031: Effective Java - Item 8

In this mini Fragment, we introduce Joshua's eighth Item. This one is a doozy, probably one of the longest items in the group of the effective Java series, but most definitely quite important.

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Stay tuned for more items from our "Effective Java for Android developers" Fragment series.

Effective Java (2nd Edition) - Joshua Bloch

Obey the general contract when overriding equals

Each instance of the class is inherently unique.
You don't care whether the class provides a "logical equality" test.
A superclass has already overridden equals, and the superclass behavior is appropriate for this class.

The equals method implement an equivalence relation which states it must be:

Reflexive
Symmetric
Transitive
Consistent
For any non-null reference x, x.equals(null) must return false.

A recipe for a high-quality equals method is as such:

Use the == operator to check for references to this object.
Use the instanceof operator to check if the argument has the correct type 
Cast to the correct type.
Check all field types and corresponding field types.
Finally, when done, ask yourself - is this method symmetric, transitive and consistent?

Caveats

Always override hashcode when you override equals
Don't be too clever!
Don't substitute another type for Object in the equals declaration.

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