Sorting in Collection in Java

We can sort the elements of:

1. String objects
2. Wrapper class objects
3. User-defined class objects

Collections class provides static methods for sorting the elements of a collection. If collection elements are of a Set type, we can use TreeSet. However, we cannot sort the elements of List. Collections class provides methods for sorting the elements of List type elements.

Method of Collections class for sorting List elements

public void sort(List list): is used to sort the elements of List. List elements must be of the Comparable type.

Note: String class and Wrapper classes implement the Comparable interface. So if you store the objects of string or wrapper classes, it will be Comparable.

Example to sort string objects

1. import java.util.*;  
2. class TestSort1{  
3. public static void main(String args[]){  
4.   
5. ArrayList<String> al=new ArrayList<String>();  
6. al.add("Viru");  
7. al.add("Saurav");  
8. al.add("Mukesh");  
9. al.add("Tahir");  
10.   
11. Collections.sort(al);  
12. Iterator itr=al.iterator();  
13. while(itr.hasNext()){  
14. System.out.println(itr.next());  
15.  }  
16. }  
17. }  

Mukesh
Saurav
Tahir
Viru
       

Example to sort string objects in reverse order

1. import java.util.*;  
2. class TestSort2{  
3. public static void main(String args[]){  
4.   
5. ArrayList<String> al=new ArrayList<String>();  
6.         al.add("Viru");    
7.         al.add("Saurav");    
8.         al.add("Mukesh");    
9.         al.add("Tahir");   
10.           
11.         Collections.sort(al,Collections.reverseOrder());  
12.         Iterator i=al.iterator();  
13.         while(i.hasNext())  
14.         {  
15.             System.out.println(i.next());  
16.         }  
17. }  
18. }  

Viru
Tahir
Saurav
Mukesh
       

Example to sort Wrapper class objects

1. import java.util.*;  
2. class TestSort3{  
3. public static void main(String args[]){  
4.   
5. ArrayList al=new ArrayList();  
6. al.add(Integer.valueOf(201));  
7. al.add(Integer.valueOf(101));  
8. al.add(230);//internally will be converted into objects as Integer.valueOf(230)  
9.   
10. Collections.sort(al);  
11.   
12. Iterator itr=al.iterator();  
13. while(itr.hasNext()){  
14. System.out.println(itr.next());  
15.  }  
16. }  
17. }  

101
201
230
              

Example to sort user-defined class objects

1. import java.util.*;  
2.   
3. class Student implements Comparable<Student> {  
4.     public String name;  
5.   public Student(String name) {  
6.     this.name = name;  
7.   }  
8.   public int compareTo(Student person) {  
9.     return name.compareTo(person.name);  
10.       
11.   }   
12. }  
13. public class TestSort4 {  
14.   public static void main(String[] args) {  
15.       ArrayList<Student> al=new ArrayList<Student>();  
16.       al.add(new Student("Viru"));  
17.       al.add(new Student("Saurav"));  
18.       al.add(new Student("Mukesh"));  
19.       al.add(new Student("Tahir"));  
20.       
21.     Collections.sort(al);  
22.     for (Student s : al) {  
23.       System.out.println(s.name);  
24.     }  
25.   }  
26. }  

Mukesh
Saurav
Tahir
Viru