Comparator and Comparable

[sk8ergirl]

Hi,
I have class of student object with the String student name and double GPA they are stored in array. My GUI have button for sort by name and sort by grade. I want to sort the name and the grade and I want to sort it using Comparator, but I don't know what is Comparator and when to use it. Also what is the diffrnet between Comparator and Comparable.

[kumar.bhatia18]

Comparator is use to sort the collections. Since you have an array, u first need to convert Array into an ArrayList<Student> in generics form.
After that you need to implement Comparator interface.

Please look at below code:

Person.java

package test;

public class Person {

    private int age;
    private String firstName;
    private String secondName;
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getFirstName() {
        return firstName;
    }
    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
    public String getSecondName() {
        return secondName;
    }
    public void setSecondName(String secondName) {
        this.secondName = secondName;
    }
}




package test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class ComparatorImpl {

    public static void main(String args[]) {
        Person person = new Person();
        person.setAge(25);
        person.setFirstName("Kumar");
        person.setSecondName("Bhatia");

        Person personOne = new Person();
        personOne.setAge(35);
        personOne.setFirstName("Ashish");
        personOne.setSecondName("Bhambhani");

        Person personTwo = new Person();
        personTwo.setAge(45);
        personTwo.setFirstName("Sameer");
        personTwo.setSecondName("Kulkarni");

        List<Person> persons = new ArrayList<Person>();
        persons.add(person);
        persons.add(personOne);
        persons.add(personTwo);

        Collections.sort(persons, new Comparator() {

            @Override
            public int compare(Object o1, Object o2) {
                // TODO Auto-generated method stub
                return ((Person) o1).getFirstName().compareTo(
                        ((Person) o2).getFirstName());
            }

        });

        for (Person person2 : persons) {
            System.out.println(person2.getFirstName());
        }
    }

}

For difference betweeen Comparator and Comparable you can visit my blog on java
(http://www.javacodeimpl.blogspot.com)

[kumar.bhatia18]

Hope it helps

[sepp2k]

[you] first need to convert Array into an ArrayList<Student>

No, he doesn't. Arrays can be sorted using Arrays.sort, which works just like Collections.sort, but for arrays.

[kumar.bhatia18]

Yaa that way also is possible. But I do not noe the implementation of that.

[sk8ergirl]

@kumar.bhatia18 thank you for your help it worked for me when I used the Array instade of arrayList

[sk8ergirl]

I have another question I'm trying to sort integer and in one of the website I found this

the only thing that I don't understand in this code is what does the 1 0 and -1 and how does this code work

static class IntComparator implements Comparator<Person> {

public int compare(Person o1, Person o2) {
int i = o1.index;
int j = o2.index;

if (i < j) {
return -1;
} else if (i == j) {
return 0;
} else {
return 1;
}
}

}

[sepp2k]

The way that compare works is that it returns a negative number if the first argument is less than the second, a positive number if the first argument is greater than the second and 0 if they're equal. So return -1 means "o1 is less than o2", return 0 means "o1 is equal to o2" and return 1 means "o1 is greater than o2".

[sk8ergirl]

compare

The way that compare works is that it returns a negative number if the first argument is less than the second, a positive number if the first argument is greater than the second and 0 if they're equal. So return -1 means "o1 is less than o2", return 0 means "o1 is equal to o2" and return 1 means "o1 is greater than o2".

but why did we chose 1 -1 and 0 does it mean that if o1 is greater than o2 then put o1 at the top

[JamesCherrill]

The normal sort is an ascending sort, the lesser objects sort before the greater, so is o2 is greater than o1 it will sort after it. Just think of numbers - you expect them to sort 1,2,3,4,5... smaller/lesser first, greater/larger after