Container Event handling with ContainerListener

Container Event handling with ContainerListener
java code

import java.applet.Applet;
import java.awt.*;
import java.awt.event.ContainerEvent;
import java.awt.event.ContainerListener;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.Vector;

public class ContainerEventDemo extends Applet 
			        implements ContainerListener,
				           ActionListener {
    TextArea textAreaObj;
    Panel panelForBtns;
    Button btnAdd, btnRemove, btnClear;
    Vector vectorOfBtns;
    static final String ADD = "add";
    static final String REMOVE = "remove";
    static final String CLEAR = "clear";
    String lineSeparator;

    public void init() {
	lineSeparator = System.getProperty("line.separator");

	//Initialize an empty list of buttons.
	vectorOfBtns = new Vector(10, 10);

	//Create all the components.
	btnAdd = new Button("Add a button");
	btnAdd.setActionCommand(ADD);
	btnAdd.addActionListener(this);

	btnRemove = new Button("Remove a button");
	btnRemove.setActionCommand(REMOVE);
	btnRemove.addActionListener(this);

	panelForBtns = new Panel();
	panelForBtns.addContainerListener(this);

	textAreaObj = new TextArea(5, 20);
	textAreaObj.setEditable(false);

	btnClear = new Button("Clear text area");
	btnClear.setActionCommand(CLEAR);
	btnClear.addActionListener(this);

	//Lay out the components.
	GridBagLayout gridbag = new GridBagLayout();
	GridBagConstraints gridBagConstraint = new GridBagConstraints();
	setLayout(gridbag);
	gridBagConstraint.fill = GridBagConstraints.BOTH; //Fill entire cell.

        gridBagConstraint.weighty = 1.0;  //Button area and message area have equal height.
	gridBagConstraint.gridwidth = GridBagConstraints.REMAINDER; //end of row
	gridbag.setConstraints(textAreaObj, gridBagConstraint);
	add(textAreaObj);

        gridBagConstraint.weighty = 0.0;  
	gridbag.setConstraints(btnClear, gridBagConstraint);
	add(btnClear);

        gridBagConstraint.weightx = 1.0;  //Add/remove buttons have equal width.
	gridBagConstraint.gridwidth = 1;  //NOT end of row
	gridbag.setConstraints(btnAdd, gridBagConstraint);
	add(btnAdd);

	gridBagConstraint.gridwidth = GridBagConstraints.REMAINDER; //end of row
	gridbag.setConstraints(btnRemove, gridBagConstraint);
	add(btnRemove);

        gridBagConstraint.weighty = 1.0;  //Button area and message area have equal height.
	gridbag.setConstraints(panelForBtns, gridBagConstraint);
	add(panelForBtns);
    }

    public void componentAdded(ContainerEvent e) {
	displayMessage(" added to ", e);
    }

    public void componentRemoved(ContainerEvent e) {
	displayMessage(" removed from ", e);
    }

    void displayMessage(String action, ContainerEvent e) {
	textAreaObj.append(((Button)e.getChild()).getLabel()
		       + " was"
		       + action
		       + e.getContainer().getClass().getName()
		       + lineSeparator);
    }

 
    public void actionPerformed(ActionEvent e) {
	String command = e.getActionCommand();

	if (command == ADD) {
	    Button newButton = new Button("Button: "
	                                  + (vectorOfBtns.size() + 1));
	    vectorOfBtns.addElement(newButton);
	    panelForBtns.add(newButton);
	    panelForBtns.validate();  
	} else if (command == REMOVE) {
	    int lastIndex = vectorOfBtns.size() - 1;
	    try {
	        Button nixedButton = (Button)vectorOfBtns.elementAt(lastIndex);
	        panelForBtns.remove(nixedButton);
		vectorOfBtns.removeElementAt(lastIndex);
	        panelForBtns.validate();  
	    } catch (ArrayIndexOutOfBoundsException exc) {
	    }
	} else if (command == CLEAR) {
	    textAreaObj.setText("");
	}
    }
}