Comparing Java objects with equals() and hashcode()

• Forgetting to override hashcode() along with the equals() method or vice versa.
• Not overriding equals() and hashcode() when using hash collections like HashSet.
• Returning a constant value in the hashcode() method instead of returning a unique code per object.
• Using == and equals interchangeably. The == compares Object references, whereas equals() compares object values.

What to remember about equals() and hashcode()

• It’s a good practice to always override equals() and hashcode() methods in your POJOs.
• Use an effective algorithm to generate a unique hashcode.
• When overriding the equals() method, always override the hashcode() method as well.
• The equals() method should compare the whole state of objects: values from fields.
• The hashcode() method could be the ID of a POJO.
• When the result of comparing two object's hashcodes is false, the equals() method should also be false.
• If equals() and hashcode() are not overridden when using hash collections, the collection will have duplicate elements.

Learn more about Java

• Get more quick code tips: Read all of Rafael's articles in the InfoWorld Java Challengers series.
• Check out the videos in Rafael's Java Challengers video playlist.
• Find even more Java Challengers on Rafael's Java Challengers blog and in his book, with more than 70 code challenges.

This story, "Comparing Java objects with equals() and hashcode() " was originally published by JavaWorld.