Question

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How do I remove repeated elements from ArrayList?

I want to replace a duplicate number (if any) that i store in an ArrayList. I obtianed the numbers for a Random variable. Here is the class.

import java.util.*;

public class RandomNumbers {
// instance variables
private ArrayList<Integer> randomNumberList = new ArrayList<Integer>(); 
private Random randomNums = new Random();

public RandomNumbers(){
    // generating and adding random numbers to the list
    for (int i=0; i<MemoryGame.totalAnswers; i++)
        randomNumberList.add(randomNums.nextInt(32));

    System.out.println("Numbers in the list: " + randomNumberList);
    System.out.println("");
}

public ArrayList<Integer> getRandomNumbers(){
    return randomNumberList;
}
}

My school book tells me how to add,remove and retrieve a number, but not how to replace one that is duplicated.

thanks.

Answer 1

If you are not limited to ArrayList, use a HashSet<Integer> instead (or better yet, a LinkedHashSet). HashSet<Integer> will garantee you not to have duplicated values in the collection, LinkedHashSet<Integer> will do the same and also preserve the ordering the the items.

If you insist on having an ArrayList, then do this :

public RandomNumbers(){
    HashSet<Integer> set = new HashSet<Integer>();

    // generating and adding random numbers to the list
    //for (int i=0; i<MemoryGame.totalAnswers; i++)
    while (set.size()<MemoryGame.totalAnswers)
        set.add(randomNums.nextInt(32));

    randomNumberList.addAll(set);  // dump the set in your ArrayList

    System.out.println("Numbers in the list: " + randomNumberList);
    System.out.println("");
}

Also, if MemoryGame.totalAnswers == 32, then you can speed up this process with a random list instead :

public RandomNumbers(){
    // generating and adding random numbers to the list
    for (int i=0; i<MemoryGame.totalAnswers; i++)
        randomNumberList.add(i);

    Collections.shuffle(randomNumberList);

    System.out.println("Numbers in the list: " + randomNumberList);
    System.out.println("");
}

** Update **

Since MemoryGame.totalAnswers == 8, forget the last snippet. I’m leaving it there if anyone would happend to need it. You can skip the use of a Set entirely by folllowing pst‘s suggestion :

public RandomNumbers(){
    // generating and adding random numbers to the list
    for (int i=0; i<32; i++)
        randomNumberList.add(i);

    Collections.shuffle(randomNumberList);

    // keep only the first ones we need
    randomNumberList.removeRange(MemoryGame.totalAnswers + 1, 32);

    System.out.println("Numbers in the list: " + randomNumberList);
    System.out.println("");
}