Inheritance - Data Structures and Problem Solving Using Java - page 159

1 package weiss.util;
2
3 /**
4 * Comparator function object interface.
5 */
6 public interface Comparator<AnyType>
7 {
8 /**
9 * Return the result of comparing lhs and rhs.
10 * @param lhs first object.
11 * @param rhs second object.
12 * @return < 0 if lhs is less than rhs,
13 * 0 if lhs is equal to rhs,
14 * > 0 if lhs is greater than rhs.
15 */
16 int compare( AnyType lhs, AnyType rhs );
17 }

figure 4.39
The Comparator
interface, originally
defined in java.util
and rewritten for the
weiss.util package

Using this interface, we can now pass a Comparator as the second
parameter to findMax . If this Comparator is cmp , we can safely make the call
cmp.compare(o1,o2) to compare any two objects as needed. A wildcard used
in the Comparator parameter to indicate that the Comparator knows how to
compare objects that are the same type or supertypes of those in the array.
It is up to the caller of findMax to pass an appropriately implemented
instance of Comparator as the actual argument.
An example is shown in Figure 4.40. findMax now takes two parameters.
The second parameter is the function object. As shown on line 11, findMax
expects that the function object implements a method named compare , and it
must do so, since it implements the Comparator interface.
Once findMax is written, it can be called in main . To do so, we need to
pass to findMax an array of SimpleRectangle objects and a function object that
implements the Comparator interface. We implement OrderRectByWidth , a new
class that contains the required compare method. The compare method returns
an integer indicating if the first rectangle is less than, equal to, or greater than
the second rectangle on the basis of widths. main simply passes an instance
of OrderRectByWidth to findMax . 7 Both main and OrderRectByWidth are shown in

7. The trick of implementing compare by subtracting works for int s as long as both are the same
sign. Otherwise, there is a possibility of overflow. This simplifying trick is also why we use
SimpleRectangle, rather than Rectangle (which stored widths as doubles ) .