Table of Contents
URL: https://www.progressiverobot.com/java-sort-list/
Sometimes we have to sort a list in Java before processing its elements. In this tutorial, we will learn how to sort a list in the natural order. We will also learn how to use our own Comparator implementation to sort a list of objects. Java List is similar to arrays except that the length of the list is dynamic and it comes in [Java Collection framework](/community/tutorials/collections-in-java-tutorial). Actually, [List](/community/tutorials/java-list) is an interface and most of the time we use one of its implementation like [ArrayList](/community/tutorials/java-arraylist) or [LinkedList](/community/tutorials/java-linkedlist-linkedlist-java) etc.
Java Sort List
Here we will learn how to sort a list of Objects in Java. We can use Collections.sort() method to sort a list in the natural ascending order. All the elements in the list must implement [Comparable](/community/tutorials/comparable-and-comparator-in-java-example) interface, otherwise IllegalArgumentException is thrown. Let's look at a quick example to sort a list of strings.
package com.journaldev.sort;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class JavaListSort {
/**
* This class shows how to sort ArrayList in java
* @param args
*/
public static void main(String[] args) {
List<String> strList = new ArrayList<String>();
strList.add("A");
strList.add("C");
strList.add("B");
strList.add("Z");
strList.add("E");
//using Collections.sort() to sort ArrayList
Collections.sort(strList);
for(String str: strList) System.out.print(" "+str);
}
}
As you can see that we are using Collections.sort() method to sort the list of Strings. The String class implements Comparable interface. Output:
Java Sort List of Objects
Let's see another example where we will sort a list of custom objects. Note that the class must implement Comparable interface.
package com.journaldev.sort;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
public class JavaSortListObject {
public static void main(String[] args) {
List<Data> dl = new ArrayList<>();
dl.add(new Data(2));
dl.add(new Data(3));
dl.add(new Data(1));
System.out.println("Original List::"+dl);
Collections.sort(dl);
System.out.println("Naturally Sorted List::"+dl);
}
}
class Data implements Comparable<Data> {
private int id;
public Data(int i) {
this.id = i;
}
@Override
public int compareTo(Data d) {
return this.id - d.getId();
}
public int getId() {
return id;
}
@Override
public String toString() {
return "Data{"+this.id+"}";
}
}
Output:
Original List::[Data{2}, Data{3}, Data{1}]
Naturally Sorted List::[Data{1}, Data{2}, Data{3}]
Sort a List in Java using Comparator
Collections.sort() method is overloaded and we can also provide our own [Comparator](/community/tutorials/comparable-and-comparator-in-java-example) implementation for sorting rules. Since Comparator is a [functional interface](/community/tutorials/java-8-functional-interfaces), we can use [lambda expressions](/community/tutorials/java-lambda-expression) to write its implementation in a single line.
Collections.sort(dl, (d1, d2) -> {
return d2.getId() - d1.getId();
});
System.out.println("Reverse Sorted List using Comparator::" + dl);
Output:
Summary
[Collections](/community/tutorials/collections-class-java-util-collections) class sort() method is used to sort a list in Java. We can sort a list in natural ordering where the list elements must implement Comparable interface. We can also pass a Comparator implementation to define the sorting rules.
You can checkout more examples from our GitHub Repository.
Reference: API Doc)
