How do I create an iterator from a regular array in Java?

Question

public class TileGrid implements Iterable<Tile> {
    private wheelSize = [a positive integer];
    private Tile[][] grid = new Tile[wheelSize * 2 + 1][wheelSize * 2 + 1]

    @Override
    public Iterator<Tile> iterator() {
        return ????????;
    }
}

I've made a TileGrid class to keep track of a hex-grid for me. It stores Tile-objects in a two dimensional array called grid. Now I want to make the TileGrid class Iterable so that I can easily loop through all Tile-objects. The problem is that there is some positions in the array that is naturally not used (due to the shape of the hex-grid) and thus contains the value null.

My question is this: How do I create an iterator that iterates through all positions in grid except the ones that is null?

I do not want to use some kind of ArrayList since I'm using the array indexes to mark the position of the Tiles.

We need more information. What have you already tried? When iterating over a standard 2D array, you can iterate rows then columns, columns then rows, or something more exotic (like a zig-zag). What iteration order do you want? — Jeffrey
– Jeffrey, Commented Aug 17, 2015 at 17:16
Create an inner class of TileGrid, e.g. TileIterator, that implements Iterator<Tile> and maintains the current position in private fields (e.g. x and y). As you advance the x,y position, you skip empty cells. — Andreas
– Andreas, Commented Aug 17, 2015 at 17:17

HopefullyHelpful · Accepted Answer · 2015-08-17 18:20:59Z

2

You have to return an Instance of an Implementation of the Iterator class. The iterator you return should be able to access your array so that the code makes sense.(http://docs.oracle.com/javase/7/docs/api/java/util/Iterator.html)

public Iterator<Tile> iterator() {
   return new TileGridIterator(grid);
}

That means you need to write a class that implements the Iterator-Interface and that implements all methods that are specified in the API of that interface.

An example of this could look like that:

import java.util.Iterator;
import java.util.NoSuchElementException;

public class TileGridIterator implements Iterator<Tile> {
    int x = 0;
    int y = 0;
    int nextX = 0;
    int nextY = -1;
    Tile[][] grid;

    public TileGridIterator(Tile[][] grid) {
        this.grid = grid;
    }

    public boolean hasNext() {
        while(nextX <= x && nextY < y) {
            nextY++;
            if(nextY == grid[nextX].length) {
               nextY = 0;
               nextX++;
            }
            if(nextX >= grid.length) {
                return false;
            }
            if(grid[nextX][nextY] != null) {
                return true;
            }
        }
        if(nextX < grid.length && nextY < grid[nextX].length && grid[nextX][nextY] != null) {
            return true;
        }
        else {
            return false;
        }
    }

    public Tile next() {
        if(hasNext()) {
            x = nextX;
            y = nextY;
            return grid[x][y];
        }else {
            throw new NoSuchElementException("no more elements left");
        }
    }
}

ps: thanks for the question, it was an interesting task for me.

edited Aug 17, 2015 at 18:20

answered Aug 17, 2015 at 17:25

HopefullyHelpful

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3 Comments

Z3wz Over a year ago

Thanks, that showed me a lot about how to make an iterator. However, your code didn't work for me, so I modified it to work. I'll post the code in a new answer if you want to see it.

HopefullyHelpful Over a year ago

@Z3wz be sure to upvote good answers and select the best answer with the check mark

Z3wz Over a year ago

Sure, I will do that!

Z3wz · Accepted Answer · 2015-08-18 16:25:43Z

@HopefullyHelpful

My version:

public Iterator<Tile> iterator() {
    return new TileIterator(grid);
}

.

class TileIterator implements Iterator<Tile> {

    int x = 0, y = -1;
    int newX, newY;
    Tile[][] grid;

    TileIterator(Tile[][] grid) {
        this.grid = grid;
        updateNewIndex();
    }

    public boolean hasNext() {
        if (newX == -1) {
            return false;
        }
        return true;
    }

    public Tile next() {
        x = newX;
        y = newY;
        updateNewIndex();
        if (x == -1) {
            throw new NoSuchElementException("no more elements left");
        }
        return grid[x][y];
    }

    private void updateNewIndex() {
        newX = x;
        newY = y;
        do {
            newY++;
            if (newY == grid[newX].length) {
                newY = 0;
                newX = newX + 1;
                if (newX == grid.length) {
                    newX = newY = -1;
                }
            }
        } while (newX != -1 && grid[newX][newY] == null);
    }
}

Thanks again for your answer as it helped me make this.

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