当JList在最后一个remove元素后变空时，为什么带有JList的JScrollPane会消失？

Question

我有一个使用BorderLayout Jlists EAST，WEST，CENTER和SOUTH的JPanel。当我从SOUTH JList删除所有元素（现在使其为空）并重新绘制UI时...从手动调用repaint（），将焦点更改为另一个窗口然后返回，或者最小化/最大化窗口，等...... JList从视图中消失。

因此，我无法将显示JLists的元素拖放到SOUTH JList中，因为它没有显示......非常烦人。

我该如何解决这个问题？

代码：

Main.java

import java.awt.BorderLayout;
import java.awt.EventQueue;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

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

    private ExampleList westItems;          
    private ExampleList eastItems;
    private ExampleList southItems;         
    private ExampleList centerItems;

    public Main(){
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {                     
                JFrame frame = new JFrame();
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                JPanel contentPane = new JPanel(new BorderLayout());
                JPanel mainPane = new JPanel(new BorderLayout());
                frame.setContentPane(contentPane);

                westItems = new ExampleList();              
                eastItems = new ExampleList();
                southItems = new ExampleList();         
                centerItems = new ExampleList();

                mainPane.add(westItems, BorderLayout.WEST);
                mainPane.add(eastItems, BorderLayout.EAST);
                mainPane.add(southItems, BorderLayout.SOUTH);
                mainPane.add(centerItems, BorderLayout.CENTER);

                JButton addButton = new JButton("Add Items");
                addButton.addActionListener(new ActionListener() {
                    @Override
                    public void actionPerformed(ActionEvent evt) {
                        for(int i = 0; i < 5; i++){
                            westItems.addItem(new String("West "+i));
                            eastItems.addItem(new String("East "+i));
                            southItems.addItem(new String("South "+i));
                            centerItems.addItem(new String("Center "+i));
                        }
                    }
                });

                JButton removeFromSouthButton = new JButton("Remove South Item");
                removeFromSouthButton.addActionListener(new ActionListener() {
                    @Override
                    public void actionPerformed(ActionEvent evt) {
                        // Remove the last element
                        int count = southItems.getModel().getSize();
                        if( count > 0 )
                            southItems.removeItemAt(0);
                    }
                });             

                JPanel buttonsPanel = new JPanel();
                buttonsPanel.add(addButton);
                buttonsPanel.add(removeFromSouthButton);

                contentPane.add(mainPane, BorderLayout.CENTER);
                contentPane.add(buttonsPanel, BorderLayout.SOUTH);

                frame.setExtendedState(frame.getExtendedState()|JFrame.MAXIMIZED_BOTH);
                frame.setVisible(true);
                frame.repaint();                
            }
        });

    }
}

ExampleList.java

import javax.swing.BorderFactory;
import javax.swing.DefaultListModel;
import javax.swing.JList;
import javax.swing.ListSelectionModel;

@SuppressWarnings("rawtypes")
public class ExampleList extends JList {
    private static final long serialVersionUID = 1L;
    private DefaultListModel<String> model;

    @SuppressWarnings("unchecked")
    public ExampleList() {

        model = new DefaultListModel<String>();
        setModel(model);
        setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
        setVisibleRowCount(1);        

        setBorder(BorderFactory.createTitledBorder("Example List"));

        setFixedCellWidth(150);
        setFixedCellHeight(150);
    }

    public void removeAllElements(){
        model.removeAllElements();
    }

    public void removeItem(String item) {
        model.removeElement(item);

    }

    public void removeItemAt(int index) {
        model.remove(index);

    }

    public void addItem(String item) {
        model.addElement(item);

    }

// End Mouse Listener Stuff
}

Answer 1

由于没有人正确回答，我想出来了。如果您没有执行“setPreferredSize（new Dimension（width，height））”，那么一旦列表中没有其他项，它将只是width = 0和height = 0。如果您使用任何非零值调用setPreferredSize（），那么它将显示。

我做到了这一点并且有效：

southItems.setPreferredSize（new Dimension（150,150））;