Finding index of recurring objects in an ArrayList in Java

Question

public class NewMain {
    public static void main(String[] args) {
        ArrayList<String> array = new ArrayList<>();
        array.add("Ace of Hearts");
        array.add("King of Clubs");
        array.add("Three of Spade");
        array.add("Ace of Hearts");
        array.add("Ten of Clubs");
        for(String s: array)
            System.out.println(s+"at postion: "+array.indexOf(s));
    }
}

IndexOf() returns the first occurence of the object in an ArrayList. Assuming my code is the hand of a poker player, and I want to know the indices of the cards he is holding. Ace of Hearts is at positon 1 and 4. But IndexOf() returns as position 1 and 1 for both occurences of Ace of Hearts.

Question:
1)Is there any other way of getting the value?
2)Should I use someother Data Structure like HashMap or something else?

Does this answer your question? [Find all indexes of 'x' in ArrayList](https://stackoverflow.com/questions/47622723/find-all-indexes-of-x-in-arraylist) — Wiciaq123, Feb 18 '20 at 08:45
Does this answer your question? [Java indexOf method for multiple matches in String](https://stackoverflow.com/questions/4988050/java-indexof-method-for-multiple-matches-in-string) — Prakhar Londhe, Feb 18 '20 at 09:04

score 1 · Accepted Answer · answered Feb 18 '20 at 08:56

The List.indesOf() method will return the first occurrence of the list. But your requirement can fulfill as following way.

    import java.util.ArrayList;

    public class NewMain {
        public static void main(String[] args) {
            ArrayList<String> array = new ArrayList<>();
            array.add("Ace of Hearts");
            array.add("King of Clubs");
            array.add("Three of Spade");
            array.add("Ace of Hearts");
            array.add("Ten of Clubs");
            for(int i =0;i<array.size();i++)
                System.out.println(array.get(i)+"at postion: "+i);
        }
    }

score 0 · Answer 2 · answered Feb 19 '20 at 05:52

This should solve your problem :

 public class NewMain {
        public static void main(String[] args) {
            String abc="Ace of Hearts";
            List<String> array = new ArrayList<String>();
            array.add("Ace of Hearts");
            array.add("King of Clubs");
            array.add("Three of Spade");
            array.add("Ace of Hearts");
            array.add("Ten of Clubs");
            List<Integer> indexList= indexOfAll(abc,array);
            System.out.println(indexList);

        }

        static <T> List<Integer> indexOfAll(T obj, List<T> list) {
            final List<Integer> indexList = new ArrayList<Integer>();
            for (int i = 0; i < list.size(); i++) {
                if (obj.equals(list.get(i))) {
                    indexList.add(i);
                }
            }
            return indexList;
        }

Well, this would further increase complexity, since indexOfAll() will be called for every turn of the player. Since card positions are maintained, I would just stick to a for loop. Thanks anyways!! — vicki, Feb 19 '20 at 10:35

Finding index of recurring objects in an ArrayList in Java

2 Answers2