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Penn CIT 594 - Comparable and Comparator

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Comparable and ComparatorNuts and boltsComparing our own objectsOutline of a Student classConstructor for StudentThe main method, version 1Using the TreeSetImplementing ComparableAn improved methodUsing a separate ComparatorOutline of StudentComparatorThe compare methodThe someComparator.equals methodThe main methodWhen to use eachSorting differentlyThe EndComparable and ComparatorNuts and Bolts2Nuts and boltsFour methods underlie many of Java’s important Collection types: equals, compare and compareTo, and hashCodeTo put your own objects into a Collection, you need to ensure that these methods are defined properlyAny collection with some sort of membership test uses equals (which, in many cases, defaults to ==)Any collection that depends on sorting requires larger/equal/smaller comparisons (compare or compareTo)Any collection that depends on hashing requires both equality testing and hash codes (equals and hashCode)Any time you implement hashCode, you must also implement equalsSome of Java’s classes, such as String, already define all of these properly for youFor your own objects, you have to do it yourself3Comparing our own objectsThe Object class provides public boolean equals(Object obj) and public int hashCode() methodsFor objects that we define, the inherited equals and hashCode methods use the object’s address in memoryWe can override these methodsIf we override hashCode, we must override equalsThe Object class does not provide any methods for “less” or “greater”—however,There is a Comparable interface in java.langThere is a Comparator interface in java.util4Outline of a Student classpublic class Student implements Comparable {public Student(String name, int score) {...}public int compareTo(Object o)throws ClassCastException {...}public static void main(String args[]) {...}}5Constructor 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 scoreThis example will use sets, but that’s irrelevant—comparisons happen between two objects, whatever kind of collection they may or may not be in6The 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); }}7Using 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?8Implementing 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 parameter9An 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;}10Using a separate ComparatorIn the program we just finished, Student implemented Comparable Therefore, it had a compareTo methodWe could sort students only by their scoreIf we wanted to sort students another way, such as by name, we are out of luckNow we will put the comparison method in a separate class that implements Comparator instead of ComparableThis is more flexible (you can use a different Comparator to sort Students by name or by score), but it’s also clumsierComparator is in java.util, not java.langComparable requires a definition of compareTo but Comparator requires a definition of compare Comparator also (sort of) requires equals11Outline 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 class12The 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 Student13The so meComparator.equals methodIgnore this 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Implementing an interface requires you to have a definition for every method in the interface--so how can this be an exception?Because you do have a definition, inherited from Object !In fact, it’s always safe to ignore this methodThe purpose is efficiency—you can replace one Comparator with an equal but faster one14The 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);15When to use eachThe Comparable interface is simpler and less work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 but slightly more workCreate as many different classes that implement Comparator


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