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Getting Random Elements from ArrayList in Java

  • Difficulty Level : Easy
  • Last Updated : 07 Jan, 2021
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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

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