Requirement for object key for HashMap

Question

Hi,
I would like to put the object to HashMap keyed by my own object. what is the requirement for the class to be key? I have a method boolean equals(Object o) defined in the key class, My key is composed by 2 ids, so in the equals method I compared the 2 ids. but it seems can't get the value out. Please help. Thanks

Answer 1

How do I supposed to do the hashCode? If myKey1.equals(myKey2) returns true, then myKey1.hashCode() must return the same value as myKey2.hashCode(). One consequence of this is that if something is not used to compuate equals(), then it must not be used to compute hashCode(). (Note that the reverse is not true. That is, if two objects are not equal, they can still return the same value from hashCode(), and hence, if some data is used for equals() it may still be left out of hashCode().)
You want hashCode to be 1) quick & easy to compute and 2) "well distributed" or "mostly unique. If you know how hashcodes are used (in the general sense, not just the Java hashCode() method) then you should understand why those properties are desirable. If not, just ask.
The most common approach is to use some subset of the object's fields in computing the hashCode. If you have a Person object with lastName, firstName, birthdate, height, weight, address, phone number, you probably wouldn't use all those fields. You could just lastName, or maybe a combination of lastName and firstName.
One generally combines multiple pieces by taking XORing (the "^") operator the individual pieces (for primitives) or their hashcodes (for objects). For example, in the Person example: public int hashCode() {
    return lastName.hashCode() ^ firstName.hashCode(); // but make sure to check for null first
}