The collections api - Data Structures and Problem Solving Using Java - page 267

1 class BaseClass
2 {
3 public BaseClass( int i )
4 { x = i; }
5
6 public boolean equals( Object rhs )
7 {
8 if( rhs == null || getClass( ) != rhs.getClass( ) )
9 return false;
10
11 return x == ( (BaseClass) rhs ).x;
12 }
13
14 private int x;
15 }
16
17 class DerivedClass extends BaseClass
18 {
19 public DerivedClass( int i, int j )
20 {
21 super( i );
22 y = j;
23 }
24
25 public boolean equals( Object rhs )
26 {
27 // Class test not needed; getClass() is done
28 // in superclass equals
29 return super.equals( rhs ) &&
30 y == ( (DerivedClass) rhs ).y;
31 }
32
33 private int y;
34 }

figure 6.34
Correct
implementation of
equals

When using a HashSet , we must also override the special hashCode method
that is specified in Object ; hashCode returns an int . Think of hashCode as pro-
viding a trusted hint of where the items are stored. If the hint is wrong, the
item is not found, so if two objects are equal, they should provide identical
hints. The contract for hashCode is that if two objects are declared equal by
the equals method, then the hashCode method must return the same value for
them. If this contract is violated, the HashSet will fail to find objects, even if
equals declares that there is a match. If equals declares the objects are not
equal, the hashCode method should return a different value for them, but this
is not required. However, it is very beneficial for HashSet performance if
hashCode rarely produces identical results for unequal objects. How hashCode
and HashSet interact is discussed in Chapter 20.

The hashCode
method must be
overridden, if
equals is overrid-
den, or the HashSet
will not work.