How to shuffle an object array in Java

Question

I want to implement a poker game in Java. So I made a card class and created 52 card objects and put them into a array. This array is the card deck.

How would I randomly shuffle this array of objects? Is there an inbuilt method or any other way?

you can check this...stackoverflow.com/questions/1519736/… — rahul pasricha
– rahul pasricha, Commented Jun 25, 2014 at 14:38
Unless your array variable is final, cardsArray = Collections.shuffle(Arrays.asList(cardsArray)).asArray(new Card[cardsArray.length]); should do it. — Bhesh Gurung
– Bhesh Gurung, Commented Jun 25, 2014 at 14:44

Jigar Joshi · Accepted Answer · 2014-06-25 14:37:38Z

6

Use ArrayList instead and use Collections.shuffle()

Collections.shuffle(yourListInstance);

answered Jun 25, 2014 at 14:37

Jigar Joshi

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Community · Accepted Answer · 2017-05-23 12:17:53Z

1

I've modified the top answer from "Random shuffling of an array" to work for an array of Card objects. I simply changed int[] ar → Card[] ar, and int a = ar[index] → Card a = ar[index]

import java.util.*;
import java.util.concurrent.ThreadLocalRandom;

// Implementing Fisher–Yates shuffle
static void shuffleArray(Card[] ar)
{
  // If running on Java 6 or older, use `new Random()` on RHS here
  Random rnd = ThreadLocalRandom.current();
  for (int i = ar.length - 1; i > 0; i--)
  {
    int index = rnd.nextInt(i + 1);
    // Simple swap
    Card a = ar[index];
    ar[index] = ar[i];
    ar[i] = a;
  }
}

edited May 23, 2017 at 12:17

CommunityBot

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answered Apr 23, 2017 at 18:00

Stevoisiak

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Comments

Lorenzo Gatti · Accepted Answer · 2014-06-25 14:42:41Z

0

Collections.shuffle isn't good enough:

Randomly permutes the specified list using a default source of randomness. All permutations occur with approximately equal likelihood. The hedge "approximately" is used in the foregoing description because default source of randomness is only approximately an unbiased source of independently chosen bits. If it were a perfect source of randomly chosen bits, then the algorithm would choose permutations with perfect uniformity.

You need a source of randomness producing a number that is exactly distributed uniformly between 1 and 52! (inclusive).

answered Jun 25, 2014 at 14:42

Lorenzo Gatti

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5 Comments

awksp Over a year ago

I'm sorry, I don't quite follow... What is wrong with Collections.shuffle() exactly? It seems to me that the description just says that a PRNG is used instead of a true RNG, which makes sense. Also, I'm not quite following your last requirement. Why do you need a source of randomness that produces exactly that range?

assylias Over a year ago

You can provide your own random generator to shuffle().

Daybreaker Over a year ago

I didn't get what u hv said. Bt it seems their is a biased situation in Collections.shuffle()

awksp Over a year ago

@Daybreaker That's more a problem with PRNGs in general than Collections.shuffle() itself. The shuffling method implemented in Collections.shuffle() can be perfectly random if you can provide a true RNG.

Lorenzo Gatti Over a year ago

If the "source of randomness" (actually the PRNG) doesn't produce each possible permutation with equal probability the result of Collections.shuffle() is biased. The quoted Javadoc excerpt appears an admission that Collections.shuffle() takes shortcuts, such as blindly applying the modulo operator to PRNG numbers rather than rejecting them as necessary, on top of not seeding the PRNG randomly enough.

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