https://teratail.com/questions/167295

DotPictView.java

class Dot {
    final Paint paint = new Paint(); {
        paint.setStyle(Paint.Style.FILL);
        paint.setColor(Color.WHITE);
    }
    final RectF rect = new RectF();
}

class DotPict {
    private static final int ROWS = 16;
    private static final int COLUMNS = 20;

    private final Dot[][] dots = new Dot[ROWS][COLUMNS]; {
        for (int row = 0; row < ROWS; ++row) {
            for (int column = 0; column < COLUMNS; ++column) {
                dots[row][column] = new Dot();
            }
        }
    }
    private final Paint framePaint = new Paint(); {
        framePaint.setStyle(Paint.Style.STROKE);
        framePaint.setStrokeWidth(2.0f);
        framePaint.setColor(Color.LTGRAY);
    }

    void setSize(int width, int height) {
        float xSpan = width / COLUMNS;
        float ySpan = height / ROWS;
        for (int row = 0; row < ROWS; ++row) {
            for (int column = 0; column < COLUMNS; ++column) {
                Dot dot = dots[row][column];
                dot.rect.set(
                        xSpan * column,
                        ySpan * row,
                        xSpan * (column + 1),
                        ySpan * (row + 1)
                );
            }
        }
    }

    void touch(PointF point) {
        for (Dot[] rowDots : dots) {
            for (Dot dot : rowDots) {
                if (dot.rect.contains(point.x, point.y)) {
                    dot.paint.setColor(Color.BLACK);
                }
            }
        }
    }

    void draw(Canvas canvas) {
        for (Dot[] rowDots : dots) {
            for (Dot dot : rowDots) {
                canvas.drawRect(dot.rect, dot.paint); // 矩形の描画
                canvas.drawRect(dot.rect, framePaint); // 枠線の描画
            }
        }
    }
}

public class DotPictView extends View {
    private final DotPict dotPict = new DotPict();

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        dotPict.setSize(w, h);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        dotPict.draw(canvas);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        dotPict.touch(new PointF(event.getX(), event.getY()));
        invalidate();
        return true;
    }
}