Java Swing：JScrollPane中列表行垂直均分空白问题求解

我是Java新手，昨天刚开始学习，请多包涵。目前我在把一组继承自JPanel的StudentRow添加到作为JScrollPane视口的JPanel中，遇到了问题：添加少量列表项时，JPanel的剩余空间会被各列表行垂直均分，效果如下：

• 添加2个项的效果：
  
• 添加3个项的效果：
  
  只有添加约10个项后，列表才会展开并支持滚动。我搜了好几个小时都没找到解决办法，求各位给点建议！

问题代码 #

StudentListView类 #

public class StudentListView extends JScrollPane {

    private  JPanel myList;
    private  EventManager events;
    private  List<StudentModel> students;
    private  StudentRow listRow;

    public StudentListView(List<StudentModel> students, EventManager events) {
        this.students = students;
        this.events = events;

        myList = new JPanel();
        myList.setLayout(new BoxLayout(myList, BoxLayout.Y_AXIS));

        for (int i = 0; i < this.students.size(); i++) {     
            listRow = new StudentRow(students.get(i), events, i);
            listRow.setAlignmentY(Component.TOP_ALIGNMENT);
            myList.add(listRow);
        }

        setViewportView(myList);
    }

    public void updateView(List<StudentModel> students) {
        myList.removeAll();

        this.students = students;
        for (int i = 0; i < this.students.size(); i++) { 
            listRow = new StudentRow(students.get(i), events, i);
            myList.add(listRow);
        }

        myList.revalidate();
        setViewportView(myList);
    }
}

StudentRow类 #

public class StudentRow extends JPanel {
    
    public int index;
    private JLabel nameLabel;
    private JLabel ageLabel;
    private JButton editButton;
    private JButton deleteButton;

    public StudentRow(StudentModel student, EventManager events, int index) {
        this.index = index;

        setPreferredSize(new Dimension(400, 30));

        nameLabel = new JLabel();
        nameLabel.setText(student.getName());
        ageLabel = new JLabel();
        ageLabel.setText(Integer.toString(student.getAge()));
        editButton = new JButton("Edit");
        deleteButton = new JButton("Delete");
        
        GridBagConstraints gbc = new GridBagConstraints();
        gbc.gridx = 0;
        gbc.gridy = 0;
        gbc.fill = GridBagConstraints.WEST;
        add(nameLabel, gbc);
        
        gbc.gridx = 1;
        gbc.gridy = 0;
        gbc.fill = GridBagConstraints.WEST;
        add(ageLabel, gbc);
        
        gbc.gridx = 2;
        gbc.gridy = 0;
        gbc.fill = GridBagConstraints.EAST;
        add(editButton, gbc);
        
        gbc.gridx = 3;
        gbc.gridy = 0;
        gbc.fill = GridBagConstraints.EAST;
        add(deleteButton, gbc);

        // Create component action listeners
        deleteButton.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                events.notify("removeStudent", student);
            }
        });
    }
}

解决方案 #

修改后的StudentListView代码如下：

public class StudentListView extends JScrollPane {

    private  JPanel myList;
    private  EventManager events;
    private  List<StudentModel> students;
    private  StudentRow listRow;
    private  JPanel BorderLayoutPanel;

    public StudentListView(List<StudentModel> students, EventManager events) {
        this.students = students;
        this.events = events;

        myList = new JPanel();
        myList.setLayout(new GridLayout(0,1));

        BorderLayoutPanel = new JPanel(new BorderLayout());
        BorderLayoutPanel.add(myList, BorderLayout.PAGE_START);

        for (int i = 0; i < this.students.size(); i++) {  
            listRow = new StudentRow(students.get(i), events, i);
            myList.add(listRow);
        }

        setViewportView(BorderLayoutPanel);
    }

    public void updateView(List<StudentModel> students) {
        myList.removeAll();

        this.students = students;
        for (int i = 0; i < this.students.size(); i++) { 
            listRow = new StudentRow(students.get(i), events, i);
            myList.add(listRow);
        }

        myList.revalidate();
        setViewportView(BorderLayoutPanel);
    }
}

为什么这样能解决问题 #

• 原来的BoxLayout.Y_AXIS布局，当视口高度大于所有行的总高度时，会自动拉伸每个行组件去填充剩余空间，导致行被均分高度。
• 改用GridLayout(0,1)：第一个参数设为0表示行数随内容自动调整，第二个参数1表示单列布局，这样每个StudentRow会保持自己设置的30px高度，不会被拉伸。
• 再把这个列表容器放到BorderLayout的PAGE_START位置，这个区域的组件只会占用自身所需的垂直空间，不会被强制填充整个视口。当内容总高度超过视口高度时，JScrollPane就会自动显示滚动条，不用等凑够10个项才触发滚动。

内容的提问来源于stack exchange，提问作者Nex