Comparable and ComparatorOutline of the Student classConstructor for StudentThe main method, version 1Using the TreeSetImplementing ComparableAn improved method*Using a separate ComparatorOutline of StudentComparatorThe compare methodThe equals methodThe main methodWhen to use eachSorting differentlyThe EndComparable and ComparatorOutline of the Student classimport java.util.*;public class Student implements Comparable {public Student(String name, int score) {...}public int compareTo(Object o)throws ClassCastException {...}public static void main(String args[]) {...}}Constructor for Student•This is the same for both methods—nothing new here•public Student(String name, int score) {this.name = name;this.score = score;}•We will be sorting students according to their scoreThe main method, version 1 public static void main(String args[]) { TreeSet set = new TreeSet(); set.add(new Student("Ann", 87)); set.add(new Student("Bob", 83)); set.add(new Student("Cat", 99)); set.add(new Student("Dan", 25)); set.add(new Student("Eve", 76)); Iterator iter = set.iterator(); while (iter.hasNext()) { Student s = (Student)iter.next(); System.out.println(s.name + " " + s.score); }}Using the TreeSet•In the main method we have the line TreeSet set = new TreeSet();•Later we use an iterator to print out the values in order, and get the following result: Dan 25Eve 76Bob 83Ann 87Cat 99•How did the iterator know that it should sort Students by score, rather than, say, by name?Implementing Comparable•public class Student implements Comparable•This means it must implement the methodpublic int compareTo(Object o)•Notice that the parameter is an Object•In order to implement this interface, our parameter must also be an Object, even if that’s not what we want•public int compareTo(Object o) throws ClassCastException { if (o instanceof Student) return score - ((Student)o).score; else throw new ClassCastException("Not a Student!");}•A ClassCastException should be thrown if we are given a non-Student parameterAn improved method*•Since casting an arbitrary Object to a Student may throw a classCastException for us, we don’t need to throw it explicitly•public int compareTo(Object o) throws ClassCastException { return score - ((Student)o).score;}•Moreover, since classCastException is a subclass of RuntimeException, we don’t even need to declare that we might throw one:•public int compareTo(Object o) { return score - ((Student)o).score;}*Suggested by Randall SidlingerUsing a separate Comparator•In the program we just finished, Student implemented Comparable –Therefore, it had a compareTo method•Now we will put the comparison method in a separate class•This is more flexible (you can use a different Comparator to sort Students by name), but it’s also clumsier•This new class will implement Comparator instead of Comparable•Comparable requires a definition of compareTo but Comparator requires a definition of compare •Comparator also (sort of) requires equalsOutline of StudentComparator import java.util.*; public class StudentComparator implements Comparator { public int compare(Object o1, Object o2) {...} public boolean equals(Object o1) {...}}•Note: When we are using this Comparator, we don’t need the compareTo method in the Student classThe compare method public int compare(Object o1, Object o2) { return ((Student)o1).score - ((Student)o2).score;}•This differs from compareTo(Object o) in Comparable in these ways:–The name is different–It takes both objects as parameters, not just one–We have to check the type of both objects–Both objects have to be cast to Student–Also note that Comparable is defined in java.lang (which is always imported automatically), but Comparator is defined in java.util.The equals method•This method is not used to compare two Students—it is used to compare two Comparators•Even though it’s part of the Comparator interface, you don’t actually need to override it, since you inherit equals from Object anyway•In fact, it’s always safe to ignore it•The purpose is efficiency—you can replace one Comparator with an equal but faster one•My opinion: ignore this method entirely!The main method•The main method is just like before, except that instead of TreeSet set = new TreeSet();We have Comparator comp = new StudentComparator(); TreeSet set = new TreeSet(comp);When to use each•The Comparable interface is simpler and less work–Say your class implements Comparable–Provide a public int compareTo(Object o) method–Use no argument in your TreeSet or TreeMap constructor–You will use the same comparison method every time•The Comparator interface is more flexible and more work–Create as many different classes that implement Comparator as you like–You can sort the TreeSet or TreeMap differently with each–For example, sort Students by score or by nameSorting differently•Suppose you have students sorted by score, in a TreeSet you call studentsByScore•Now you want to sort them again, this time by name Comparator myStudentNameComparator = new MyStudentNameComparator(); TreeSet studentsByName = new TreeSet(myStudentNameComparator); studentsByName.addAll(studentsByScore);The
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