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.
Contact #
- @fragmentedcast [twitter.com]
- @donnfelker [twitter.com]
- @kaushikgopal [twitter.com]
