code for water billing system

import javax .swing.*;import java.awt.event.*;import java.awt.*;import javax.swing.border.*;
public class calcBills4 extends JFrame implements ActionListener
{
                       
            public static void main(String [] args)
               {
                new calcBills4();
               }
                 
            CardLayout cl = new CardLayout();  
            JButton calcButton = new JButton("Calculate");
            JButton clearButton = new JButton("Clear");
            JButton viewButton = new JButton("View");
            JButton quit = new JButton("Quit");
            JButton back = new JButton("Back");
            JButton save = new JButton("Save");
            JTextField nameField = new JTextField(20);
            JTextField mtField = new JTextField(10); 
            JTextField rdField = new JTextField(10); 
            JTextField prField = new JTextField(10);
            JTextField rtField = new JTextField("2",10);
            JLabel bill = new JLabel();  
            JTextArea ta=new JTextArea(18,25);
            JScrollPane scroll=new JScrollPane(ta);
            double name,rd,pr,rt,value;
            String mm;
           
             public calcBills4()
             {
          this.setLocation(300,250);
   
          JPanel myPanel=new JPanel();
          JPanel conPanel=new JPanel();
          JPanel viewPanel=new JPanel();                                      
          JPanel panel1 = new JPanel();         
          JPanel panel2 = new JPanel();                     
          JPanel panel3 = new JPanel();
          myPanel.setLayout(new BorderLayout());
          panel2.setLayout(new GridLayout(2,0));                                                
          panel3.setLayout(new GridBagLayout());
          panel3.setBackground(Color.lightGray);
          viewPanel.setLayout(new BorderLayout());    
                  
                  
          this.setContentPane(conPanel);
          conPanel.setLayout(cl);
         
          conPanel.add("opt1",myPanel);
          conPanel.add("opt2",viewPanel);                   
         
                                
          JLabel label1 = new JLabel("WATER BILLING SYSTEM");
          label1.setForeground(Color.blue);
          label1.setFont(new Font("algerian",Font.BOLD | Font.ITALIC, 18));  
        
          Border b1 =BorderFactory.createTitledBorder("PROVIDE DETAILS BELOW");
          panel3.setBorder(b1);
         
          addItem(panel3,new JLabel("Customer account"),0,0,1, 1, GridBagConstraints.EAST);                       
          addItem(panel3,nameField,1,0,1, 1, GridBagConstraints.WEST);                   
          addItem(panel3,new JLabel("Customer name"),0,1,1, 1, GridBagConstraints.EAST);                       
          addItem(panel3,mtField,1,1,1, 1, GridBagConstraints.WEST);                      
          addItem(panel3, new JLabel("Metre reading"),0,2,1, 1, GridBagConstraints.EAST);                     
          addItem(panel3,rdField,1,2,1, 1, GridBagConstraints.WEST);                       
          addItem(panel3, new JLabel("previous reading"),0,3,1, 1, GridBagConstraints.EAST);                      
          addItem(panel3,prField,1,3,1, 1, GridBagConstraints.WEST);
          addItem(panel3, new JLabel("Rate"),0,4,1, 1, GridBagConstraints.EAST);
          addItem(panel3,rtField,1,4,1, 1, GridBagConstraints.WEST);
          save.setEnabled(false);        
          panel1.add(label1);
                                             
         myPanel.add(panel1,BorderLayout.NORTH);
          myPanel.add(panel2,BorderLayout.SOUTH);
          myPanel.add(panel3,BorderLayout.CENTER);
         
          Box box = Box.createHorizontalBox();
          box.add(new JLabel("Customer Database"));
          box.add(Box.createHorizontalStrut(450)); 
          box.add(back);
          viewPanel.add(new JLabel("water Billing Sytem"),BorderLayout.NORTH);
          viewPanel.add(scroll,BorderLayout.CENTER);
          viewPanel.add(box,BorderLayout.SOUTH);
          ta.setEditable(false);
  
           Box box1 = Box.createHorizontalBox();   
           box1.add(calcButton);
           box1.add(Box.createHorizontalStrut(5));                       
           box1.add(clearButton);               
         //  box1.add(Box.createHorizontalStrut(250));                                   //  box1.add(viewButton);         //  box1.add(Box.createHorizontalStrut(5));               // box1.add(save);           box1.add(Box.createHorizontalStrut(5));
           box1.add(quit);        
          
           Box box2 = Box.createHorizontalBox();
           box2.add(bill);        
                      
          panel2.add(box1);
         panel2.add(box2);                                               
          calcButton.addActionListener(this);
          clearButton.addActionListener(this);
          viewButton.addActionListener(this);
          quit.addActionListener(this);
          back.addActionListener(this);
          save.addActionListener(this);
         
      this.setTitle("water billing system");             
      this.pack();
      this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      this.setVisible(true);
      this.setResizable(false);
                          
                 
   }                   


   public void actionPerformed(ActionEvent e){
      
        if (e.getSource() == calcButton){
              
                 mm=mtField.getText();
              

               try{
                  name = Double.parseDouble(nameField.getText());
               }
                   catch(NumberFormatException evt){
                     nameField.setText("Illegal Data Entered");
                     JOptionPane.showMessageDialog(this,"Enter a valid Account Number please!",
                     "Bill System",JOptionPane.WARNING_MESSAGE);
                     nameField.setText("");
                     nameField.requestFocus();
                     return; 
                  }             

               if((mm.length()==0)||(mm.startsWith(" ")==true)){                            
               JOptionPane.showMessageDialog(this,"Enter a valid Name please!",
               "Bill System",JOptionPane.WARNING_MESSAGE);          
               mtField.requestFocus();return;

               }    
            
              
                try{
               rd = Double.parseDouble(rdField.getText());
            } 
                catch(NumberFormatException evt)   {
                  rdField.setText("Illegal Data Entered");
                  JOptionPane.showMessageDialog(this,"Enter a valid value for metre reading !",
                  "Bill System",JOptionPane.WARNING_MESSAGE);
                  rdField.setText("");
                  rdField.requestFocus();
                  return;
               }

               try{
               pr = Double.parseDouble(prField.getText());
            } 
                catch(NumberFormatException evt)   {
                  prField.setText("Illegal Data Entered");
                  JOptionPane.showMessageDialog(this,"Enter a valid value for previous reding !",
                  "Bill System",JOptionPane.WARNING_MESSAGE);
                  prField.setText("");
                  prField.requestFocus();                                       
                  return;
               }

                 try{
                  rt = Double.parseDouble(rtField.getText());
               }
                     catch(NumberFormatException evt){
                     rtField.setText("Illegal Data Entered");
                     JOptionPane.showMessageDialog(this,"Enter a valid rate!",
                     "Bill System",JOptionPane.WARNING_MESSAGE);
                     rtField.setText("");
                     rtField.requestFocus();
                     return; 
                  }
                 
                  if((rd-pr)<0){
                  JOptionPane.showMessageDialog(this,"Current reading less than previous Re-enter readings!",
                 "Bill System",JOptionPane.ERROR_MESSAGE);
                  rdField.setText("");
                  prField.setText("");
                  } 
                  else {
                 
                  value = calc(rd,pr,rt);                                                                                 
                  JOptionPane.showMessageDialog(null,"BILL IS MK "+value+"\n\n THANK YOU");                
                  save.setEnabled(true);
                  }                           
               }
       
              
         else if (e.getSource() == clearButton){
                           nameField.setText("");
                           mtField.setText("");
                           rdField.setText("");
                           prField.setText("");
                           rtField.setText("");
                           bill.setText("");
                           save.setEnabled(false);                                                                        
                                      
               }
           
  
         else
               { 
               System.exit(0);              
               } 
              
                       
      } 
   /*    */                              //GridBag method                                        public static void addItem(JPanel p, JComponent c,
  int x, int y, int width, int height,
  int align)
    {
        GridBagConstraints gc = new GridBagConstraints();
        gc.gridx = x;
        gc.gridy = y;
        gc.gridwidth = width;
        gc.gridheight = height;
        gc.weightx = 100.0;
        gc.weighty = 100.0;
        gc.insets = new Insets(5, 5, 5, 5);
        gc.anchor = align;
        gc.fill = GridBagConstraints.NONE;
        p.add(c, gc);
    }
    public static double calc(double rd, double pr, double rt)
{

      double value;                               
      value = ((rd-pr)*rt);                                                                                   
      return value;

}  
   
            
}