Randomly Select Items from a List in Java
Last Updated :
12 Feb, 2025
In this article, we will explore how to efficiently select an element from a list in Java. The basic approach involves generating a random index between 0 and the size of the list, and then using that index to retrieve the item.
Different Ways to Select Items from a List
Below are the different approaches to selecting random elements from a list.
1. Select a Single Random item
The easiest way to select a random item is by generating a random index using the Java Random class.
Example: This example demonstrates how to select a single random element.
Java
// Randomly select an element from a list
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
public class Geeks {
public static void main(String[] args)
{
// creating a list of integer type
List<Integer> l = new ArrayList<>();
l.add(10);
l.add(20);
l.add(30);
l.add(40);
l.add(50);
Geeks o = new Geeks();
// take a random element from list and print them
System.out.println("Random Element: "
+ o.getRandomElement(l));
}
// Function select an element base on index
// and return an element
public int getRandomElement(List<Integer> l)
{
Random r = new Random();
return l.get(r.nextInt(l.size()));
}
}
2. Selecting a Random Element in a Multi-threaded Environment
In multi-threaded applications, if multiple threads use the same Random object, they might interfere with each other and cause unexpected results. To fix, this Java provides ThreadLocalRandom class, which gives each thread its own random number generator and prevent any interference.
Example: This example demonstrates how to randomly select an element from a list using ThreadLocalRandom.
Java
// Java Program to demonstrate the working
// of ThreadLocalRandom class
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ThreadLocalRandom;
public class Geeks {
public static void main(String[] args) {
List<Integer> l = new ArrayList<>();
l.add(10);
l.add(20);
l.add(30);
l.add(40);
l.add(50);
Geeks o = new Geeks();
System.out.println("Random Element: " + o.getRandomElement(l));
}
public int getRandomElement(List<Integer> l) {
return l.get(ThreadLocalRandom.current().nextInt(l.size()));
}
}
3. Selecting Multiple Random Items With Repetitions
Sometimes, we want to select multiple random items from a list, allowing repetitions. In this case, we can keep selecting random elements without removing them from the list.
Example: This example demonstrates how to randomly select multiple elements from a list.
Java
// Select randomly multiple elements
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
public class Geeks {
public static void main(String[] args) {
List<Integer> l = new ArrayList<>();
l.add(10);
l.add(20);
l.add(30);
l.add(40);
l.add(50);
Geeks o = new Geeks();
System.out.println("Random Elements: "
+ o.getRandomElements(l, 3));
}
public List<Integer> getRandomElements(List<Integer> l, int totalItems) {
Random r = new Random();
List<Integer> newList = new ArrayList<>();
for (int i = 0; i < totalItems; i++) {
int randomIndex = r.nextInt(l.size());
newList.add(l.get(randomIndex));
}
return newList;
}
}
OutputRandom Elements: [50, 10, 50]
4. Selecting Multiple Random Items Without Repetitions
In some cases, we may want to select multiple random items without repetitions. To do this, we can remove elements from the list after selecting them or use a Set to track previously selected items.
Example: This example demonstrates how to select random element from a list without repetition by removing the selected items from the original list.
Java
// Select random elements from a
// list without repetition
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
public class Geeks {
public static void main(String[] args) {
List<Integer> l = new ArrayList<>();
l.add(10);
l.add(20);
l.add(30);
l.add(40);
l.add(50);
Geeks o = new Geeks();
System.out.println("Random Elements: "
+ o.getRandomElements(l, 3));
}
public List<Integer> getRandomElements(List<Integer> l, int totalItems) {
Random r = new Random();
List<Integer> newList = new ArrayList<>();
for (int i = 0; i < totalItems; i++) {
int randomIndex = r.nextInt(l.size());
newList.add(l.get(randomIndex));
l.remove(randomIndex); // Remove selected item
}
return newList;
}
}
OutputRandom Elements: [30, 10, 50]
Similar Reads
Java List Interface The List Interface in Java extends the Collection Interface and is a part of the java.util package. It is used to store the ordered collections of elements. In a Java List, we can organize and manage the data sequentially. Key Features:Maintained the order of elements in which they are added.Allows
15+ min read
AbstractList in Java with Examples The AbstractList class in Java is a part of the Java Collection Framework and implements the Collection interface and the AbstractCollection class. AbstractList class provides a skeletal implementation of the List interface to minimize the effort required to implement this interface backed by a Rand
7 min read
ArrayList in Java Java ArrayList is a part of the collections framework and it is a class of java.util package. It provides us with dynamic-sized arrays in Java. The main advantage of ArrayList is that, unlike normal arrays, we don't need to mention the size when creating ArrayList. It automatically adjusts its capac
9 min read
LinkedList in Java Linked List is a part of the Collection framework present in java.util package. This class is an implementation of the LinkedList data structure, which is a linear data structure where the elements are not stored in contiguous locations, and every element is a separate object with a data part and an
12 min read
Immutable List in Java ImmutableList, as suggested by the name, is a type of List which is immutable. It means that the content of the List are fixed or constant after declaration, that is, they are read-only. If any attempt made to add, delete and update elements in the List, UnsupportedOperationException is thrown. An I
5 min read
CopyOnWriteArrayList in Java CopyOnWriteArrayList class is introduced in JDK 1.5, which implements the List interface. It is an enhanced version of ArrayList in which all modifications (add, set, remove, etc) are implemented by making a fresh copy. It is found in java.util.concurrent package. It is a data structure created to b
6 min read
Custom ArrayList in Java Before proceeding further let us quickly revise the concept of the arrays and ArrayList quickly. So in java, we have seen arrays are linear data structures providing functionality to add elements in a continuous manner in memory address space whereas ArrayList is a class belonging to the Collection
8 min read
Difference Between Synchronized ArrayList and CopyOnWriteArrayList in Java Collection As we know that the ArrayList is not synchronized, if multiple threads try to modify an ArrayList at the same time, then the final outcome will be non-deterministic. Hence synchronizing the ArrayList is a must to achieve thread safety in a multi-threaded environment. In order to make List objects we
6 min read
How to remove a SubList from a List in Java Given a list in Java, the task is to remove all the elements in the sublist whose index is between fromIndex, inclusive, and toIndex, exclusive. The range of the index is defined by the user. Example: Input list = [1, 2, 3, 4, 5, 6, 7, 8], fromIndex = 2, endIndex = 4 Output [1, 2, 5, 6, 7, 8] Input
2 min read
Randomly Select Items from a List in Java In this article, we will explore how to efficiently select an element from a list in Java. The basic approach involves generating a random index between 0 and the size of the list, and then using that index to retrieve the item. Different Ways to Select Items from a ListBelow are the different appro
3 min read