How returns XxxSize from JComponent(s) added to the JLabel

问题

how can I correctly returns XxxSize from JComponent(s) added to the JLabel

1st. figure >> lets LayoutManager works like as for JPanel, JLabel returns Size(0, 0)

2nd. figure >> added some PreferredSize to the JLabel

3rd. figure >> calculated PreferredSize from JComponent(s) added to the JLabel

4th. figure >> lets LayoutManager works changed JLabel to JPanel, now LayoutManager correctly calculated Dimension without using any XxxSize

notice sice there is used Nimbus L&F, same output is there for all accesible L&F

import java.awt.*;
import java.awt.event.*;
import java.util.LinkedList;
import java.util.Queue;
import javax.swing.*;

public class NimbusBorderPainterDemo extends JFrame {

    private static final long serialVersionUID = 1L;
    private JFrame frame = new JFrame();
    private JPanel fatherPanel = new JPanel(), titlePanel = new JPanel();
    private JLabel buttonPanel = new JLabel();


    //figure  ---> 4th. switch JLabel with JPanel
    //private JPanel buttonPanel = new JPanel();
    private Queue<Icon> iconQueue = new LinkedList<Icon>();

    public NimbusBorderPainterDemo() {
        iconQueue.add(UIManager.getIcon(\"OptionPane.errorIcon\"));
        iconQueue.add(UIManager.getIcon(\"OptionPane.informationIcon\"));
        iconQueue.add(UIManager.getIcon(\"OptionPane.warningIcon\"));
        JButton button0 = createButton();
        JButton button1 = createButton();
        JButton button2 = createButton();
        button2.addActionListener(new ActionListener() {

            public void actionPerformed(ActionEvent e) {
                System.exit(1);
            }
        });
        int gap = 5;
        buttonPanel.setLayout(new GridLayout(0, 3, gap, 0));
        buttonPanel.add(button0);
        buttonPanel.add(button1);
        buttonPanel.add(button2);

        // figure 1st. --->  without PreferredSize

        // figure 2nd. --->
        //buttonPanel.setPreferredSize(new Dimension(160, 30));

        // figure 3rd. --->
        /*Dimension dim = button0.getPreferredSize();
        int w = dim.width;
        int h = dim.height;
        w = (w + 5) * 3;
        h += 4;
        dim = new Dimension(w, h);
        buttonPanel.setPreferredSize(dim);*/

        titlePanel.setLayout(new BorderLayout());
        titlePanel.add(new JLabel(nextIcon()), BorderLayout.WEST);
        titlePanel.add(new JLabel(\"My Frame\"), BorderLayout.CENTER);
        titlePanel.setBorder(BorderFactory.createLineBorder(Color.GRAY));
        titlePanel.add(buttonPanel, BorderLayout.EAST);
        fatherPanel.setLayout(new BorderLayout());
        fatherPanel.add(titlePanel, BorderLayout.CENTER);
        frame.setUndecorated(true);
        frame.add(fatherPanel);
        frame.setLocation(50, 50);
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
        frame.setVisible(true);
    }

    private JButton createButton() {
        JButton button = new JButton();
        button.setBorderPainted(false);
        button.setBorder(null);
        button.setFocusable(false);
        button.setMargin(new Insets(0, 0, 0, 0));
        button.setContentAreaFilled(false);
        button.setIcon(nextIcon());
        //button.setRolloverIcon(nextIcon());
        //button.setPressedIcon(nextIcon());
        //button.setDisabledIcon(nextIcon());
        nextIcon();
        return button;
    }

    private Icon nextIcon() {
        Icon icon = iconQueue.peek();
        iconQueue.add(iconQueue.remove());
        return icon;
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(\"com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel\");
                } catch (Exception fail) {
                }
                UIManager.getLookAndFeelDefaults().put(\"nimbusFocus\", Color.RED);
                NimbusBorderPainterDemo nimbusBorderPainterDemo = new NimbusBorderPainterDemo();
            }
        });
    }
}

回答1:

The default preferred size calculation is to use the layout manager to determine the preferred size of a component. This means the layout manager iterates through all the child components to determine the preferred size of each. For a JPanel, which is meant to be used as a Container this calculation is used.

However, for other Swing components, the getPreferredSize() method is always overridden to provide a reasonable size for the given component.

In the case of a JLabel, the preferred size calculation takes into account the text and the icon used. Since you didn't provide either the preferred size is zero. Of course if you manually override this calculation by using the setPreferredSize() method then the component will have a preferred size.

So even though Swing allows you to add components to any component and use a layout manager to layout the child components, these child components are not used in the preferred size calculation.

This is not just a Nimbus issue.

来源：https://stackoverflow.com/questions/8575641/how-returns-xxxsize-from-jcomponents-added-to-the-jlabel