Collections and Generics - SCJP Sun Certified Programmer for Java Platform SE6 - page 435

Notice that the two Product objects are equal if they have the same id , and two equal
Product objects generate the same hashCode .
compareTo Consistent with equals
If you write a class that implements Comparable , you introduce new business logic
for determining equality. The compareTo method returns 0 if two objects are equal,
while your equals method returns true if two objects are equal. A natural ordering
that uses compareTo is said to be consistent with equals if and only if x.equals(y) is
true whenever x.compareTo(y) equals 0 . You are strongly encouraged to make your
Comparable classes consistent with equals because not all collection classes behave
predictably if the compareTo and equals methods are not consistent. For example, the
following Product class defi nes a compareTo method that is consistent with equals :
public class Product implements Comparable<Product> {
int id;
public boolean equals(Object obj) {
if(!(obj instanceof Product)) {
return false;
}
Product other = (Product) obj;
return this.id == other.id;
}
public int compareTo(Product obj) {
return this.id - obj.id;
}
}
If two Product objects are equal, they have the same id . Therefore, the return value of
compareTo is 0 when comparing two equal Product objects, so this compareTo method is
consistent with equals .
Now that we have discussed the various types of collections in the Collections
Framework, let's put this knowledge to use by instantiating and using the collection classes
in the next section. Because all of the collections classes use generics and the exam requires
knowledge of generics, the next section discusses both topics simultaneously.