Keep BoxLayout From Expanding Children

问题

I want to stack some JComponents vertically inside a JPanel so they stack at the top and any extra space is at the bottom. I'm using a BoxLayout. The components will each contain a JTextArea that should allow the text to wrap if necessary. So, basically, I want the height of each of these components to be the minimum necessary for displaying the (possibly wrapped) text.

Here's a contained code example of what I'm doing:

import javax.swing.*;
import java.awt.*;
public class TextAreaTester {
    public static void main(String[] args){
        new TextAreaTester();
    }
    public TextAreaTester(){
        JFrame frame = new JFrame();
        JPanel panel = new JPanel();
        panel.setLayout(new BoxLayout(panel,BoxLayout.PAGE_AXIS));
        panel.setPreferredSize(new Dimension(100,400));
        for(int i = 0; i<3; i++){
            JPanel item = new JPanel(new BorderLayout());
            JTextArea textarea = new JTextArea("this is a line of text I want to wrap if necessary");
            textarea.setWrapStyleWord(true);
            textarea.setLineWrap(true);
            textarea.setMaximumSize( textarea.getPreferredSize() );
            item.add(textarea,BorderLayout.NORTH);
            panel.add(item);
        }
        panel.add(Box.createGlue());
        frame.add(panel);
        frame.setVisible(true);
        frame.pack();  
    }
}

The child JPanels are expanding to fill the vertical space. I tried using glue because I thought that's what glue was for, but it seems to do nothing at all. Any help?

Note: I have found questions that look almost identical, but none with answers I can apply.

回答1:

One solution: nest JPanels with the outer JPanel using Borderlayout and adding the BoxLayout using JPanel to this one BorderLayout.NORTH, also known as BorderLayout.PAGE_START:

Edit for Kleopatra:

import javax.swing.*;
import java.awt.*;

public class TextAreaTester {
   public static void main(String[] args) {
      new TextAreaTester();
   }

   public TextAreaTester() {
      JFrame frame = new JFrame();
      JPanel panel = new JPanel();
      panel.setLayout(new BoxLayout(panel, BoxLayout.PAGE_AXIS));
      // panel.setPreferredSize(new Dimension(100,400));
      for (int i = 0; i < 3; i++) {
         JPanel item = new JPanel(new BorderLayout());
         // item.setLayout(new BoxLayout(item,BoxLayout.LINE_AXIS));
         JTextArea textarea = new JTextArea(
               "this is a line of text I want to wrap if necessary", 3, 35);
         textarea.setWrapStyleWord(true);
         textarea.setLineWrap(true);
         // textarea.setMaximumSize(textarea.getPreferredSize());
         // item.setMaximumSize( item.getPreferredSize() );
         item.add(new JScrollPane(textarea), BorderLayout.NORTH);
         panel.add(item);
      }
      panel.add(Box.createGlue());

      JPanel mainPanel = new JPanel(new BorderLayout()) {
         private final int prefW = 100;
         private final int prefH = 400;

         @Override
         public Dimension getPreferredSize() {
            return new Dimension(prefW, prefH);
         }
      };
      // mainPanel.setPreferredSize(new Dimension(100, 400));
      mainPanel.add(panel, BorderLayout.PAGE_START);

      frame.add(mainPanel);
      frame.setVisible(true);
      // frame.getContentPane().add(jp);
      frame.pack();
   }
}

回答2:

Alternatively, you can use Box.Filler. Just replace your call to panel.add(Box.createGlue()) with

panel.add(new Box.Filler(new Dimension(0, 0),
    new Dimension(0, Short.MAX_VALUE),
    new Dimension(0, Short.MAX_VALUE)));

If you want to achieve the same for a horizontal layout, just use Short.MAX_VALUE for width instead of height in the Dimension call.

来源：https://stackoverflow.com/questions/14010864/keep-boxlayout-from-expanding-children