How to Write an Equality Method in Java (2009)

• Auto

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  A collection of source code generators for Java.

  

For immutable value types, AutoValue[1] is a nice library for automating this. Lombok[2] is also popular, but a lot of orgs avoid it because it has some pretty deep dependencies on implementation details of the JDK that aren't guaranteed to be stable across releases. It seems well-maintained in practice, thoguh. Once Java records[3] are out of preview, they will probably be the preferred approach, but it will be a while before most teams are on a version of the JDK that supports them.

For mutable objects, the notion of equality is hairy in general, because what you want often depends on why you'd be comparing the two objects. You might have two mutable objects that can be meaningfully compared in your business logic, but absolutely should not be used as keys in a map. Often you can satisfy your goals by implementing a separate method that does not attempt to conform to the 'equals' contract, which ensures it will not unintentionally be used for e.g. data structure ordering.

[1] https://github.com/google/auto/blob/master/value/userguide/i...

[2] https://projectlombok.org/features/EqualsAndHashCode

[3] https://blogs.oracle.com/javamagazine/records-come-to-java

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