Update a JFrame.

Question

Hi, I am new to java GUI. I created a JFrame containing 3 JPanels and a JLable, the JLabel contains a icon which is a graphic I generated. But as the program runs, the user can update some variables and the graph should change and some other labels in the Jpanels should change too, but I am not sure how can I update the JFrame after I change the value in the variables.
another quich Q is that I have a textfield, and whenever the user enters something in there and press the "entery" key, I should do some action, which listener should I use for the "enter" key? thanx

Answer 1

import java.awt.*;
import java.awt.event.*;
import java.awt.font.*;
import java.util.Random;
import javax.swing.*;
public class UpdateDemo {
  static String msg = "";
  static Random seed = new Random();
  static int
    topCount = 1,
    midCount = 1,
    lowCount = 1;
  public static void main(String[] args) {
    final JLabel label = new JLabel() {
      Font font = new Font("lucida sans regular", Font.PLAIN, 18);
      public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setFont(font);
        g2.setPaint(Color.blue);
        float width = getWidth();
        float height = getHeight();
        FontRenderContext frc = g2.getFontRenderContext();
        LineMetrics metrics = font.getLineMetrics(msg, frc);
        float msgWidth = (float)font.getStringBounds(msg, frc).getWidth();
        float descent = metrics.getDescent();
        g2.drawString(msg, (width - msgWidth)/2,
                           (height + metrics.getHeight())/2 - descent);
    label.setPreferredSize(new Dimension(300,30));
    label.setOpaque(true);
    label.setBackground(Color.white);
    final GridBagLayout gridbag = new GridBagLayout();
    final JPanel centerPanel = new JPanel(gridbag);
    final GridBagConstraints gbc = new GridBagConstraints();
    gbc.gridwidth = gbc.REMAINDER;
    Dimension panelSize = new Dimension(300,100);
    final JPanel
      topPanel = new JPanel(),
      midPanel = new JPanel(),
      lowPanel = new JPanel();
    final JPanel[] panels = {
      topPanel, midPanel, lowPanel
    final JLabel
      topLabel = new JLabel("topPanel color 1"),
      midLabel = new JLabel("midPanel color 1"),
      lowLabel = new JLabel("lowPanel color 1");
    JLabel[] labels = {
      topLabel, midLabel, lowLabel
    for(int i = 0; i < 3; i++) {
      panels.setPreferredSize(panelSize);
panels[i].setBackground(getPanelColor());
panels[i].add(labels[i]);
centerPanel.add(panels[i], gbc);
final JTextField tf = new JTextField(18);
tf.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
msg = tf.getText();
label.repaint();
final JButton
topButton = new JButton("top"),
midButton = new JButton("mid"),
lowButton = new JButton("low");
ActionListener l = new ActionListener() {
public void actionPerformed(ActionEvent e) {
JButton button = (JButton)e.getSource();
if(button == topButton) {
topPanel.setBackground(getPanelColor());
topLabel.setText("topPanel color " + ++topCount);
if(button == midButton) {
midPanel.setBackground(getPanelColor());
midLabel.setText("midPanel color " + ++midCount);
if(button == lowButton) {
lowPanel.setBackground(getPanelColor());
lowLabel.setText("lowPanel color " + ++lowCount);
centerPanel.repaint();
topButton.addActionListener(l);
midButton.addActionListener(l);
lowButton.addActionListener(l);
JPanel southPanel = new JPanel(gridbag);
gbc.weightx = 1.0;
gbc.insets = new Insets(5,0,5,0);
southPanel.add(tf, gbc);
gbc.gridwidth = 1;
southPanel.add(topButton, gbc);
gbc.gridwidth = gbc.RELATIVE;
southPanel.add(midButton, gbc);
gbc.gridwidth = gbc.REMAINDER;
southPanel.add(lowButton, gbc);
JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.getContentPane().add(label, "North");
f.getContentPane().add(centerPanel, "Center");
f.getContentPane().add(southPanel, "South");
f.pack();
f.setLocation(400,200);
f.setVisible(true);
private static Color getPanelColor() {
int r = seed.nextInt(256);
int g = seed.nextInt(256);
int b = seed.nextInt(256);
return new Color(r,g,b);