Java Comparable and Comparator Interface
The Collections and Arrays can sorted using the method in the API. Java.util.Colections,sort() and Arrays.sort() method is used to sort the Collections and Arrays. Remember that argument passed to Collections.sort() method must implement the Comparable interface otherwise we will get a compilation error.
The Comparable Interface:
· This interface is used by Collections.sort() and Arrays.sort() method to sort the list and array of objects.
· To implement this interface , a class must implement a single method compareTo().
· Let us see the invocation of compareTo() method:
int x= thisObject.compareTo(anotherObject);
Here compareTo() to method returns an int with the following meaning
· negative ----- if thisObject < anotherObject
· zero ----- if thisObject == anotherObject
· positive ----- if thisObject > anotherObject
· Before generics the compareTo() method takes an Object rather than a specific type. Here we have to cast object to specific type. For example see below code
class DVDInfo implements Comparable {
//some code
public int compareTo(Object o){
DVDInfo d= (DVDInfo) o ;
return title.compareTo(d.getTitle));
}
}
The Comparator Interface
· The Comparator interface gives you the capability to sort collection in different ways.
· This interface is used sort instance of any class even if classes you can't modify.
· To implement this interface , a class must implement a single method compare().
· Comparator.compare() method returns an int whose meaning is same as Comparable.compareTo() method's return value.
Comparing Comparable to Comparator :
|
Java.lana.Comparable |
Java.util.Comparator |
|
Int objeone.compareTo(objTwo); |
Int compare(objOne, objTwo) |
|
Returns
negative if objOne<objTwo
zero if objOne == objTwo
positive if objOne > objTwo |
Same as Comparable |
|
We must modify the class whose instances we want to sort. |
We build a class separate from the class whose instances we want to sort |
|
Only one sort sequence can be created |
Many sort sequence can be created |
|
Implemented frequently in the API by:
String, Wrapper classes, Date, Calender.... |
Meant to be implemented to sort instances of third party classes.
|
Let us see the example for both Comparator and Comparable.
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
public class Student implements Comparable<Student> {
private String name;
private int score;
public Student(String name, int score) {
this.name = name;
this.score = score;
}
public int compareTo(Student obj){
Integer i=new Integer(score);
return i.compareTo(obj.score);
}
public static void main(String args[]) {
ArrayList<Student> list = new ArrayList<Student>();
list.add(new Student("Ann", 87));
list.add(new Student("Bob", 83));
list.add(new Student("Cat", 99));
list.add(new Student("Dan", 25));
list.add(new Student("Eve", 76));
Iterator<Student> iter = list.iterator();
while (iter.hasNext()) {
Student s = iter.next();
System.out.println(s.name + " " + s.score);
}
System.out.println("Sorted List");
Collections.sort(list);
Iterator<Student> iter1 = list.iterator();
while (iter1.hasNext()) {
Student s = iter1.next();
System.out.println(s.name + " " + s.score);
}
StudentName st1= new StudentName();
Collections.sort(list,st1);
Iterator<Student> iter2 = list.iterator();
while (iter2.hasNext()) {
Student s = iter2.next();
System.out.println(s.name + " " + s.score);
}
}
static class StudentName implements Comparator<Student>{
public int compare(Student o1, Student o2) {
return o1.name.compareTo(o2.name);
}
}
}
