The Object.hashCode() method returns a number that is effectively random. This number is also implementation- and instance-dependent. This has a couple consequences:

1 – The order of iteration for elements in a Hashtable will be unpredictable, and will generally differ from one program invocation to the next.

2 – A class that overrides Object.equals() must also overrideObject.hashCode(). It is safe, if inefficient, if the hashCode() method returns the same value for unequal objects. It is unsafe if the hashCode() method returns different values for two objects that are, in fact, equal.

Solution: If a repeatable order of enumeration over the elements in a Hashtable is important to your program, then any object used as a key in the Hashtable should have a class-specific hashCode method. This hashCode method should return a value computed from data fields of the object. If any of your classes define an equals method, they must also define a hashCode method. This means that for any two objects a,b of that class, a.equals(b) implies that a.hashCode() == b.hashCode(). Note that hashCode should not depend on any mutable property. If an object