Getting Random Elements from ArrayList in Java

ArrayList is a part of collection framework and is present in java.util package. It provides us with dynamic arrays in Java.

Generating random numbers from the list have a good utility value 

There are various methods to get a random element from the ArrayList:

1. Using Math.random()
2. Using ArrayList Shuffle
3. Using Random class

Method 1: Using Math.random()

Syntax :

public static double random()

Return :
This method returns a pseudorandom double greater than or equal to 0.0 and less than 1.0.

• Math.random() generates a random value between 0 and 1. 
• Multiply this value with the size of ArrayList and take only the integer part of this value.
• Now the number that is generated after performing the above steps can be taken as the index of the ArrayList.
• Use the get() method to return a random element from the ArrayList using the above-generated index.

Note: Each time the Math.random() method is called it generates a new random value however original order of elements in the ArrayList does not get disturbed.

Java

// Java program for Getting Random Elements
// from ArrayList using Math.random() function

import java.io.*;
import java.util.ArrayList;
class GFG {
    public static void main(String[] args)
    {
        // creating ArrayList
        ArrayList<Integer> my_list = new ArrayList<Integer>();

        // adding elements
        my_list.add(10);
        my_list.add(80);
        my_list.add(30);
        my_list.add(70);
        my_list.add(5);
        my_list.add(90);
        my_list.add(19);
        my_list.add(25);

        // loop to print elements at randonm
        for (int i = 0; i < my_list.size(); i++)
        {
           // generating the index using Math.random()
            int index = (int)(Math.random() * my_list.size());

            System.out.println("Random Element is :"
                               + my_list.get(index));
        }
    }
}

Output

Random Element is :70
Random Element is :25
Random Element is :90
Random Element is :5
Random Element is :70
Random Element is :30
Random Element is :70
Random Element is :10

Method 2: Using ArrayList Shuffle

• Collections.shuffle() method of Java shuffles the order of elements in ArrayList.
• Now we can simply iterate over the ArrayList and return elements as they are in random order now.

Note: The actual ArrayList itself gets shuffled and original ordering is lost.

Java

// Java program for Getting Random Elements
// from ArrayList using Collections.shuffle()

import java.io.*;
import java.util.*;
import java.util.ArrayList;
class GFG {
    public static void main(String[] args)
    {
        // creating ArrayList
        ArrayList<Integer> my_list = new ArrayList<Integer>();

        // adding elements
        my_list.add(10);
        my_list.add(80);
        my_list.add(30);
        my_list.add(70);
        my_list.add(5);
        my_list.add(90);
        my_list.add(19);
        my_list.add(25);

        // using collections.shuffle to shuffle elements of
        // ArrayList
        Collections.shuffle(my_list);

        // using for each loop to print values at random
        System.out.println("Random values :");

        for (Integer random_values : my_list)
        {
            System.out.print(random_values + " ");
        }
    }
}

Output

Random values :
19 5 25 90 10 30 80 70

Method 3: Using Random Class function

• nextInt() method of Random class can be used to generate a random value between 0 and the size of ArrayList.
• Now use this number as an index of the ArrayList.
• Use get () method to return a random element from the ArrayList using number generated from nextInt() method.

Note: The original order of the ArrayList remains the same.

Java

// Java program for Getting Random Elements
// from ArrayList using nextInt() function

import java.io.*;
import java.util.ArrayList;
import java.util.Random;
class GFG {
    public static void main(String[] args)
    {
        // creating ArrayList
        ArrayList<Integer> my_list = new ArrayList<Integer>();

        // adding elements
        my_list.add(10);
        my_list.add(80);
        my_list.add(30);
        my_list.add(70);
        my_list.add(5);
        my_list.add(90);
        my_list.add(19);
        my_list.add(25);

        // initializing random class
        Random random_method = new Random();

        // loop for generation random number
        for (int i = 0; i < my_list.size(); i++)
        {
            // generating random index with the help of
            // nextInt() method
            int index = random_method.nextInt(my_list.size());

            System.out.println("Random Element is :"
                               + my_list.get(index));
        }
    }
}

Output

Random Element is :5
Random Element is :10
Random Element is :19
Random Element is :5
Random Element is :30
Random Element is :70
Random Element is :25
Random Element is :90