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POinstance variable's effect is hidden in Java
 

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  1. POinstance variable's effect is hidden in Java
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    <p>I'm creating a simple face using GObject class which will use GRect and GOval. I want GRect and GOval to be anchored to the same coordinates for which I described the getWidth() and getHeight() as Instance Variable. I'm not shown any error when I do so, but there is no result on the canvas. I get result only when I describe the getWidth() and getHeight() as local variables. Why are the instance variables' effect not showing up ?</p> <pre><code>/* * This is section 2 problem 2 to draw a face using GRect amnd GOval. The face is centred in the canvas. * PS: The face is scalable w.r.t. canvas. */ package Section_2; import java.awt.Color; import java.awt.Graphics; import acm.graphics.GRect; import acm.program.GraphicsProgram; /* * creates a robot face which is centered in the canvas. */ public class RobotFace extends GraphicsProgram{ private static final long serialVersionUID = 7489531748053730220L; //canvas dimensions for centering int _canvasWidth = getWidth(); int _canvasHeight = getHeight(); public void run(){ removeAll(); //to make sure multiple instances of graphic are not drawn during resize as an effect of overriding Java paint method //draw objects createFace(); } //currently only createFace() is implemented /* * creates a rect which is centred in the canvas */ private void createFace() { //canvas dimensions for centering //int _canvasWidth = getWidth(); //int _canvasHeight = getHeight(); //make the face scalable int _faceWidth = _canvasWidth / 6; int _faceHeight = _canvasHeight / 4; //to center the face int _faceX = (_canvasWidth - _faceWidth)/2; int _faceY = (_canvasHeight - _faceHeight)/2; GRect _face = new GRect(_faceX , _faceY, _faceWidth, _faceHeight); _face.setFilled(true); _face.setColor(Color.BLUE); add(_face); } /* * (non-Javadoc) * @see java.awt.Container#paint(java.awt.Graphics) * to override Java inherited paint method to retain graphic after resizing */ public void paint(Graphics g) { this.run(); super.paint(g); } } </code></pre> <p>If you uncomment the local variables for getWidth() and getHeight(), you get the Rect , else no effect on the canvas.</p>
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    1. USAkash Kumar Sharma
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    1. COMost likely `getWidth()` and `getHeight()` return 0 when the instance variables are initialized, and when `createFace()` is called they have the actual value.
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      1. POinstance variable's effect is hidden in Java
      1. USMark Rotteveel
    2. COWhat happens if you set _canvasWidth and _canvasHeight to hardcoded values (instead of calling the getWidth()/getHeight() methods) ? Do you see the same problem?
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      1. POinstance variable's effect is hidden in Java
      1. USmatt freake
    3. COwith hardcoded values it works fine. The issue is with getWidth() and getHeight() only as noted by @mark-rotteveel
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      1. USAkash Kumar Sharma
 

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