We can create custom view by extending View class. The most important part of custom view is its appearance. We can give any shape to a custom view by overriding onDraw(Canvas canvas) method. Canvas class provide us many method which help us to draw any time of shape. For draw anything via Canvas class we need the object of Paint class.

Paint paint = new Paint();
paint.setColor(Color.WHITE);
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeWidth(1);

Example of some of the methods of Canvas class
// to draw a rectangle

canvas.drawRect(startX, startY, width, height, paint);

where
startX: The left side of the rectangle to be drawn
startY: The top side of the rectangle to be drawn
width: The right side of the rectangle to be drawn
height: The bottom side of the rectangle to be drawn
paint: The paint used to draw the rect
// to draw a oval

canvas.drawOval (float left, float top, float right, float bottom, Paint paint)

// to draw a circle

canvas.drawCircle (float cx, float cy, float radius, Paint paint)

where
cx: The x-coordinate of the center of the cirle to be drawn
cy: The y-coordinate of the center of the cirle to be drawn
radius: The radius of the cirle to be drawn
paint: The paint used to draw the circle
// to draw a line

canvas.drawLine (float startX, float startY, float stopX, float stopY, Paint paint)

where
startX: The x-coordinate of the start point of the line
startY: The y-coordinate of the start point of the line
stopX: The x-coordinate of the end point of the line
stopY: The y-coordinate of the end point of the line
paint: The paint used to draw line

Example of drawing a Matrix in a custom view
Create a custom view with the name "DrawMatrix"

public class DrawMatrix extends View implements KeyInterface {

    private final int startX = 10, startY = 20, width = 330, height = 350;

    public int getRow() {
        return row;
    }

    public void setRow(int row) {
        this.row = row;
    }

    public int getColumn() {
        return column;
    }

    public void setColumn(int column) {
        this.column = column;
    }

    private int row = 3, column = 3;
    Paint paint = new Paint();

    public DrawMatrix(Context context, int row, int column) {
        super(context);
        // this.whichMatrix = whichMatrix;
        this.row = row;
        this.column = column;
    }


    private void drawRectangle(Canvas canvas) {
        canvas.drawRect(startX, startY, width, height, paint);
    }

    private void drawHorizontalLine(Canvas canvas, int no) {
        for (int i = 1; i < no; i++) {
            canvas.drawLine(startX, i * (height / no) + 10, width, i * (height / no) + 10, paint);
        }
    }

    private void drawVerticalLine(Canvas canvas, int no) {
        for (int i = 1; i < no; i++) {
            canvas.drawLine(i * (width / no) + 10, startY, i * (width / no) + 10, height, paint);
        }
    }

    @Override
    public void onDraw(Canvas canvas) {
        paint.setColor(Color.WHITE);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(1);
        drawRectangle(canvas);
        drawHorizontalLine(canvas, row);
        drawVerticalLine(canvas, column);

    }

}

Then use this custom view in your activity like this:-

 private DrawMatrix drawMatrix;
// initialize drawMatrix inside onCreate method
drawMatrix = new DrawMatrix(this, 3, 3); // creating 3*3 Matrix
// add drawMatrix in your view or the container
matrixContainer.addView(drawMatrix); // here matrixContainer is any layout that hold your custom view

To redraw again:-

drawMatrix.setRow(8);
drawMatrix.setColumn(5);
drawMatrix.invalidate();

The above code will redraw matrix of 8*5 order