How to debug this java code ? What is the error in this code?

import javax.swing.*; // For JPanel, etc.
import java.awt.*;           // For Graphics, etc.
im开发者_开发知识库port java.awt.geom.*;      // For Ellipse2D, etc.
import java.awt.event.*; 


public class ShapeExample extends JPanel {
  private Rectangle2D.Double square =
    new Rectangle2D.Double(50, 50, 100, 100);
  private Rectangle2D.Double square1 =
    new Rectangle2D.Double(10, 10, 200, 200);

  public void paintComponent(Graphics g) {
    clear(g);
    Graphics2D g2d = (Graphics2D)g;
    g2d.fill(square);
    g2d.draw(square1);
  }
 g.add(b = new Button("Bonjour"), BorderLayout.NORTH); 
    b.setActionCommand("Good Morning"); 
    b.addActionListener(a); 
    g.add(b = new Button("Good Day"), BorderLayout.CENTER); 
    b.addActionListener(a); 
    g.add(b = new Button("Aurevoir"), BorderLayout.SOUTH); 
    b.setActionCommand("Exit"); 
    b.addActionListener(a); 
    g.pack(); 
    g.show(); 
} 
  // super.paintComponent clears offscreen pixmap,
  // since we're using double buffering by default.

  protected void clear(Graphics g) {
    super.paintComponent(g);
  }

  protected Rectangle2D.Double getsquare() {
    return(square);
  }

  public static void main(String[] args) {
    WindowUtilities.openInJFrame(new ShapeExample(), 100, 100);
    Button b; 

  }
}

How to debug this code?

The error I get is this

ShapeExample.java:19: <identifier> expected
 g.add(b = new Button("Bonjour"), BorderLayout.NORTH); 
      ^
ShapeExample.java:19: <identifier> expected
 g.add(b = new Button("Bonjour"), BorderLayout.NORTH); 
        ^
ShapeExample.java:19: ';' expected
 g.add(b = new Button("Bonjour"), BorderLayout.NORTH); 
          ^
ShapeExample.java:19: invalid method declaration; return type required
 g.add(b = new Button("Bonjour"), BorderLayout.NORTH); 
               ^
ShapeExample.java:19: illegal start of type
 g.add(b = new Button("Bonjour"), BorderLayout.NORTH); 
                      ^
ShapeExample.java:19: ';' expected
 g.add(b = new Button("Bonjour"), BorderLayout.NORTH); 
                                ^
ShapeExample.java:20: <identifier> expected
    b.setActionCommand("Good Morning"); 
                      ^
ShapeExample.java:20: illegal start of type
    b.setActionCommand("Good Morning"); 
                       ^
ShapeExample.java:21: <identifier> expected
    b.addActionListener(a); 
                       ^
ShapeExample.java:21: <identifier> expected
    b.addActionListener(a); 
                         ^
ShapeExample.java:22: <identifier> expected
    g.add(b = new Button("Good Day"), BorderLayout.CENTER); 
         ^
ShapeExample.java:22: <identifier> expected
    g.add(b = new Button("Good Day"), BorderLayout.CENTER); 
           ^
ShapeExample.java:22: ';' expected
    g.add(b = new Button("Good Day"), BorderLayout.CENTER); 
             ^
ShapeExample.java:22: invalid method declaration; return type required
    g.add(b = new Button("Good Day"), BorderLayout.CENTER); 
                  ^
ShapeExample.java:22: illegal start of type
    g.add(b = new Button("Good Day"), BorderLayout.CENTER); 
                         ^
ShapeExample.java:22: ';' expected
    g.add(b = new Button("Good Day"), BorderLayout.CENTER); 
                                    ^
ShapeExample.java:23: <identifier> expected
    b.addActionListener(a); 
                       ^
ShapeExample.java:23: <identifier> expected
    b.addActionListener(a); 
                         ^
ShapeExample.java:24: <identifier> expected
    g.add(b = new Button("Aurevoir"), BorderLayout.SOUTH); 
         ^
ShapeExample.java:24: <identifier> expected
    g.add(b = new Button("Aurevoir"), BorderLayout.SOUTH); 
           ^
ShapeExample.java:24: ';' expected
    g.add(b = new Button("Aurevoir"), BorderLayout.SOUTH); 
             ^
ShapeExample.java:24: invalid method declaration; return type required
    g.add(b = new Button("Aurevoir"), BorderLayout.SOUTH); 
                  ^
ShapeExample.java:24: illegal start of type
    g.add(b = new Button("Aurevoir"), BorderLayout.SOUTH); 
                         ^
ShapeExample.java:24: ';' expected
    g.add(b = new Button("Aurevoir"), BorderLayout.SOUTH); 
                                    ^
ShapeExample.java:25: <identifier> expected
    b.setActionCommand("Exit"); 
                      ^
ShapeExample.java:25: illegal start of type
    b.setActionCommand("Exit"); 
                       ^
ShapeExample.java:26: <identifier> expected
    b.addActionListener(a); 
                       ^
ShapeExample.java:26: <identifier> expected
    b.addActionListener(a); 
                         ^
ShapeExample.java:27: <identifier> expected
    g.pack(); 
          ^
ShapeExample.java:28: <identifier> expected
    g.show(); 
          ^
ShapeExample.java:33: class, interface, or enum expected
  protected void clear(Graphics g) {
            ^
ShapeExample.java:35: class, interface, or enum expected
  }
  ^
ShapeExample.java:39: class, interface, or enum expected
  }
  ^
ShapeExample.java:41: class, interface, or enum expected
  public static void main(String[] args) {
                ^
ShapeExample.java:43: class, interface, or enum expected
    Button b; 
    ^
ShapeExample.java:45: class, interface, or enum expected
  }

how to debug this?

You have code randomly strewn about inside your class. All non-declaration code within a class must be within a method of some sort. Do you mean that code to go inside the paintComponent method? if so, you've got an extraneous }

Change that method to:

public void paintComponent(Graphics g) {
    clear(g);
    Graphics2D g2d = (Graphics2D)g;
    g2d.fill(square);
    g2d.draw(square1);
    Button bonjourButton = new Button("Bonjour");
    g.add(bonjourButton, BorderLayout.NORTH); 
    b.setActionCommand("Good Morning"); 
    b.addActionListener(a);
    Button goodDayButton = new Button("Good Day"); 
    g.add(goodDayButton, BorderLayout.CENTER); 
    b.addActionListener(a); 
    Button aurevoirButton = new Button("Aurevoir");
    g.add(aurevoirButton, BorderLayout.SOUTH); 
    b.setActionCommand("Exit"); 
    b.addActionListener(a); 
    g.pack(); 
    g.show(); 
} 

And also note that I am using local Button instances instead here as well.

edit: and also you'll need to define the action listener a as there's nothing named a anywhere in the code, much less in the scope of this function.

The main mistake is here:

g.add(b = new Button("Aurevoir"), BorderLayout.SOUTH); 

either you should use:

 g.add(new Button("Aurevoir"), BorderLayout.SOUTH);

or confirming to your remaing code:

Button b = new Button("Aurevoir")
 g.add(b, BorderLayout.SOUTH); 

This will maybe also solve the other marked errors.