Check out example codes for "Write a simple java swing application that will display rectangle graphics as shown in the picture below:". It will help you in understanding the concepts better.

Code Example 1

public void paintComponent(Graphics g)

Code Example 2

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import javax.swing.JPanel;

/**
 * This panel displays 25 copies of a message.  The color and 
 * position of each message is selected at random.  The font
 * of each message is randomly chosen from among five possible
 * fonts.  The messages are displayed on a black background.
 * <p>Note:  The style of drawing used here is bad, because every
 * time the paintComponent() method is called, new random values are
 * used.  This means that a different picture will be drawn each
 * time.  This is particularly bad if only part of the panel
 * needs to be redrawn, since then the panel will contain
 * cut-off pieces of messages.
 * <p>This panel is meant to be used as the content pane in
 * either an applet or a frame.
 */
public class RandomStringsPanel extends JPanel {

   private String message;  // The message to be displayed.  This can be set in
                            // the constructor.  If no value is provided in the 
                            // constructor, then the string "Java!" is used.
   
   private Font font1, font2, font3, font4, font5;  // The five fonts.
   
   /**
    * Default constructor creates a panel that displays the message "Java!".
    *
    */
   public RandomStringsPanel() {
      this(null);  // Call the other constructor, with parameter null.
   }
   
   /**
    * Constructor creates a panel to display 25 copies of a specified message.
    * @param messageString The message to be displayed.  If this is null,
    * then the default message "Java!" is displayed.
    */
   public RandomStringsPanel(String messageString) {
   
      message = messageString;
      if (message == null)
          message = "Java!";
         
      font1 = new Font("Serif", Font.BOLD, 14);
      font2 = new Font("SansSerif", Font.BOLD + Font.ITALIC, 24);
      font3 = new Font("Monospaced", Font.PLAIN, 30);
      font4 = new Font("Dialog", Font.PLAIN, 36);
      font5 = new Font("Serif", Font.ITALIC, 48);
      
      setBackground(Color.BLACK);
      
   }
   
   /**
    * The paintComponent method is responsible for drawing the content of the panel.
    * It draws 25 copies of the message string, using a random color, font, and
    * position for each string.
    */
   public void paintComponent(Graphics g) {
   
      super.paintComponent(g);  // Call the paintComponent method from the 
                                // superclass, JPanel.  This simply fills the 
                                // entire panel with the background color, black.
      
      int width = getWidth();
      int height = getHeight();
     
      for (int i = 0; i < 25; i++) {

          // Draw one string.  First, set the font to be one of the five
          // available fonts, at random.  

          int fontNum = (int)(5*Math.random()) + 1;
          switch (fontNum) {
             case 1:
                g.setFont(font1);
                break;
             case 2:
                g.setFont(font2);
                break;
             case 3:
                g.setFont(font3);
                break;
             case 4:
                g.setFont(font4);
                break;
             case 5:
                g.setFont(font5);
                break;
           } // end switch

           // Set the color to a bright, saturated color, with random hue.

           float hue = (float)Math.random();
           g.setColor( Color.getHSBColor(hue, 1.0F, 1.0F) );

           // Select the position of the string, at random.

           int x,y;
           x = -50 + (int)(Math.random()*(width+40));
           y = (int)(Math.random()*(height+20));

           // Draw the message.

           g.drawString(message,x,y);

      } // end for
  
   } // end paintComponent()
   

}  // end class RandomStringsPanel

Code Example 3

Color myColor = new Color(r,g,b);

Code Example 4

public void paintComponent(Graphics g) {
   super.paintComponent(g);
   Graphics2D g2;
   g2 = (Graphics2D)g;
    .
    . // Draw on the component using g2.
    .
}

Code Example 5

public void paintComponent(Graphics g) {
   super.paintComponent(g);
   int width =  getWidth();   // Find out the width of this component.
   int height = getHeight();  // Find out its height.
   . . .   // Draw the content of the component.
}

Code Example 6

Font plainFont = new Font("Serif", Font.PLAIN, 12);
Font bigBoldFont = new Font("SansSerif", Font.BOLD, 24);

Code Example 7

public void repaint();

Code Example 8

// Graphics context's current color.
void setColor(Color c)
Color getColor()
 
// Graphics context's current font.
void setFont(Font f)
Font getFont()

// Set/Get the current clip area. Clip area shall be rectangular and no rendering is performed outside the clip area.
void setClip(int xTopLeft, int yTopLeft, int width, int height)
void setClip(Shape rect)
public abstract void clipRect(int x, int y, int width, int height) // intersects the current clip with the given rectangle
Rectangle getClipBounds()  // returns an Rectangle
Shape getClip()            // returns an object (typically Rectangle) implements Shape

Code Example 9

Color randomColor = Color.getHSBColor( (float)Math.random(), 1.0F, 1.0F );

Code Example 10

public void paintComponent(g) {
   super.paintComponent(g);
   . . . // Draw the content of the component.
}

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