Advanced Data Structures - Java Concepts - page 978

Whenever you use a hash set, you need to make sure that an appropriate hash function
exists for the type of the objects that you add to the set. Check the equals method
of your class. It tells you when two objects are considered equal. There are two
possibilities. Either equals has been defined or it has not been defined. If equals
has not been defined, only identical objects are considered equal. In that case, don't
define hashCode either. However, if the equals method has been defined, look at
its implementation. Typically, two objects are considered equal if some or all of the
instance fields are equal. Sometimes, not all instance fields are used in the
comparison. Two Student objects may be considered equal if their studentID
fields are equal. Define the hashCode method to combine the hash codes of the
fields that are compared in the equals method.
In a hash map, only the keys are hashed.
When you use a HashMap , only the keys are hashed. They need compatible
hashCode and equals methods. The values are never hashed or compared. The
reason is simpleȌthe map only needs to find, add, and remove keys quickly.
What can you do if the objects of your class have equals and hashCode methods
defined that don't work for your situation, or if you don't want to define an
appropriate hashCode method? Maybe you can use a TreeSet or TreeMap
instead. Trees are the subject of the next section.
ch16/hashcode/Coin.java
1 /**
2 A coin with a monetary value.
3 */
4 public class Coin
5 {
6 /**
7 Constructs a coin.
8 @param aValue the monetary value of the coin
9 @param aName the name of the coin
10 */
11 public Coin( double aValue, String aName)
12 {