Problem about canvas repaint!

Question

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.util.*;
import java.io.*;
class DrawPie extends Canvas{
Container cont;
public DrawPie(Container parent){
cont=parent;
public void addinContainer(){
this.setSize(200,200);
cont.add(this,BorderLayout.SOUTH);
public void paint(Graphics g){
g.drawString("This is test",20,20);
public class WelFrame extends JFrame {
JPanel contentPane;
BorderLayout borderLayout1 = new BorderLayout();
// Construct the frame
public WelFrame() {
enableEvents(AWTEvent.WINDOW_EVENT_MASK);
try {
jbInit();
catch (Exception e) {
e.printStackTrace();
// Component initialization
private void jbInit() throws Exception {
Button btn=new Button("Open");
contentPane = (JPanel) this.getContentPane();
contentPane.setLayout(borderLayout1);
btn.addActionListener(new java.awt.event.ActionListener() {
public void actionPerformed(ActionEvent e) {
btn_actionPerformed(e);
contentPane.add(btn,BorderLayout.NORTH);
this.setTitle("Welcome to JBuilder");
this.setSize(new Dimension(400, 300));
//Overridden so we can exit when window is closed
protected void processWindowEvent(WindowEvent e) {
super.processWindowEvent(e);
if (e.getID() == WindowEvent.WINDOW_CLOSING) {
System.exit(0);
//File | Exit action performed
public void jMenuFileExit_actionPerformed(ActionEvent e) {
System.exit(0);
void btn_actionPerformed(ActionEvent e) {
DrawPie dp=new DrawPie(contentPane);
dp.addinContainer();
public static void main(String[] args){
WelFrame wf=new WelFrame();
wf.show();
I can't get the text painted when click the button open.But when i resize the Frame,the text appeared.
how can i get the text painted when i click the button open.Thanks a lot.

Answer 1

When you add a component to another component, call it the parent component, AFTER YOU HAVE CONSTRUCTED THE PARENT COMPONENT, you need to call the method validate()on the parent component to insure that this component, the parent component, has a valid layout. In your code, you're adding your component which extends Canvas to the parent component which extends JFrame AFTER instanciating it. Hence, you need to call validate() on this parent right after adding the child component. In other words, your method btn_actionPerformed should look like the following:
void btn_actionPerformed(ActionEvent e) {
DrawPie dp=new DrawPie(contentPane);
dp.addinContainer();
validate(); //or this.validate();
calling validate() will insure re-arrenging all children correctly according to the parent layout. When you resize your form, validate is called automatically and that's why the child component shows up.
Without calling validate(), your newly added child component is not resized and located according to the parent layout. it's probably with the size 0,0 and that's why you're not seeing the text.
I hope I explained it clearly enough