override equals() and hashCode() methods

Saw the hashCode() method used in the FarmerWolfGoatState (a state representation) implementation, but do not know why there should be such an overriding.

equals() and hashCode() are the default methods of java class Object.

By default, if two objects are equal, then

public boolean equals(Object obj) {

    return (this == obj);

  }

Also, if two objects are equal according to the equals(Object) method, then calling the

hashCode

method on each of the two objects must produce the same integer result.

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What would happen if

Integer

did not override

equals()

and

hashCode()

? Nothing, if we never used an

Integer

as a key in a

HashMap

or other hash-based collection. However, if we were to use such an

Integer

object for a key in a

HashMap

, we would not be able to reliably retrieve the associated value, unless we used the exact same

Integer

instance in the

get()

call as we did in the

put()

call. This would require ensuring that we only use a single instance of the

Integer

object corresponding to a particular integer value throughout our program. Needless to say, this approach would be inconvenient and error prone.

Why does our root object class need

hashCode()

, when its discriminating ability is entirely subsumed by that of

equals()

? The

hashCode()

method exists purely for efficiency.

Deciding whether, and how, to override

equals()

requires a little more judgment. In the case of simple immutable value classes, such as

Integer

(and in fact for nearly all immutable classes), the choice is fairly obvious -- equality should be based on the equality of the underlying object state. In the case of

Integer

, the object's only state is the underlying integer value.

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So sometimes if we want to change the default equality definition of object, and only use their state, instead of every part of ojbect, as the rule to judege their equality, we need to override the equals() method. Then we need also to override hashCode() so as to keep the consistence.

It seems to be enough to provide equlas() method, adding another one hashCode() is just in order to improve the efficiency for the for the benefit of hashtables such as those provided by

java.util.Hashtable.

reference:

http://www.ibm.com/developerworks/java/library/j-jtp05273.html

http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Object.html#hashCode()