In this article, we will discuss how to sort LinkedList elements in ascending & descending order using Collections.sort() method
1. Collections.sort() method
- Sorts the specified list into ascending order, according to the natural ordering of its elements
- All elements in the list must implement the Comparable interface
- Furthermore, all elements in the list must be mutually comparable
- That is, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list
- This sort is guaranteed to be stable: equal elements will not be reordered as a result of the sort
- String and wrapper classes implements comparable interface, so when we store elements of String type then we can use Collections’ sort() method to sort elements in ascending order of alphabets
- Note: there shouldn’t be any NULL objects, otherwise NullPointerException will be thrown
- Method signature :- public static <T extends Comparable<? super T>> void sort(List<T> list)
1.1 Sorting LinkedList of Integer numbers
- A LinkedList contains integer numbers in insertion-order
- We are sorting these integer numbers in natural order (or ascending order) using Collections.sort() method
SortLinkedListOfIntegerUsingCollectionsSortMethod.java
package net.bench.resources.sort.linkedlist.java;
import java.util.Collections;
import java.util.Comparator;
import java.util.LinkedList;
import java.util.List;
public class SortLinkedListOfIntegerUsingCollectionsSortMethod {
public static void main(String[] args) {
// 1. create LinkedList
List<Integer> numbers = new LinkedList<>();
// 1.2 add integer numbers to LL
numbers.add(87);
numbers.add(18);
numbers.add(98);
numbers.add(64);
numbers.add(25);
numbers.add(77);
numbers.add(54);
// 1.3 original LinkedList in insertion order
System.out.println("1. Original LinkedList<Integer> "
+ "as per insertion-order :- \n");
// 1.4 Iterating using enhanced for-loop
for(int num : numbers){
System.out.println(num);
}
// 2. Collections.sort() - natural order
Collections.sort(numbers);
// 2.1 print to console
System.out.println("\n2. Natural order of "
+ "LinkedList<Integer> :- \n");
// 2.2 Iterating using enhanced for-loop
for(int num : numbers){
System.out.println(num);
}
// 3. Collections.sort() - reverse order
Collections.sort(numbers, Comparator.reverseOrder());
// 3.1 print to console
System.out.println("\n3. Reverse order of "
+ "LinkedList<Integer> :- \n");
// 3.2 Iterating using enhanced for-loop
for(int num : numbers){
System.out.println(num);
}
}
}
Output:
1. Original LinkedList<Integer> as per insertion-order :-
87
18
98
64
25
77
54
2. Natural order of LinkedList<Integer> :-
18
25
54
64
77
87
98
3. Reverse order of LinkedList<Integer> :-
98
87
77
64
54
25
18
1.2 Sorting LinkedList of String elements
- A LinkedList contains String elements in insertion-order
- We are sorting these String elements in alphabetical order using Collections.sort() method
SortLinkedListOfStringUsingCollectionsSortMethod.java
package net.bench.resources.sort.linkedlist.java;
import java.util.LinkedList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
public class SortLinkedListOfStringUsingCollectionsSortMethod {
public static void main(String[] args) {
// 1. create LinkedList
List<String> names = new LinkedList<>();
// 1.1 add names to LL
names.add("Karthik");
names.add("Nagarjuna");
names.add("Ramki");
names.add("Surya");
names.add("Ajith");
names.add("Prasanna");
// 1.2 original LinkedList as per insertion order
System.out.println("1. Original LinkedList<String> "
+ "as per insertion-order :- \n");
// 1.3 Iterating using enhanced for-loop
for(String str : names){
System.out.println(str);
}
// 2. Collections.sort() - alphabetical order
Collections.sort(names);
// 2.1 print to console
System.out.println("\n2. Alphabetically sorted-order "
+ "of LinkedList<String> :- \n");
// 2.2 Iterating using enhanced for-loop
for(String str : names){
System.out.println(str);
}
// 3. Collections.sort() - reverse alphabetical order
Collections.sort(names, new Comparator<String>() {
@Override
public int compare(String str1, String str2) {
return str2.compareTo(str1);
}
});
// 3.1 print to console
System.out.println("\n3. Reverse alphabetically sorted-order "
+ "of LinkedList<String> :- \n");
// 3.2 Iterating using enhanced for-loop
for(String str : names){
System.out.println(str);
}
}
}
Output:
1. Original LinkedList<String> as per insertion-order :-
Karthik
Nagarjuna
Ramki
Surya
Ajith
Prasanna
2. Alphabetically sorted-order of LinkedList<String> :-
Ajith
Karthik
Nagarjuna
Prasanna
Ramki
Surya
3. Reverse alphabetically sorted-order of LinkedList<String> :-
Surya
Ramki
Prasanna
Nagarjuna
Karthik
Ajith
2. Comparable interface & Collections.sort() method
- Compares this object with the specified object for order
- Returns a negative integer, zero, or a positive integer as this object is less than, equal to, or greater than the specified object
- Method signature :- int compareTo(T object)
2.1 Sorting LinkedList of Student objects
- A LinkedList contains Student objects with attributes such as Id, name and their age in insertion-order
- We are sorting these Student objects in natural order according to their age using Collections.sort() method
Student.java
package net.bench.resources.sort.linkedlist.java;
public class Student implements Comparable<Student> {
// member variables
private int id;
private String name;
private int age;
// 3-arg parameterized constructor
// getters & setters
// toString()
// compareTo()
@Override
public int compareTo(Student s) {
return Integer.compare(this.age, s.getAge());
}
}
SortLinkedListOfIntegerUsingStream.java
package net.bench.resources.sort.linkedlist.java;
import java.util.Collections;
import java.util.Comparator;
import java.util.LinkedList;
import java.util.List;
public class SortLinkedListOfStudentsUsingCollectionsSortMethod {
public static void main(String[] args) {
// 1. create LinkedList
List<Student> students = new LinkedList<>();
// 1.1 add Students to LL
students.add(new Student(2063, "Pravin", 35));
students.add(new Student(2097, "Shweta", 32));
students.add(new Student(2019, "Sachin", 28));
students.add(new Student(2023, "Kalpana", 27));
students.add(new Student(2045, "Rajdeepan", 31));
students.add(new Student(2073, "Sneha", 24));
// 1.2 original LinkedList in insertion order
System.out.println("1. Original LinkedList "
+ "as per insertion-order :- \n");
// 1.3 Iterating using enhanced for-loop
for(Student student : students){
System.out.println(student);
}
// 2. Collections.sort() - increasing-order of Age
Collections.sort(students);
// 2.1 print to console
System.out.println("\n2. Sorted Student LinkedList -"
+ " according to Age :- \n");
// 2.2 Iterating using enhanced for-loop
for(Student student : students){
System.out.println(student);
}
// 3. Collections.sort() - decreasing-order of Age
Collections.sort(students, new Comparator<Student>() {
@Override
public int compare(Student stud1, Student stud2) {
return stud2.getAge() - stud1.getAge();
}
});
// 3.1 print to console
System.out.println("\n3. Reverse sorted Student LinkedList -"
+ " according to Age :- \n");
// 3.2 Iterating using enhanced for-loop
for(Student student : students){
System.out.println(student);
}
}
}
Output:
1. Original LinkedList as per insertion-order :-
Student [id=2063, name=Pravin, age=35]
Student [id=2097, name=Shweta, age=32]
Student [id=2019, name=Sachin, age=28]
Student [id=2023, name=Kalpana, age=27]
Student [id=2045, name=Rajdeepan, age=31]
Student [id=2073, name=Sneha, age=24]
2. Sorted Student LinkedList - according to Age :-
Student [id=2073, name=Sneha, age=24]
Student [id=2023, name=Kalpana, age=27]
Student [id=2019, name=Sachin, age=28]
Student [id=2045, name=Rajdeepan, age=31]
Student [id=2097, name=Shweta, age=32]
Student [id=2063, name=Pravin, age=35]
3. Reverse sorted Student LinkedList - according to Age :-
Student [id=2063, name=Pravin, age=35]
Student [id=2097, name=Shweta, age=32]
Student [id=2045, name=Rajdeepan, age=31]
Student [id=2019, name=Sachin, age=28]
Student [id=2023, name=Kalpana, age=27]
Student [id=2073, name=Sneha, age=24]
3. Comparator interface & Collections.sort() method
- Compares its two arguments for order
- Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second argument
- Method signature :- int compare(T object1, T object2)
3.1 Sorting LinkedList of Student objects
- A LinkedList contains Student objects with attributes such as Id, name and their age in insertion-order
- We are sorting these Student objects in natural order according to their name using Collections.sort() method
Student.java
package net.bench.resources.sort.linkedlist.java;
public class Student {
// member variables
private int id;
private String name;
private int age;
// 3-arg parameterized constructor
// getters & setters
// toString()
}
StudentComparator.java
package net.bench.resources.sort.linkedlist.java;
import java.util.Comparator;
public class StudentComparator implements Comparator<Student> {
// name comparison in alphabetical order
@Override
public int compare(Student student1, Student student2) {
return student1.getName().compareTo(student2.getName());
}
}
SortLinkedListOfStudentsUsingComparator.java
package net.bench.resources.sort.linkedlist.java;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
public class SortLinkedListOfStudentsUsingComparator {
public static void main(String[] args) {
// 1. create LinkedList
List<Student> students = new LinkedList<>();
// 1.1 add Students to LL
students.add(new Student(2063, "Pravin", 35));
students.add(new Student(2097, "Abhijeet", 32));
students.add(new Student(2019, "Sachin", 28));
students.add(new Student(2023, "Kalpana", 27));
students.add(new Student(2045, "Rajdeepan", 31));
students.add(new Student(2073, "Nikita", 24));
// 1.2 original LinkedList in insertion order
System.out.println("1. Original LinkedList "
+ "as per insertion-order :- \n");
// 1.3 Iterating using enhanced for-loop
for(Student student : students){
System.out.println(student);
}
// 2. Collections.sort() - alphabetical order of Name
Collections.sort(students, new StudentComparator());
// 2.1 print to console
System.out.println("\n2. Sorted Student LinkedList -"
+ " according to Name :- \n");
// 2.2 Iterating using enhanced for-loop
for(Student student : students){
System.out.println(student);
}
// 3. Collections.sort() - reverse alphabetical order of Name
Collections.sort(students, (new StudentComparator()).reversed());
// 3.1 print to console
System.out.println("\n3. Reverse sorted Student LinkedList -"
+ " according to Name :- \n");
// 3.2 Iterating using enhanced for-loop
for(Student student : students){
System.out.println(student);
}
}
}
Output:
1. Original LinkedList as per insertion-order :-
Student [id=2063, name=Pravin, age=35]
Student [id=2097, name=Abhijeet, age=32]
Student [id=2019, name=Sachin, age=28]
Student [id=2023, name=Kalpana, age=27]
Student [id=2045, name=Rajdeepan, age=31]
Student [id=2073, name=Nikita, age=24]
2. Sorted Student LinkedList - according to Name :-
Student [id=2097, name=Abhijeet, age=32]
Student [id=2023, name=Kalpana, age=27]
Student [id=2073, name=Nikita, age=24]
Student [id=2063, name=Pravin, age=35]
Student [id=2045, name=Rajdeepan, age=31]
Student [id=2019, name=Sachin, age=28]
3. Reverse sorted Student LinkedList - according to Name :-
Student [id=2019, name=Sachin, age=28]
Student [id=2045, name=Rajdeepan, age=31]
Student [id=2063, name=Pravin, age=35]
Student [id=2073, name=Nikita, age=24]
Student [id=2023, name=Kalpana, age=27]
Student [id=2097, name=Abhijeet, age=32]
Related Articles:
- Sorting ArrayList using Comparable and Comparator
- Sorting ArrayList in descending order
- How to sort LinkedList using Collections.sort() method
- How to sort Vector using Collections.sort()
- Sorting list of objects on multiple fields using Comparator in Java
- Sorting HashSet contents in ascending and descending order
- How to Sort HashSet in Java – 2 ways
- How to sort HashSet in Java 8 ?
- How to sort LinkedHashSet contents in Java ?
- How to sort TreeSet in descending order using Comparator ?
- Sorting Collection of String, StringBuffer and StringBuilder in Java
References:
- https://docs.oracle.com/javase/tutorial/collections/intro/
- https://docs.oracle.com/javase/7/docs/api/java/util/Collections.html
- https://docs.oracle.com/javase/tutorial/collections/interfaces/collection.html
- https://docs.oracle.com/javase/7/docs/api/java/util/LinkedList.html
- https://docs.oracle.com/javase/7/docs/api/java/lang/Comparable.html
- https://docs.oracle.com/javase/7/docs/api/java/util/Comparator.html
Happy Coding !!
Happy Learning !!
