Fast way of iterate over trues in boolean array

I have a big array of booleans and i need just to work with the true elements, and need theyer index.

for (int x = 0; x < cells.length; x++) {
        for (int y = 0; y < cells[0].length; y++) {

                if (cells[x][y]) {
                    g.fillRect(x * cellSize, y * cellSize, cellSize, cellSize);
                }

        }
    }

so how can i do that without the two foor loops and save the time?