Objects sorting in java

Here I explain how to sort java objects using Comparator and  Comparable interfaces.

We want to sort students by marks.
Rahul: 45,
Sameera: 95,
Amith: 28,
Anju: 60,
Lahiru: 86

Method 1: Comparator 

Now create student class

Student.java

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1. package com.codestore.sorting;  
2.   
3. /** 
4.  * @author Sameera Jayasekara 
5.  */  
6. public class Student {  
7.     String name;  
8.     int totalMarks;  
9.   
10.     public Student(String name, int totalMarks) {  
11.         this.name = name;  
12.         this.totalMarks = totalMarks;  
13.   
14.     }  
15.   
16.     @Override  
17.     public String toString() {  
18.         return name + ": " + totalMarks;  
19.     }  
20. }  

package com.codestore.sorting;

/**
 * @author Sameera Jayasekara
 */
public class Student {
    String name;
    int totalMarks;

    public Student(String name, int totalMarks) {
        this.name = name;
        this.totalMarks = totalMarks;

    }

    @Override
    public String toString() {
        return name + ": " + totalMarks;
    }
}

Create comparator class and override compare method

StudentMarksComparator.java

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1. import java.util.Comparator;  
2.   
3. /** 
4.  * @author Sameera Jayasekara 
5.  */  
6. public class StudentMarksComparator implements Comparator<Student> {  
7.   
8.     @Override  
9.     public int compare(Student s1, Student s2) {  
10.         Student student1 = s1;  
11.         Student student2 = s2;  
12.         return student2.totalMarks - student1.totalMarks;  
13.     }  
14.   
15. }  

import java.util.Comparator;

/**
 * @author Sameera Jayasekara
 */
public class StudentMarksComparator implements Comparator<Student> {

    @Override
    public int compare(Student s1, Student s2) {
        Student student1 = s1;
        Student student2 = s2;
        return student2.totalMarks - student1.totalMarks;
    }

}

Main.java

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1. package com.codestore.sorting;  
2.   
3. import java.util.*;  
4.   
5. /** 
6.  * @author Sameera Jayasekara 
7.  */  
8. public class Main {  
9.   
10.     public static void main(String[] args) {  
11.   
12.         List<Student> students = new ArrayList<Student>();  
13.         students.add(new Student("Rahul", 45));  
14.         students.add(new Student("Sameera", 95));  
15.         students.add(new Student("Amith", 28));  
16.         students.add(new Student("Anju", 60));  
17.         students.add(new Student("Lahiru", 86));  
18.   
19.         System.out.println("Before Sort: " + students);  
20.   
21.         StudentMarksComparator smc = new StudentMarksComparator();  
22.         Collections.sort(students, smc);  
23.   
24.         System.out.println("After Sort: " + students);  
25.   
26.     }  
27.   
28. }  
29.    

package com.codestore.sorting;

import java.util.*;

/**
 * @author Sameera Jayasekara
 */
public class Main {

    public static void main(String[] args) {

        List<Student> students = new ArrayList<Student>();
        students.add(new Student("Rahul", 45));
        students.add(new Student("Sameera", 95));
        students.add(new Student("Amith", 28));
        students.add(new Student("Anju", 60));
        students.add(new Student("Lahiru", 86));

        System.out.println("Before Sort: " + students);

        StudentMarksComparator smc = new StudentMarksComparator();
        Collections.sort(students, smc);

        System.out.println("After Sort: " + students);

    }

}
 

Output:
Before Sort: [Rahul: 45, Sameera: 95, Amith: 28, Anju: 60, Lahiru: 86]
After Sort: [Sameera: 95, Lahiru: 86, Anju: 60, Rahul: 45, Amith: 28]

You can directly add a comparator class as a method parameter without creating a separate class.

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1. package com.codestore.sorting;  
2.   
3. import java.util.*;  
4.   
5. /** 
6.  * @author Sameera Jayasekara 
7.  */  
8. public class Main {  
9.   
10.  public static void main(String[] args) {  
11.   
12.   List<Student> students = new ArrayList<Student>();  
13.   students.add(new Student("Rahul", 45));  
14.   students.add(new Student("Sameera", 95));  
15.   students.add(new Student("Amith", 28));  
16.   students.add(new Student("Anju", 60));  
17.   students.add(new Student("Lahiru", 86));  
18.   
19.   System.out.println("Before Sort: " + students);  
20.   
21.   // StudentMarksComparator smc = new StudentMarksComparator();  
22.   // Collections.sort(students, smc);  
23.   
24.   Collections.sort(students, new Comparator<Student>() {  
25.   
26.    @Override  
27.    public int compare(Student s1, Student s2) {  
28.     Student student1 = s1;  
29.     Student student2 = s2;  
30.     return student2.totalMarks - student1.totalMarks;  
31.    }  
32.   
33.   });  
34.   
35.   System.out.println("After Sort: " + students);  
36.   
37.  }  
38.   
39. }  

package com.codestore.sorting;

import java.util.*;

/**
 * @author Sameera Jayasekara
 */
public class Main {

 public static void main(String[] args) {

  List<Student> students = new ArrayList<Student>();
  students.add(new Student("Rahul", 45));
  students.add(new Student("Sameera", 95));
  students.add(new Student("Amith", 28));
  students.add(new Student("Anju", 60));
  students.add(new Student("Lahiru", 86));

  System.out.println("Before Sort: " + students);

  // StudentMarksComparator smc = new StudentMarksComparator();
  // Collections.sort(students, smc);

  Collections.sort(students, new Comparator<Student>() {

   @Override
   public int compare(Student s1, Student s2) {
    Student student1 = s1;
    Student student2 = s2;
    return student2.totalMarks - student1.totalMarks;
   }

  });

  System.out.println("After Sort: " + students);

 }

}

Method 2: Comparable

Student.java

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1. package com.codestore.sorting.comparable;  
2.   
3. /** 
4.  * @author Sameera Jayasekara 
5.  */  
6. public class Student implements Comparable<Student> {  
7.   
8.     String name;  
9.     int totalMarks;  
10.   
11.     public Student(String name, int totalMarks) {  
12.         this.name = name;  
13.         this.totalMarks = totalMarks;  
14.     }  
15.   
16.     @Override  
17.     public int compareTo(Student s) {  
18.         return this.totalMarks - s.totalMarks;  
19.     }  
20.   
21.     @Override  
22.     public String toString() {  
23.         return name + ": " + totalMarks;  
24.     }  
25.   
26. }  
27.    

package com.codestore.sorting.comparable;

/**
 * @author Sameera Jayasekara
 */
public class Student implements Comparable<Student> {

    String name;
    int totalMarks;

    public Student(String name, int totalMarks) {
        this.name = name;
        this.totalMarks = totalMarks;
    }

    @Override
    public int compareTo(Student s) {
        return this.totalMarks - s.totalMarks;
    }

    @Override
    public String toString() {
        return name + ": " + totalMarks;
    }

}
 

Main.java

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1. package com.codestore.sorting.comparable;  
2.   
3. import java.util.*;  
4.   
5. /** 
6.  * @author Sameera Jayasekara 
7.  */  
8. public class Main {  
9.   
10.     public static void main(String[] args) {  
11.   
12.         List<Student> students = new ArrayList<Student>();  
13.         students.add(new Student("Rahul", 45));  
14.         students.add(new Student("Sameera", 95));  
15.         students.add(new Student("Amith", 28));  
16.         students.add(new Student("Anju", 60));  
17.         students.add(new Student("Lahiru", 86));  
18.   
19.         System.out.println("Before Sort: " + students);  
20.   
21.         Collections.sort(students);  
22.   
23.         System.out.println("After Sort: " + students);  
24.   
25.     }  
26.   
27. }  
28.    

package com.codestore.sorting.comparable;

import java.util.*;

/**
 * @author Sameera Jayasekara
 */
public class Main {

    public static void main(String[] args) {

        List<Student> students = new ArrayList<Student>();
        students.add(new Student("Rahul", 45));
        students.add(new Student("Sameera", 95));
        students.add(new Student("Amith", 28));
        students.add(new Student("Anju", 60));
        students.add(new Student("Lahiru", 86));

        System.out.println("Before Sort: " + students);

        Collections.sort(students);

        System.out.println("After Sort: " + students);

    }

}
 

Output:
Before Sort: [Rahul: 45, Sameera: 95, Amith: 28, Anju: 60, Lahiru: 86]
After Sort: [Sameera: 95, Lahiru: 86, Anju: 60, Rahul: 45, Amith: 28]