Java AWT multiple screens don't show up

Question

I'm working on a task-planning AWT applet for my dev team, and I'm running into a problem.

I'm using a screen system, where the main class has a "current screen" variable that it uses to paint other screens. When the applet starts, it loads the "main screen" which has a "Chatroom" button. When you click the button, it should open the chatroom screen.

My problem is that it displays the main screen just fine, but when you click the button everything just goes blank and the chatroom does not show up at all. What am I doing wrong?

Each screen is a subclass of the Screen class, which is a subclass of Container.

Main Class:

public class TPApplet extends Applet
{
private static final long serialVersionUID = 7611084043153150559L;
private static final int WIDTH = 400;
private static final int HEIGHT = 350;
private static final String TITLE = "TaskPlanner v";
private static final double VERSION = 0.01;

private boolean setup = false;

public Screen currentScreen;

public void init()
{
    setLayout(null);

    setScreen(new MainScreen(this));
}

public void stop()
{
}

public void setScreen(Screen s)
{
    if (currentScreen != null)
    {
        currentScreen.destroy();
        remove(currentScreen);
    }

    currentScreen = s;

    if (currentScreen != null)
    {
        currentScreen.init();
        add(currentScreen);
    }
}

public void paint(Graphics g)
{
    if (!setup)
    {
        setSize(WIDTH, HEIGHT);
        setName(TITLE + VERSION);

        currentScreen.setLocation(0, 0);
        currentScreen.setSize(WIDTH, HEIGHT);

        setup = true;
    }

    if (currentScreen != null)
    {
        currentScreen.paint(g);
    }
}
}

Main Menu class:

public class MainScreen extends Screen
{
private static final long serialVersionUID = -993648854350389881L;

private TPApplet applet;

private Button todoButton;
private Button chatButton;

private boolean setup = false;

public MainScreen(TPApplet tpApplet)
{
    applet = tpApplet;
}

@Override
public void init()
{
    setLayout(null);

    todoButton = createButton("To-Do List");
    chatButton = createButton("Chatroom");
}

@Override
public void destroy()
{
    removeAll();
}

@Override
public void paint(Graphics g)
{
    if (!setup)
    {
        todoButton.setLocation(25, 50);
        todoButton.setSize(100, 40);

        chatButton.setLocation(135, 50);
        chatButton.setSize(100, 40);

        setup = true;
    }
}

@Override
public void actionPerformed(ActionEvent e)
{
    if (e.getSource() instanceof Button)
    {
        Button button = (Button) e.getSource();

        if (button.getLabel() == chatButton.getLabel())
        {
            applet.setScreen(new ChatScreen(applet));
        }
    }
}
}

Chatroom Class:

public class ChatScreen extends Screen
{
private static final long serialVersionUID = -8774060448361093669L;

private TPApplet applet;

private ScrollPane chatWindow;
private TextField textField;
private Button sendButton;

private boolean setup = false;

public ChatScreen(TPApplet tpApplet)
{
    applet = tpApplet;
}

@Override
public void init()
{
    setLayout(null);

    sendButton = createButton("Send");

    chatWindow = new ScrollPane();
    textField = new TextField();

    add(chatWindow);
    add(textField);
}

@Override
public void destroy()
{
    removeAll();
}

@Override
public void paint(Graphics g)
{
    if (!setup)
    {
        chatWindow.setLocation(20, 20);
        chatWindow.setSize(100, 100);

        textField.setLocation(150, 150);
        textField.setSize(60, 20);

        sendButton.setLocation(220, 150);
        sendButton.setSize(40, 20);

        setup = true;
    }
}

@Override
public void actionPerformed(ActionEvent e)
{
    if (e.getSource() instanceof Button)
    {
        Button button = (Button) e.getSource();

        if (button.getLabel() == sendButton.getLabel())
        {
            String text = textField.getText();
        }
    }
}
}

Thank you in advance for your help!

Answer 1

I suspect that seen as you've chosen to discard the use of layout managers, when you add a new screen, the screen is being added with a 0x0 size

public void setScreen(Screen s)
{

    //...//

    if (currentScreen != null)
    {
        currentScreen.init();
        // Look ma, I have no size...
        add(currentScreen);
    }
}

One of the jobs of a layout manger is to decide how any new components should be laid out.

Try setting the applet's layout manager to something like BorderLayout.

The next problem is that the child screens suffer from the same problem, so even though the screen will be sized (based on the needs of the layout manager), the screens themselves also have no layout manager, so the components you add to them have no size and it will appear that the screen hasn't been updated.

I'd also recommend that you take a look at Andrew's example of CardLayout

You could also check out A Visual Guide to Layout Managers and Using Layout Managers for more details...

Answer 2

You will need to invalidate() the applet in your setScreen method. The new screen component needs to be laid out again to compute the sizes of its children.

It's a shame this isn't done automatically when adding!

Also, consider doing this using a LayoutManager if possible. Would a CardLayout work for you?