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 ]]