Question

In performance critical onDraw() method of a View, would it be better to store the instance in a variable and reuse it later or should i just refer my instance as this?

EX:

public class ScheduleListView extends ListView {

    private Paint paint = new Paint();

    public ScheduleListView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        paint.setColor(Color.GRAY);
        paint.setAlpha(100);
        paint.setColor(Color.parseColor("#47B3EA"));
        canvas.drawRect(this.getLeft(), 10, this.getRight(), 10, paint);
        super.onDraw(canvas);
        canvas.restore();
    }

}

the this is referred in this.getRight() and this.getLeft(), would i have any performance improvement if I do the following modification:

private ScheduleListView scheduleListView  = this;

... ...

@Override
protected void onDraw(Canvas canvas) {
    ....
    //refer to my instance using scheduleListView instead of this
    scheduleListView.getLeft() 
    ....
}

is this good practice? Will this have any performance gain? if not, what is a better way of doing this?

PS: I’m asking this because in jQuery(javascript), if I were to use an object multiple times, I would cache the object in a variable, don’t really know would this be helpful in Java or not….

EX:

var myDiv = $(".myDiv");
myDiv.html();//do stuff

Answer 1

No, that wouldn’t have any improvement.

The most improvement you could make from your code is to move the calls to your paint object to the View’s constructor:

public ScheduleListView(Context context, AttributeSet attrs) {
    super(context, attrs);
    paint.setColor(Color.GRAY);
    paint.setAlpha(100);
    paint.setColor(Color.parseColor("#47B3EA"));

}

@Override
protected void onDraw(Canvas canvas) {
    canvas.drawRect(this.getLeft(), 10, this.getRight(), 10, paint);
    super.onDraw(canvas);
    canvas.restore();
}