Overriding hashCode() and equals()

• equals(), hashCode(), and toString() are public.
• Override toString() so that System.out.println() or other methods can see something useful, like your object's state.
• Use == to determine if two reference variables refer to the same object.
• Use equals() to determine if two objects are meaningfully equivalent.
• If you don't override equals(), your objects won't be useful hashing keys.
• If you don't override equals(), different objects can't be considered equal.
• Strings and wrappers override equals() and make good hashing keys.
• When overriding equals(), use the instanceof operator to be sure you're evaluating an appropriate class.
• When overriding equals(), compare the objects' significant attributes.
• Highlights of the equals() contract:
• Reflexive: x.equals(x) is true.
• Symmetric: If x.equals(y) is true, then y.equals(x) must be true.
• Transitive: If x.equals(y) is true, and y.equals(z) is true, then z.equals(x) is true.
• Consistent: Multiple calls to x.equals(y) will return the same result.
• Null: If x is not null, then x.equals(null) is false.
• If x.equals(y) is true, then x.hashCode() == y.hashCode() is true.
• If you override equals(), override hashCode().
• HashMap, HashSet, Hashtable, LinkedHashMap, & LinkedHashSet use hashing.
• An appropriate hashCode() override sticks to the hashCode() contract.
• An efficient hashCode() override distributes keys evenly across its buckets.
• An overridden equals() must be at least as precise as its hashCode() mate.
• To reiterate: if two objects are equal, their hashcodes must be equal.
• It's legal for a hashCode() method to return the same value for all instances (although in practice it's very inefficient).
• Highlights of the hashCode() contract:
• Consistent: multiple calls to x.hashCode() return the same integer.
• If x.equals(y) is true, x.hashCode() == y.hashCode() is true.
• If x.equals(y) is false, then x.hashCode() == y.hashCode() can be either true or false, but false will tend to create better efficiency.
• transient variables aren't appropriate for equals() and hashCode().