equals : Java Glossary

• Note that your method must work even when passed a null or an Object of the wrong type.
• Note that the method signature specifies Object. If you change it to something else, you won’t be overriding the official equals method, but creating a new unrelated method that will only be used in some contexts.
• Whenever you implement an equals method, you must implement a corresponding hashCode method.
• The parameter to equals must not be null and, of course, the first Object must not be null either. That holds for Object.equals and String.equals.
• The equals method implements an equivalence relation. In other words, your equals method must behave in a civilised way, behaving as people would expect a method called equals ought to behave.
  1. It must be reflexive: x.equals(x) should return true.
  2. It must be symmetric: x.equals(y) should return true if and only if y.equals(x).
  3. It must be transitive: if x.equals(y) and y.equals(z), then x.equals(z) should return true.
  4. It must be consistent: multiple invocations of x.equals(y) must consistently return true or consistently return false, provided no information used in equals comparisons on the object is modified.
  What happens if your equals method violates these rules or your hashCode method does not match your equals method? Usually all that happens is the various collections such as Hashtable, HashMap and HashSet don’t work. Java can’t find the objects you put in the collections. Unfortunately, you won’t get a warning message explaining what you did wrong, so best to write a little test code to make sure your methods behave according to the four rules.
• equals is a method of Object so every object implements it. There is no corresponding interface, so generics do not apply.
• Nearly always you should override equals ( Object other ) not equals ( SomethingElse other ). Otherwise methods like HashSet. contains or HashMap. get which use equals ( Object other ), will use the default == implementation of equals rather than yours. You can get this wrong and your code will still almost work.

Strategy

1. Check for equality of the cached hashcodes. If they are not equal you know right away the two Objects can’t be equal.
2. You then compare the two addresses/references. If equal, you know the entire Objects must be equal (the same).
3. Failing that, you check the Object sizes. If they are not the same you know the two can’t be equal. There is no method to give you the aggregate Object size. By that I mean the sum of the lengths of the variable parts of the Object that participate in the equality test.
4. Failing that, you do your byte by byte comparison.

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