Java 7 for Absolute Beginners

CHAPTER 4 ■ OPERATORS

Listing 4-32. Book comparator class

package com.apress.javaforabsolutebeginners .examples.comparing;

import java.util.Comparator;

public class BookComparator implements Comparator<Person> {

public int compare(Person p1, Person p2) {

return p1.favoriteBook.compareTo(p2.favoriteBook);

}

}

In this case, all we have to do is use the String class's compareTo method. Note that we must tell the

comparator what kind of thing to compare (with Comparator<Person>). Otherwise, we have to accept

arguments of type Object and cast to objects of type Person.

■ Note You don't need comparators for arrays of primitives or for collections of any objects that implement

java.lang.Comparable (such as String and Integer). Those objects are already comparable.

Finally, Listing 4-33 shows CompareTest, expanded to use both kinds of comparison.

Listing 4-33. CompareTest using both comparisons

package com.apress.javaforabsolutebeginners .examples.comparing;

import java.util.ArrayList;

import java.util.Collections;

public class CompareTest {

public static void main(String[] args) {

Person samSpade = new Person("Sam", "Spade", "The Maltese Falcon");

Person sherlockHolmes =

new Person("Sherlock", "Holmes", "The Sign of the Four");

Person johnWatson = new Person("John", "Watson", "A Study in Scarlet");

Person drWatson = new Person("John", "Watson", "A Study in Scarlet");

// compare the two that are really equal

System.out.println(johnWatson == drWatson);

System.out.println(johnWatson.equals(drWatson));

System.out.println();

System.out.println("Sorting by name");

// Make a collection from our characters and sort them

ArrayList<Person> characters = new ArrayList<Person>();

characters.add(samSpade);

characters.add(sherlockHolmes);

characters.add(johnWatson);