为什么我所有的组件都聚集在JFrame的中心

Question

I'm using a grid bag layout to create a layout that looks like this:

but what I have is this:

Why is this happening? I've specified left alignment and to take up all horizontal space but i still end up with this. Here's my code:

public DepotView()
{
    setSize(FRAME_WIDTH,FRAME_HEIGHT);
    setLocationRelativeTo ( null );

    getContentPane ().setLayout ( new GridBagLayout());

    JPanel workerPanel = createTextAreaPanel("Processing: ",workerArea);
    GridBagConstraints c = new GridBagConstraints();
    c.gridx = 0;
    c.gridy = 0;
    c.gridwidth = 2;
    c.gridheight = 1;
    c.weightx = 1;
    c.fill = GridBagConstraints.HORIZONTAL;
    c.anchor = GridBagConstraints.FIRST_LINE_START;
    getContentPane().add ( workerPanel );


    JPanel customerPanel = createTextAreaPanel("Customers: ",textArea);
    c = new GridBagConstraints();
    c.gridx = 0;
    c.gridy = 1;
    c.gridwidth = 1;
    c.gridheight = 1;
    c.weightx = 0.5;
    c.insets = new Insets(5,5,5,5);
    getContentPane().add ( customerPanel );

    JPanel depotPanel = createTextAreaPanel("Depot: ",depotArea);
    c = new GridBagConstraints();
    c.gridx = 1;
    c.gridy = 1;
    c.gridwidth = 1;
    c.gridheight = 1;
    c.weightx = 0.5;
    c.insets = new Insets(5,5,5,5);
    getContentPane().add ( depotPanel );


    //pack();
    setDefaultCloseOperation ( JFrame.EXIT_ON_CLOSE );

Answer 1

The problem is that the size of your frame is smaller than the total preferred size of your content pane. From there, you get a swcrewed layout.

A few more things:

• use pack() instead of setSize() on your JFrame to get an appropriate frame size.
• avoid using gridx/gridy , they tend to make constraint complex and hard to maintain
• anchor/fill should almost always be combined with weightx and/or weighty bigger than 0
• instead of using the default FlowLayout of JPanel , use a LayoutManager which will take advantage of extra avilable space (for example BorderLayout )
• don't use static variables, it is just evil
• your textarea variables are always empty.

Here is a piece of code which seems to work quite well:

import java.awt.BorderLayout;
import java.awt.Font;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;

import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.SwingUtilities;

public class DepotView extends JFrame {

    private JTextArea textArea;
    private JTextArea depotArea;
    private JTextArea workerArea;

    public DepotView() {

        getContentPane().setLayout(new GridBagLayout());

        JPanel workerPanel = createTextAreaPanel("Processing: ", workerArea = new JTextArea());
        GridBagConstraints c = new GridBagConstraints();
        c.gridwidth = GridBagConstraints.REMAINDER;
        c.weightx = 1.0;
        c.weighty = 1.0;
        c.fill = GridBagConstraints.BOTH;
        c.anchor = GridBagConstraints.FIRST_LINE_START;
        getContentPane().add(workerPanel, c);

        JPanel customerPanel = createTextAreaPanel("Customers: ", textArea = new JTextArea());
        c = new GridBagConstraints();
        c.insets = new Insets(5, 5, 5, 5);
        c.fill = GridBagConstraints.BOTH;
        c.weightx = 1.0;
        c.weighty = 1.0;
        c.anchor = GridBagConstraints.FIRST_LINE_START;
        getContentPane().add(customerPanel, c);

        JPanel depotPanel = createTextAreaPanel("Depot: ", depotArea = new JTextArea());
        c = new GridBagConstraints();
        c.weightx = 1.0;
        c.weighty = 1.0;
        c.gridwidth = GridBagConstraints.REMAINDER;
        c.insets = new Insets(5, 5, 5, 5);
        c.fill = GridBagConstraints.BOTH;
        getContentPane().add(depotPanel, c);

        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        pack();
        setLocationRelativeTo(null);
    }

    private JPanel createTextAreaPanel(String label, JTextArea textArea) {
        JPanel customerQueuePanel = new JPanel(new BorderLayout());

        customerQueuePanel.add(new JLabel(label), BorderLayout.NORTH);
        textArea.setRows(15);
        textArea.setColumns(20);
        textArea.setFont(new Font(Font.MONOSPACED, Font.PLAIN, 14));
        textArea.setEditable(false);

        JScrollPane scrollPane = new JScrollPane(textArea);
        customerQueuePanel.add(scrollPane);
        return customerQueuePanel;
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new DepotView().setVisible(true);
            }
        });
    }

}

Answer 2

It looks like you just forgot to include the GridBadConstriants when adding the panels to the frame. Just change getContentPane().add(panel); to getContentPane().add(panel, c); and it should work.

Answer 3

The layout of the JPanel isn't defined properly. You're currently using JPanel 's default layout FlowLayout , which is a poor choice in your case.

• It doesn't resize the components to match the size of the container (the JPanel ).
• Depending on the size of the JPanel , it would place the text are next to the label.

The best choice would probably be to use BorderLayout .

Also set a minimum size for the JPanel :

JPanel customerQueuePanel = new JPanel(new BorderLayout());
customerQueuePanel.setMinimumSize(new Dimension(250, 150));
...
customerQueuePanel.add ( new JLabel(label), BorderLayout.NORTH);
customerQueuePanel.add( scrollPane, BorderLayout.CENTER); 

EDIT: The code above is an edit to your createTextArea method which you removed from your question.

Also add the constraints as a parameter, like you did before.

getContentPane().add(panel, c);