How to use the Collections.shuffle method in Java

The Collections.shuffle method is defined in Java’s built-in java.util.Collections class. As its name suggests, the method​ shuffles the elements of a given list by randomly permuting them.

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Code

The following code snippet creates a list of strings and then shuffles them​ randomly using Collections.shuffle:

import java.util.*;



class Program {

    public static void main( String args[] ) {

      List<String> myList = new ArrayList<String>();



      myList.add("Hippo");

      myList.add("Panda");

      myList.add("Brown Bear");

      myList.add("Koala");

      myList.add("Polar Bear");  



      System.out.println("Original list: " + myList);

      // Randomly shuffle the list.

      Collections.shuffle(myList);

      System.out.println("List after shuffle: " + myList);

    }

}

One can also define the degree of randomness with which the elements are shuffled. This can be used to make shuffling a deterministic process (if that is a requirement for the program)​. Hence, when the degree of randomness is defined,​ shuffling with a fixed random seed will always produce the same random order for a particular list. The following code snippet demonstrates the usage of shuffling with a specific degree of randomness:

import java.util.*;
class Program {
    public static void main( String args[] ) {
      List<String> myList = new ArrayList<String>();
      myList.add("Hippo");
      myList.add("Panda");
      myList.add("Brown Bear");
      myList.add("Koala");
      myList.add("Polar Bear");  
      myList.add("Elephant");
      myList.add("Owl");
      System.out.println("Original list: " + myList);
      // Randomly shuffle the list with a random number.
      Collections.shuffle(myList, new Random());
      System.out.println("List after shuffle with Random(): " + myList);
      Collections.shuffle(myList, new Random(5));
      System.out.println("List after shuffle with Random(5): " + myList);
    }
}

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