How to convert Arraylist into 2D double[][] array

Question

I have an ArrayList which has objects of Coords which are x and y:

ArrayList<Coords> positionsArrayList = new ArrayList<>(values);

class Coords {
    int x;
    int y;

    public boolean equals(Object o) {
        Coords c = (Coords) o;
        return c.x == x && c.y == y;
    }

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

    public int hashCode() {
        return new Integer(x + "0" + y);
    }
}

Now i want to extract this x and y in form of 2D array double[][]. Just like

double[][] vector={{0,0},{1,1},{2,2}}

I have tried this code:

for (Coords value : positionsArrayList) {
    positions = new double[][]{{value.x, value.y}};
}

But it does enter only the last entry. New to java please help

NiVeR · Accepted Answer · 2019-03-18 14:28:47Z

2

The statement:

positions = new double[][]{{value.x, value.y}};

reassigns the reference each time. Finally, positions will "point to" (contain) the last tuple, that's why you see it as the result.

answered Mar 18, 2019 at 14:28

NiVeR

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Comments

SpaceyBot · Accepted Answer · 2019-03-18 14:39:07Z

1

What you can do is use traditional for loop to achieve the result:

positions = new double[positionsArrayList.size()][2];
for(int i =0; i < positionsArrayList.size(); i++){
    positions[i][0] = positionsArrayList.get(i).getX();
    positions[i][1] = positionsArrayList.get(i).getY();
}

answered Mar 18, 2019 at 14:39

SpaceyBot

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Comments

Ruslan · Accepted Answer · 2019-03-18 14:35:13Z

1

You could assign each coordinates value for each index like:

double[][] positions = new double[positionsArrayList.size()][2];

for (int i = 0; i < positionsArrayList.size(); i++) {
    positions[i][0] = positionsArrayList.get(i).x;
    positions[i][1] = positionsArrayList.get(i).y;
}

Or if you're familiar with stream:

double[][] positions= positionsArrayList.stream()
            .map(coords -> new double[]{coords.x, coords.y})
            .toArray(double[][]::new);

answered Mar 18, 2019 at 14:35

Ruslan

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