Using Map in Java. How to not overwrite Value?

Question

    for(int i = 0; i < Board.NUM_OF_ROWS; i++) {
        for(int j = 0; j < Board.NUM_OF_COLS; j++) {
            if(piece.canMove(board, piece.getX(), piece.getY(), i, j)) {
                mappedPosition.put(i, j);
            }
        }
    }

In this code, I am trying to add a pair of (x,y) coordinate of a movable Position of a chess "piece".

For example, I was expecting it to add [2,2], [2,4], [3,1], [3,5], [5,1], [5,5], [6,2], [6,4]

But when I use put, it overwrites the Value when it has the same Key. So [2,2] just becomes [2,4] eventually.

How can I get the full list of the pair without overwriting it?

Answer 1

Map uses the unique key identifiers thus it's impossible to have the same key twice and more.

Create the class holding the two coordinate values you need.

public class BoardPoint {

    private int x;
    private int y;

    public BoardPoint(int x, int y) {
        this.x = x;
        this.y = y;
    }

    // getters & setters
}

The class above will be useful in case you need to scale: implement more variables or perform some operations over the pair of values. If you need just a POJO (plain-old Java object) the class java.awt.Point should be enough as @XtremeBaumer said id the comments.

Answer 2

Map by definition is key --> value. And each key can only have one value at each moment. You can either use set when the values are pair of (x,y) or just a list, it depends on how you wish to use this information later.

Answer 3

You can, use Guava's Multimap.

Multimap<Integer, String> multimap = ArrayListMultimap.create();
multimap.put(1, "rohit");
multimap.put(1, "jain");

System.out.println(multimap.get(1));  // Prints - [rohit, jain]

Answer 4

In that case you can also use a Multimap. The API is similar, but the value is always a Collection.

http://google.github.io/guava/releases/23.0/api/docs/com/google/common/collect/Multimap.html

Answer 5

You can use Apache MultiMap for holding several values per key

MultiMap mhm = new MultiValueMap();
mhm.put(key, "A"); mhm.put(key, "B");
mhm.put(key, "C");
Collection coll = (Collection) mhm.get(key);

coll will be a collection containing "A", "B", "C".