Comparator and Comparable Interfaces

Comparable:-

1. It is inside java.lang package. 
2. The class whose elements need to be sorted must implement this interface and override the compareTo(Object obj) method.
3. The sort is done using either Collection.sort(List<T> list) or Arrays.sort(Array[] array)

Example 
package com.arsoft.tests;

public class Student implements Comparable<Student>{ //class is implementing Comparable
    private String name;
    private float marks;
   
    public Student (String name, float marks){
        this.name = name;
        this.marks = marks;
    }
   
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public float getMarks() {
        return marks;
    }
    public void setMarks(float marks) {
        this.marks = marks;
    }
   
    public int compareTo(Student student){ //Method needs to be overridden
        return this.name.compareTo(student.getName());
    } 
   
}

package com.arsoft.tests;

import java.util.ArrayList;
import java.util.Collections;

public class StudentDemo {
    public static void main(String args[]){
        ArrayList<Student> students =  new ArrayList<Student>();
        Student student = new Student("Abhishek", 95);
        students.add(student);
        student = new Student("Anshul",93);
        students.add(student);
        student = new Student("Akhil",86);
        students.add(student);
        student = new Student("Rajeev",99);
        students.add(student);
        student = new Student("Ruchika", 67);
        students.add(student);
        for(Student st : students) {
            System.out.println(st.getMarks() + " : " + st.getName());
        }
        System.out.println("##################################");
        Collections.sort(students);
        for(Student st : students) {
            System.out.println(st.getMarks() + " : " + st.getName());
        }
       
    }
}


Comparator:-

1. It is inside java.util package.
2. The class whose elements need to be sorted must NOT implement this interface instead we can create a separate class implementing this interface and the override the compare(Object obj, Object obj1) method.
3. The sort is done using either Collection.sort(List<T> list, Comparator comparator) or Arrays.sort(Array[] array, Comparator comparator) 

package com.arsoft.tests;

import java.util.Comparator;

public class NewStudent { //Class is not implementing Comparator
    private String name;
    private float marks;
   
    public NewStudent (String name, float marks){
        this.name = name;
        this.marks = marks;
    }
   
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public float getMarks() {
        return marks;
    }
    public void setMarks(float marks) {
        this.marks = marks;
    }
   
}

package com.arsoft.tests;

import java.util.Comparator;

public class NameComparator implements Comparator<NewStudent>{ //New comparator which //implements comparator is created

    @Override
    public int compare(NewStudent student1, NewStudent student2) { //Method is overridden
        return student1.getName().compareTo(student2.getName());
    }

}




package com.arsoft.tests;

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

public class NameStudentDemo {
    public static void main(String args[]){
        ArrayList<NewStudent> students =  new ArrayList<NewStudent>();
        NewStudent student = new NewStudent("Abhishek", 95);
        students.add(student);
        student = new NewStudent("Anshul",93);
        students.add(student);
        student = new NewStudent("Akhil",86);
        students.add(student);
        student = new NewStudent("Rajeev",99);
        students.add(student);
        student = new NewStudent("Ruchika", 67);
        students.add(student);
        for(NewStudent st : students) {
            System.out.println(st.getMarks() + " : " + st.getName());
        }
        System.out.println("##################################");
        NameComparator nc = new NameComparator();
        Collections.sort(students, nc); //an object of custom comparator class is being used in sorting
        for(NewStudent st : students) {
            System.out.println(st.getMarks() + " : " + st.getName());
        }
       
    }

}