Tips for Implementing hashCode

You must override hashCode in every class that overrides equals.

Tips for converting primitive types to int:

• boolean: v ? 1 : 0.
• byte, char, or short: (int) v.
• long: (int) (v ^ (v >>> 32)).
• float: Float.floatToIntBits(v).
• double: Double.doubleToLongBits(v) and then convert the result as above.
• null: Usually use 0 here.
• Object reference:
  • If equals is defined as a recursive function, compute hashCode recursively, too.
  • Otherwise, compute a “canonical representation” and then invoke hashCode on it.
• Array: Treat it as if each element was a separate member field.

An okay hash function:

// Initial value of code is usually non-zero, say 17.
code = 31 * code + hashValue;

Write unit tests to verify that equal instances have equal hash codes.

You may cache the hash code of immutable objects.