operator wouldn't return false.



Thanks in advance......



Also, to the moderators.....i wasn't sure which forum this question would be best served in, so forgive me if I broke any rules.
Rob Spoor
Sheriff

Joined: Oct 27, 2005
Posts: 19722
    
  20

instanceof can be dangerous; if used improperly, you can break one of the rules for equals: if x.equals(y), then y.equals(x) should be true.

Consider the following piece of code:

Now if class A is final, or the equals method is final, there's no big issue. But otherwise, it is very well possible to break the contract in a sub class. You might not do it, but someone else still can.

Now if you use the Class comparison, then a.equals(b) will be false as well, therefore not breaking the contract.


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Derek Harper
Greenhorn

Joined: Aug 02, 2006
Posts: 25
Thanks Rob!
 
It is sorta covered in the JavaRanch Style Guide.
 
subject: conditional check code usage in equals & hashCode