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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.

Show Notes

Obey the general contract when overriding equals

When to not override 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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