Question

Possible Duplicate:
Overriding equals and hashCode in Java

If I have

class A {
    int x = 1;
}
...
A a1 = new A();
A a2 = new A();
a1.equals(a2);

If I compare 2 instances of A without override the equals method, will I get expected result?

Answer 1

If I compare 2 instances of A without override the equals method, will I get expected result?

That depends on what you expect 🙂

The default implementation will give you reference equality – in other words, when you compare two references, equals will only return true if they’re references to the same object.

You would normally override equals to implement “value equality” – where two distinct objects are deemed equal, usually by virtue of having equal field values themselves. The exact meaning of equality will depend on your design – the two objects could still be distinguishable in other ways, for example.

If you override equals, you should also override hashCode to be consistent with equals, such that if a.equals(b) is true, then a.hashCode() == b.hashCode(). This will allow instances of your class to be used as keys in hash-based collections (e.g. HashMap) so that you can look up a value based on a key which is equal to the original one, rather than having to use a reference to the exact original key object.