Introducing Graphics - AWT Classes

Introducing Graphics

The AWT includes several methods that support graphics. All graphics are drawn relative to a window. This can be the main window of an applet, a child window of an applet, or a stand-alone application window. (These methods are also supported by Swing-based windows.)

The origin of each window is at the top-left corner and is 0,0. Coordinates are specified in pixels. All output to a window takes place through a graphics context.

A graphics context is encapsulated by the Graphics class. Here are two ways in which a graphics context can be obtained:

        It is passed to a method, such as paint( ) or update( ), as an argument.

        It is returned by the getGraphics( ) method of Component.

Among other things, the Graphics class defines a number of methods that draw various types of objects, such as lines, rectangles, and arcs. In several cases, objects can be drawn edge-only or filled. Objects are drawn and filled in the currently selected color, which is black by default. When a graphics object is drawn that exceeds the dimensions of the window, output is automatically clipped. A sampling of the drawing methods supported by Graphics is presented here.

Drawing Lines

Lines are drawn by means of the drawLine( ) method, shown here: void drawLine(int startX, int startY, int endX, int endY )

drawLine( ) displays a line in the current drawing color that begins at startX, startY and ends at endX, endY.

Drawing Rectangles

The drawRect( ) and fillRect( ) methods display an outlined and filled rectangle, respectively. They are shown here:

void drawRect(int left, int top, int width, int height) void fillRect(int left, int top, int width, int height)

The upper-left corner of the rectangle is at left, top. The dimensions of the rectangle are specified by width and height.

To draw a rounded rectangle, use drawRoundRect( ) or fillRoundRect( ), both shown here:

void drawRoundRect(int left, int top, int width, int height, int xDiam, int yDiam)

void fillRoundRect(int left, int top, int width, int height, int xDiam, int yDiam)

A rounded rectangle has rounded corners. The upper-left corner of the rectangle is at left, top. The dimensions of the rectangle are specified by width and height. The diameter of the rounding arc along the X axis is specified by xDiam. The diameter of the rounding arc along the Y axis is specified by yDiam.

Drawing Ellipses and Circles

To draw an ellipse, use drawOval( ). To fill an ellipse, use fillOval( ). These methods are shown here:

void drawOval(int left, int top, int width, int height) void fillOval(int left, int top, int width, int height)

The ellipse is drawn within a bounding rectangle whose upper-left corner is specified by left, top and whose width and height are specified by width and height. To draw a circle, specify a square as the bounding rectangle.

Drawing Arcs

Arcs can be drawn with drawArc( ) and fillArc( ), shown here:

void drawArc(int left, int top, int width, int height, int startAngle, int sweepAngle)

void fillArc(int left, int top, int width, int height, int startAngle, int sweepAngle)

The arc is bounded by the rectangle whose upper-left corner is specified by left, top and whose width and height are specified by width and height. The arc is drawn from startAngle through the angular distance specified by sweepAngle. Angles are specified in degrees. Zero degrees is on the horizontal, at the three o’clock position. The arc is drawn counterclockwise if sweepAngle is positive, and clockwise if sweepAngle is negative. Therefore, to draw an arc from twelve o’clock to six o’clock, the start angle would be 90 and the sweep angle 180.

Drawing Polygons

It is possible to draw arbitrarily shaped figures using drawPolygon( ) and fillPolygon( ), shown here:

void drawPolygon(int x[ ], int y[ ], int numPoints) void fillPolygon(int x[ ], int y[ ], int numPoints)

The polygon’s endpoints are specified by the coordinate pairs contained within the x and y arrays. The number of points defined by these arrays is specified by numPoints. There are alternative forms of these methods in which the polygon is specified by a Polygon object.

Demonstrating the Drawing Methods

The following program demonstrates the drawing methods just described.

// Draw graphics elements.

import java.awt.*;

import java.applet.*; /*

<applet code="GraphicsDemo" width=350 height=700> </applet>

*/

public class GraphicsDemo extends Applet {

public void paint(Graphics g) {

     Draw lines. g.drawLine(0, 0, 100, 90); g.drawLine(0, 90, 100, 10); g.drawLine(40, 25, 250, 80);

     Draw rectangles. g.drawRect(10, 150, 60, 50); g.fillRect(100, 150, 60, 50);

g.drawRoundRect(190, 150, 60, 50, 15, 15); g.fillRoundRect(280, 150, 60, 50, 30, 40);

     Draw Ellipses and Circles g.drawOval(10, 250, 50, 50); g.fillOval(90, 250, 75, 50); g.drawOval(190, 260, 100, 40);

     Draw Arcs

g.drawArc(10, 350, 70, 70, 0, 180); g.fillArc(60, 350, 70, 70, 0, 75);

// Draw a polygon

int xpoints[] = {10, 200, 10, 200, 10}; int ypoints[] = {450, 450, 650, 650, 450}; int num = 5;

g.drawPolygon(xpoints, ypoints, num);

}

}

Sample output is shown in Figure 25-2.

Sizing Graphics

Often, you will want to size a graphics object to fit the current size of the window in which it is drawn. To do so, first obtain the current dimensions of the window by calling getSize( ) on the window object. It returns the dimensions of the window encapsulated within a Dimension object. Once you have the current size of the window, you can scale your graphical output accordingly.

To demonstrate this technique, here is an applet that will start as a 200´200-pixel square and grow by 25 pixels in width and height with each mouse click until the applet gets larger than 500´500. At that point, the next click will return it to 200´200, and the process starts over.

Within the window, a rectangle is drawn around the inner border of the window; within that rectangle, an X is drawn so that it fills the window. This applet works in appletviewer, but it may not work in a browser window.

// Resizing output to fit the current size of a window.

import java.applet.*;

import java.awt.*;

import java.awt.event.*; /*

<applet code="ResizeMe" width=200 height=200> </applet>

*/

public class ResizeMe extends Applet { final int inc = 25;

int max = 500; int min = 200; Dimension d;

public ResizeMe() { addMouseListener(new MouseAdapter() {

public void mouseReleased(MouseEvent me) {

int w = (d.width + inc) > max?min :(d.width + inc);

int h = (d.height + inc) > max?min :(d.height + inc);

setSize(new Dimension(w, h));

}

});

}

public void paint(Graphics g) { d = getSize();

g.drawLine(0, 0, d.width-1, d.height-1); g.drawLine(0, d.height-1, d.width-1, 0); g.drawRect(0, 0, d.width-1, d.height-1);

}

}