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Java Example 2: Memory, Graphics, and Coordinate Systems

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Object Memory Layout

Rectangle Class

Consider the following class definition:

public class Rectangle
{
	String shapeType; 
	int x;
	int y;
	double width;
	double height;
}

Below is the structure (sequence and number of bytes for each data member) of object $t$ created using:

Rectangle t = new Rectangle("Table Top", 100, 100, 300, 200);
Data Member Type Size (B)in bytes Content
shapeType String 18 (2 bytes per character) Table Top
x int 4 100
y int 4 100
width double 8 300
height double 8 200

Polygon Class

Below is the structure (sequence and number of bytes for each data member) of object $p$ created using:

public class Polygon {
	private int xCenter;
	private int yCenter;
	private int radius;
	private int nSides;
	private int[] x;
	private int[] y;

    public Polygon() {
        this(0, 0, 100, 6);
    }

    public Polygon(int xc, int yc, int r, int n) {
        xCenter = xc;
        yCenter = yc;
        radius  = r;
        nSides = n;
        x = new int[n];
        y = new int[n];
    }
}

Below is the structure and content of object $p$ created as follows:

Polygon p = new Polygon();
Data member Type Size in bytes Content
xCenter int 4 0
yCenter int 4 0
radius int 4 100
nSides int 4 6
x int[] 24 [0,0,0,0,0,0]
y int[] 24 [0,0,0,0,0,0]

AWT/Swing Graphics Examples

Drawing Shapes with Border Spacing

Below is a paint method that draws a shape in the middle of the screen. It leaves 20 pixels from the borders and assumes that the title bar is 80 pixels high.

public void paint(Graphics g) {
    int ww = (int) this.getWidth();
    int wh = (int) this.getHeight();  	
    switch (command) {
        case "Problem 3": {
            g.drawRect(20, 40, ww - 40, wh - 60);
            g.drawLine(20, 10 + wh/2, ww/2, 40);
            g.drawLine(ww/2, 40, ww - 20, 10 + wh/2);
            g.drawLine(ww - 20, 10 + wh/2, ww/2, wh - 20);
            g.drawLine(ww/2, wh - 20, 20, 10 + wh/2);
            g.drawLine(ww/2, 40, ww/2, wh - 20);
            g.drawLine(20, 10 + wh/2, ww - 20, 10 + wh/2);
            break;
        }
        
        case "Problem 4": {
            g.drawRect(20, 40, ww - 40, wh - 60);
            g.drawLine(20, (int)((1/(double)8) * wh + 10), ww - 20, (int)((1/(double)8) * wh + 10));
            g.drawLine(20, (int)((2/(double)8) * wh + 10), ww - 20, (int)((2/(double)8) * wh + 10));
            g.drawLine(20, (int)((3/(double)8) * wh + 10), ww - 20, (int)((3/(double)8) * wh + 10));
            g.drawLine(20, (int)((4/(double)8) * wh + 10), ww - 20, (int)((4/(double)8) * wh + 10));
            g.drawLine(20, (int)((5/(double)8) * wh + 10), ww - 20, (int)((5/(double)8) * wh + 10));
            g.drawLine(20, (int)((6/(double)8) * wh + 10), ww - 20, (int)((6/(double)8) * wh + 10));
            g.drawLine(20, (int)((7/(double)8) * wh + 10), ww - 20, (int)((7/(double)8) * wh + 10));
            
            g.drawLine((int)((1/(double)8) * ww), 40, (int)((1/(double)8) * ww), wh - 20);
            g.drawLine((int)((3/(double)8) * ww), 40, (int)((3/(double)8) * ww), wh - 20);
            g.drawLine((int)((5/(double)8) * ww), 40, (int)((5/(double)8) * ww), wh - 20);
            g.drawLine((int)((7/(double)8) * ww), 40, (int)((7/(double)8) * ww), wh - 20);
            break;
        }
        
        case "Problem 5": {
            for (int r = 20; r < wh - 40 && r < ww; r += 40) {
                g.drawOval(ww/2 - r/2, wh/2 + 15 - r/2, r, r);
            }
            break;
        }
    }

    // current width of window
    int ww = (int) this.getWidth(); 
    // current height of window
    int wh = (int) this.getHeight();
}

Responsive Grid Drawing

Below is a paint method that draws a grid based on the current width and height of the visible frame. It leaves 20 pixels space around the border.

public void paint(Graphics g) {
    int ww = (int) this.getWidth();
    int wh = (int) this.getHeight();

    g.drawRect (20, 20, ww-40, wh-40);

    for (int i = 1; i <= 10; i++) {
        g.drawLine(20, 20 + i * (wh - 40) / 10, ww - 20, 20 + i * (wh - 40) / 10);
    }

    for (int j = 1; j <= 10; j++) {
        g.drawLine(20 + j * (ww - 40) / 10, 20, 20 + j * (ww - 40) / 10, wh - 20);
    }
}

Concentric Circles

Below is a paint method that draws concentric circles separated by 20 pixels based on the current width and height of the visible screen. It leaves 20 pixels on the borders of visible window and considers the relationship between width and height of window.

public void paint(Graphics g) {
    int ww = (int) this.getWidth();
    int wh = (int) this.getHeight();

    int diameter = Math.min(ww, wh) - 40;

    for (int r = diameter; r > 0; r -= 40) {
        g.drawOval((ww - r) / 2, (wh - r) / 2, r, r);
    }
}