//file: Calculator.javaimport java.awt.*;import java.awt.event.*;import javax.swing.*;public class Calculator extends JPanel implements ActionListener {  GridBagConstraints gbc = new GridBagConstraints(  );  JTextField theDisplay = new JTextField(  );  public Calculator(  ) {    gbc.weightx = 1.0;  gbc.weighty = 1.0;    gbc.fill = GridBagConstraints.BOTH;    ContainerListener listener = new ContainerAdapter(  ) {      public void componentAdded(ContainerEvent e) {        Component comp = e.getChild(  );        if (comp instanceof JButton)          ((JButton)comp).addActionListener(Calculator.this);      }    };    addContainerListener(listener);    gbc.gridwidth = 4;    addGB(this, theDisplay, 0, 0);    // make the top row    JPanel topRow = new JPanel(  );    topRow.addContainerListener(listener);    gbc.gridwidth = 1;    gbc.weightx = 1.0;    addGB(topRow, new JButton("C"), 0, 0);    gbc.weightx = 0.33;    addGB(topRow, new JButton("%"), 1, 0);    gbc.weightx = 1.0;    addGB(topRow, new JButton("+"), 2, 0 );    gbc.gridwidth = 4;    addGB(this, topRow, 0, 1);    gbc.weightx = 1.0;  gbc.gridwidth = 1;    // make the digits    for(int j=0; j<3; j++)        for(int i=0; i<3; i++)            addGB(this, new JButton("" + ((2-j)*3+i+1) ), i, j+2);    // -, x, and divide    addGB(this, new JButton("-"), 3, 2);    addGB(this, new JButton("x"), 3, 3);    addGB(this, new JButton("\u00F7"), 3, 4);    // make the bottom row    JPanel bottomRow = new JPanel(  );    bottomRow.addContainerListener(listener);    gbc.weightx = 1.0;    addGB(bottomRow, new JButton("0"), 0, 0);    gbc.weightx = 0.33;    addGB(bottomRow, new JButton("."), 1, 0);    gbc.weightx = 1.0;    addGB(bottomRow, new JButton("="), 2, 0);    gbc.gridwidth = 4;    addGB(this, bottomRow, 0, 5);  }  void addGB(Container cont, Component comp, int x, int y) {    if ((cont.getLayout(  ) instanceof GridBagLayout) == false)      cont.setLayout(new GridBagLayout(  ));    gbc.gridx = x; gbc.gridy = y;    cont.add(comp, gbc);  }  public void actionPerformed(ActionEvent e) {    if (e.getActionCommand(  ).equals("C"))      theDisplay.setText("");    else      theDisplay.setText(theDisplay.getText(  )                         + e.getActionCommand(  ));  }  public static void main(String[] args) {    JFrame f = new JFrame("Calculator");    f.addWindowListener(new WindowAdapter(  ) {      public void windowClosing(WindowEvent e) { System.exit(0); }    });    f.setSize(200, 250);    f.setLocation(200, 200);    f.setContentPane(new Calculator(  ));    f.setVisible(true);  }}