Android-Activity for drawing

CanvasActivity.java

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.Path;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.view.MotionEvent;
import android.view.View;
import android.widget.Toast;

public class CanvasActivity extends AppCompatActivity {
    PaintView paintView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        paintView = new PaintView(this);
        setContentView(paintView);

        mPaint = new Paint();
        mPaint.setAntiAlias(true);
        mPaint.setDither(true);
        mPaint.setColor(0xFF000000);
        mPaint.setStyle(Paint.Style.STROKE);
        mPaint.setStrokeJoin(Paint.Join.ROUND);
        mPaint.setStrokeCap(Paint.Cap.ROUND);
        mPaint.setStrokeWidth(80);
    }

    private Paint mPaint;

    public class PaintView extends View {
        private Bitmap mBitmap;
        private Canvas mCanvas;
        private Path mPath;
        private Paint mBitmapPaint;
        private float mX, mY;
        private static final float TOUCH_TOLERANCE = 4;

        public void removeLastStroke() {

        }

        public void clearCanvas() {
            mCanvas.drawColor(Color.WHITE);
            invalidate();
        }

        public PaintView(Context c) {
            super(c);
            mPath = new Path();
            mBitmapPaint = new Paint(Paint.DITHER_FLAG);
        }

        @Override
        protected void onSizeChanged(int w, int h, int oldw, int oldh) {
            super.onSizeChanged(w, h, oldw, oldh);
            mBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
            mCanvas = new Canvas(mBitmap);
            mCanvas.drawColor(Color.WHITE);
        }

        @Override
        protected void onDraw(Canvas canvas) {
            canvas.drawColor(Color.WHITE);

            canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);

            canvas.drawPath(mPath, mPaint);
        }

        private void touch_start(float x, float y) {
            mPath.reset();
            mPath.moveTo(x, y);
            mX = x;
            mY = y;
        }
        private void touch_move(float x, float y) {
            float dx = Math.abs(x - mX);
            float dy = Math.abs(y - mY);
            if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
                mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
                mX = x;
                mY = y;
            }
        }

        private void touch_up() {
            mPath.lineTo(mX, mY);
            // commit the path to our offscreen
            mCanvas.drawPath(mPath, mPaint);
            // kill this so we don't double draw
            mPath.reset();
            if(mX != 0 &&  mY != 0) {
                Toast.makeText(CanvasActivity.this, "(" + mX + ", " + mY + ")", Toast.LENGTH_SHORT).show();
            }
            mX = 0;
            mY = 0;
        }

        @Override
        public boolean onTouchEvent(MotionEvent event) {
            float x = event.getX();
            float y = event.getY();
            switch (event.getAction()) {
                case MotionEvent.ACTION_DOWN:
                    touch_start(x, y);
                    invalidate();
                    break;
                case MotionEvent.ACTION_MOVE:
                    touch_move(x, y);
                    invalidate();
                    break;
                case MotionEvent.ACTION_UP:
                    touch_up();
                    invalidate();
                    break;
            }
            return true;
        }
    }

    @Override
    public void onBackPressed() {
        //removeLastStroke();
        clearCanvas();
    }

    public void removeLastStroke() {
        paintView.removeLastStroke();
    }

    public void clearCanvas() {
        paintView.clearCanvas();
    }
}