java use size of JPanel to size components

Is it possible to use the size of a JPanel to set the size of components inside the JPanel? When I try to use getHeight() or getWidth() on the JPanel it always returns 0. I know that it gets it's size once the JFrame is packed, but how would one go about using the dimensions of the JPanel and applying it to a component inside it? Something like this

JPanel panel = new JPanel();
JLabel label = new JLabel();
label.setWidth(panel.getWidth());
panel.add(label);

EDIT: See sample code below. What should I do if I want my Jlabel to be as wide as my JPanel? Is it wrong to use boxlayout in this case?

public class Main extends JFrame{

public Main(){
    JPanel panel = new JPanel();
    panel.setLayout(new BoxLayout(panel, BoxLayout.PAGE_AXIS));
    panel.setBorder(BorderFactory.createLineBorder(Color.blue));

    JLabel label = new JLabel();
    label.setBorder(BorderFactory.createLineBorder(Color.red));
    label.setText("label1");
    label.setMinimumSize(panel.getPreferredSize());
    label.setPreferredSize(panel.getPreferredSize());
    panel.add(label);


    add(panel);
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    pack();
    setSize(500,500);
    setVisible(true);
}

public static void main(String[] arg开发者_如何学Pythons)
{
    new Main();
}

}

Is it possible to use the size of a JPanel to set the size of components inside the JPanel?

This is the job of the layout manager.

When I try to use getHeight() or getWidth() on the JPanel it always returns 0

When the layout manager is invoked the Container will have a valid size so the layout manager can do its job properly.

There is no reason for you to be playing with sizes. Leave it to the layout managers to do their jobs.

Update:

What should I do if I want my Jlabel to be as wide as my JPanel? Is it wrong to use boxlayout in this case?

The BoxLayout attempts to respect the minimum/maximum size of the component. In you case you should be able to do something like:

JLabel label new JLabel("some text");
label.setBorder(....);
Dimension d = label.getPreferredSize();
d.width = 32767;
label.setMaximumSize( d );

Or maybe a simpler approach is to start with a BorderLayout. You can add the label to the NORTH. Then create another panel and add it to the CENTER.

Of course you can, but be careful that the result depends on the layout manager of your JPanel and on the number of its child components.

Setting the width ignores resizing. LayoutManagers typically ask components for their preferred widths and heights, and expand / contract items to maintain a visually appealing position after the frame has been resized. So, your best option is to leverage the layout manager and report a "preferred width".

You can use setPreferredWidth(...); but, if your preferred width is to change over the run of the program (due to window size changes), you will need to listen to the panel in question and update your button's preferred width as the panel's preferred width changes (this assumes it changes, which might not be true).