Item 10: Obey the general contract when overriding equals

When not to override equals

• Each instance of the class is inherently unique.
• There is no need for the class to provide a "logical equality" test.
• A superclass has already overridden equals, and the superclass behavor is appropriate for this class.
• The class is private or package-private, and it is certain that its equals method will never be invoked.

When to override equals

• A class has a notion of logical equality that differs from object identity and its superclass has not overridden equals.
• Most value classes.
  • Exceptions
    • Class with instance control (item 01)
    • Enum types (item 34)

Contract of equals

For all non-null object x, this method implements an equivalent relation:

• Reflexive: x.equals(x) == true
• Symmetric: x.equals(y) == y.equals(x)
• Transitive: x.equals(y) && y.equals(z) => x.equals(z)
• Consistent: result is consistent
• Non-null: x.equals(null) returns false

There is no way to extend an instantiable class and add a value component while preserving the equals contract.