MainActivity.java

PreviewView mCameraView;
SurfaceHolder holder;
SurfaceView surfaceView;
Canvas canvas;
Paint paint;
int cameraHeight, cameraWidth, xOffset, yOffset, boxWidth, boxHeight;

private ListenableFuture<ProcessCameraProvider> cameraProviderFuture;
private ExecutorService executor = Executors.newSingleThreadExecutor();

/**
 * Starting Camera 
 */
void startCamera(){
    mCameraView = findViewById(R.id.previewView);

    cameraProviderFuture = ProcessCameraProvider.getInstance(this);

    cameraProviderFuture.addListener(new Runnable() {
        @Override
        public void run() {
            try {
                ProcessCameraProvider cameraProvider = cameraProviderFuture.get();
                MainActivity.this.bindPreview(cameraProvider);
            } catch (ExecutionException | InterruptedException e) {
                // No errors need to be handled for this Future.
                // This should never be reached.
            }
        }
    }, ContextCompat.getMainExecutor(this));
}

/**
 * 
 * Binding to camera
 */
private void bindPreview(ProcessCameraProvider cameraProvider) {
    Preview preview = new Preview.Builder()
            .build();

    CameraSelector cameraSelector = new CameraSelector.Builder()
            .requireLensFacing(CameraSelector.LENS_FACING_BACK)
            .build();

    preview.setSurfaceProvider(mCameraView.createSurfaceProvider());
    Camera camera = cameraProvider.bindToLifecycle((LifecycleOwner)this, cameraSelector,preview);

}


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //Start Camera
    startCamera();

    //Create the bounding box 
    surfaceView = findViewById(R.id.overlay);
    surfaceView.setZOrderOnTop(true);
    holder = surfaceView.getHolder();
    holder.setFormat(PixelFormat.TRANSPARENT);
    holder.addCallback(this);

}

/**
 *
 * For drawing the rectangular box
 */
private void DrawFocusRect(int color) {
    DisplayMetrics displaymetrics = new DisplayMetrics();
    getWindowManager().getDefaultDisplay().getMetrics(displaymetrics);
    int height = mCameraView.getHeight();
    int width = mCameraView.getWidth();

    cameraHeight = height;
    cameraWidth = width;

    int left, right, top, bottom, diameter;

    diameter = width;
    if (height < width) {
        diameter = height;
    }

    int offset = (int) (0.05 * diameter);
    diameter -= offset;

    canvas = holder.lockCanvas();
    canvas.drawColor(0, PorterDuff.Mode.CLEAR);
    //border's properties
    paint = new Paint();
    paint.setStyle(Paint.Style.STROKE);
    paint.setColor(color);
    paint.setStrokeWidth(5);

    left = width / 2 - diameter / 3;
    top = height / 2 - diameter / 3;
    right = width / 2 + diameter / 3;
    bottom = height / 2 + diameter / 3;

    xOffset = left;
    yOffset = top;
    boxHeight = bottom - top;
    boxWidth = right - left;
    //Changing the value of x in diameter/x will change the size of the box ; inversely proportionate to x
    canvas.drawRect(left, top, right, bottom, paint);
    holder.unlockCanvasAndPost(canvas);
}

/**
 * Callback functions for the surface Holder
 */

@Override
public void surfaceCreated(SurfaceHolder holder) {

}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {

    DrawFocusRect(Color.parseColor("#b3dabb"));
}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {

}