Java Collections sort() Method

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Description

The Java Collections sort(List<T>) method is used to sort the specified list into ascending order, according to the natural ordering of its element.

Declaration

Following is the declaration for java.util.Collections.sort() method.

public static <T extends Comparable<? super T>> void sort(List<T> list)

Parameters

list − This is the list to be sorted.

Return Value

NA

Exception

• ClassCastException − Throws if the list contains elements that are not mutually comparable (for example, strings and integers).

• UnsupportedOperationException − Throws if the specified list's list-iterator does not support the set operation.

Java Collections sort(List, Comparator) Method

Description

The sort(List<T>,Comparator<? super T>) method is used to sort the specified list according to the order induced by the specified comparator.

Declaration

Following is the declaration for java.util.Collections.sort() method.

public static <T> void sort(List<T> list,Comparator<? super T> c)

Parameters

• list − This is the list to be sorted.

• c − This is the comparator to determine the order of the list.

Return Value

NA

Exception

• ClassCastException − Throws if the list contains elements that are not mutually comparable using the specified comparator.

• UnsupportedOperationException − Throws if the specified list's list-iterator does not support the set operation.

Sorting List of Integers Example

The following example shows the usage of Java Collection sort(List) method to get a sorted list using natural order. We've created a List object with some integers. Using sort(List) method, we've sorted the list and then it is printed.

package com.tutorialspoint;

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

public class CollectionsDemo {

   public static void main(String[] args) {
      List<Integer> list = new ArrayList<>(Arrays.asList(1,4,3,6,5,2));

      System.out.println("Initial collection value: " + list);
      // sort the list.
      Collections.sort(list);
      System.out.println("Final collection value: " + list);
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Initial collection value: [1, 4, 3, 6, 5, 2]
Final collection value: [1, 2, 3, 4, 5, 6]

Sorting List of Comparable Objects Example

The following example shows the usage of Java Collection sort(List) method to get a sorted list using natural order. We've created a List object with some Student objects. Using sort(List) method, we've sorted the list and then it is printed.

package com.tutorialspoint;

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

public class CollectionsDemo {

   public static void main(String[] args) {
      List<Student> list = new ArrayList<>(Arrays.asList(new Student(1, "Julie"), new Student(3, "Adam"), new Student(2, "Robert")));

      System.out.println("Initial collection value: " + list);
      // sort the list.
      Collections.sort(list);
      System.out.println("Final collection value: " + list);
   }
}
class Student implements Comparable<Student> {
   int rollNo;
   String name;

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

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   
   @Override
   public boolean equals(Object obj) {
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }

   @Override
   public int compareTo(Student student) {
      return this.rollNo - student.rollNo;
   }
}

Output

Let us compile and run the above program, this will produce the following result.

Initial collection value: [[ 1, Julie ], [ 3, Adam ], [ 2, Robert ]]
Final collection value: [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ]]

Sorting List of Objects using Comparator Example

The following example shows the usage of Java Collection sort(List, Comparator) method to get a sorted list using given comparator. We've created a List object with some Student objects. Using sort(List) method, we've sorted the list and then it is printed.

package com.tutorialspoint;

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

public class CollectionsDemo {

   public static void main(String[] args) {
      List<Student> list = new ArrayList<>(Arrays.asList(new Student(1, "Julie"), new Student(3, "Adam"), new Student(2, "Robert")));

      System.out.println("Initial collection value: " + list);
	  RollNoComparator comparator = new RollNoComparator();
      // sort the list.
      Collections.sort(list,comparator);
      System.out.println("Final collection value: " + list);
   }
}
class Student {
   int rollNo;
   String name;

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

   @Override
   public String toString() {
      return "[ " + this.rollNo + ", " + this.name + " ]";
   }
   
   @Override
   public boolean equals(Object obj) {
      Student s = (Student)obj;
      return this.rollNo == s.rollNo && this.name.equalsIgnoreCase(s.name);
   }

   public int getRollNo() {
      return rollNo;
   }

   public void setRollNo(int rollNo) {
      this.rollNo = rollNo;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }
}

class RollNoComparator implements Comparator<Student>{

   @Override
   public int compare(Student o1, Student o2) {
      return o1.getRollNo()-o2.getRollNo();
   }	
}

Output

Let us compile and run the above program, this will produce the following result.

Initial collection value: [[ 1, Julie ], [ 3, Adam ], [ 2, Robert ]]
Final collection value: [[ 1, Julie ], [ 2, Robert ], [ 3, Adam ]]