JList没有出现在JScrollPane上

Question

我试图使用JFilechooser打开一个文本文件，并将字符串放在JList中。我认为所有的字符串都会出现在列表中，但我不知道为什么这些字符串没有出现在JScrollPane上。团体赛有什么问题吗？我不知道该换什么..。

import javax.swing.*;

import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.io.*;
import java.util.*;
import java.util.List; 

public class WordFinder extends JFrame implements ActionListener {

    private WordList words = new WordList();

    private JScrollPane scroll;
    private JLabel label;
    private JLabel word;
    private JTextField textArea;
    private JButton button;

    private JMenuBar menuBar;
    private JMenu menu;
    private JMenuItem menuItem, menuItem2;

    private JFileChooser fc;

    private JList list;


    static private final String newline = "\n";

    private int lineCount;



    public WordFinder() {
        super("Word Finder");

        fc = new JFileChooser();
        fc.setFileSelectionMode(JFileChooser.FILES_AND_DIRECTORIES);

        menuBar = new JMenuBar();
        menu = new JMenu("File");
        menuBar.add(menu);

        menuItem = new JMenuItem("Open...");
        menuItem.addActionListener(this);

        menuItem2 = new JMenuItem("Exit");
        menuItem2.addActionListener(this);

        menu.add(menuItem);
        menu.add(menuItem2);
        setJMenuBar(menuBar);

        label = new JLabel("Find: ");

        word = new JLabel(lineCount + " words total");

        textArea = new JTextField();
        textArea.setEditable(true);
        textArea.setPreferredSize(new Dimension(200, 20));


        button = new JButton("Clear");
        button.addActionListener(new ActionListener() {
                    @Override
                    public void actionPerformed(ActionEvent e) {
                        textArea.setText("");
                    }
                });

        scroll = makeListView();
        scroll.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
        scroll.setPreferredSize(new Dimension(200, 230));

        GroupLayout layout = new GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setAutoCreateGaps(true);
        layout.setAutoCreateContainerGaps(true);

        layout.setHorizontalGroup(layout.createSequentialGroup()
                .addComponent(label)
                .addGroup(layout.createParallelGroup(GroupLayout.Alignment.LEADING)
                        .addComponent(textArea)
                        .addComponent(word)
                        .addComponent(scroll))
                .addGroup(layout.createParallelGroup(GroupLayout.Alignment.LEADING)
                        .addComponent(button)));

        layout.setVerticalGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(GroupLayout.Alignment.BASELINE)
                        .addComponent(label)
                        .addComponent(textArea)
                        .addComponent(button))
                .addGroup(layout.createParallelGroup(GroupLayout.Alignment.LEADING)
                        .addComponent(word))
                .addGroup(layout.createParallelGroup(GroupLayout.Alignment.LEADING))
                        .addComponent(scroll));

        setVisible(true);
        pack();
        // call System.exit() when user closes the window
        setDefaultCloseOperation(EXIT_ON_CLOSE);

    }


    private JScrollPane makeListView() {
    //      String[] labels = {"1", "2", "3"};
    //      list = new JList(labels);

        JScrollPane listScroller = new JScrollPane(list);
        return listScroller;
    }

    private void updateListView(DefaultListModel listModel) {
        list = new JList(listModel);
        scroll = new JScrollPane(list);
    }

    public void actionPerformed(ActionEvent e) {

        DefaultListModel listModel = new DefaultListModel();

        if (e.getSource() == menuItem) {
            int returnVal = fc.showOpenDialog(WordFinder.this);

            if (returnVal == JFileChooser.APPROVE_OPTION) {
                File file = fc.getSelectedFile();
                String fileName = file.getAbsolutePath();

                try {
                    FileReader files = new FileReader(fileName);
                    BufferedReader br = new BufferedReader(files);

                    String str;                 
                    while((str = br.readLine()) != null) {
                        listModel.addElement(str);
                        //System.out.println(str);
                        lineCount++;
                    }
                    System.out.println(lineCount);

                    updateListView(listModel);
                    br.close();
                } catch (Exception ex) {
                    ex.printStackTrace(System.out);
                    System.out.println("can't read file");
                }

                System.out.println("Opening: " + file.getName() + newline);
            }
        } else if (e.getSource() == menuItem2) {
            setVisible(false);
            dispose();
        }
    }


    /**
     * Main method.  Makes and displays a WordFinder window.
     * @param args Command-line arguments.  Ignored.
     */
    public static void main(String[] args) {
        // In general, Swing objects should only be accessed from
        // the event-handling thread -- not from the main thread
        // or other threads you create yourself.  SwingUtilities.invokeLater()
        // is a standard idiom for switching to the event-handling thread.
        SwingUtilities.invokeLater(new Runnable() {
            public void run () {
                // Make and display the WordFinder window.
                new WordFinder();
            }
        });
    }    
}

Answer 1

当您调用makeListView时，JList是null，因为它还没有初始化yet...thus，您基本上是说scroll = new JScrollPane(null);...which并不特别有用.

接下来，当您调用updateListView时，您将创建JList和JScrollPane的一个新实例，并且对它们不做任何操作.

private void updateListView(DefaultListModel listModel) {
    list = new JList(listModel);
    scroll = new JScrollPane(list);
}

所以它们永远不会显示在屏幕上..。

为了纠正这个问题，你需要做一些修改.

1. 在创建JList之前，创建一个JScrollPane实例并将其分配给实例字段list
2. 不要创建list和scroll的新实例，只需使用JList#setModel

您可能还想看看Should I avoid the use of set(Preferred|Maximum|Minimum)Size methods in Java Swing?

使用JList，可以通过使用JList#setVisibleRowCount和JList#setPrototypeCellValue来影响JScrollPane的大小。