滚动条未出现在JList上

Question

我正在使用GridBagLayout，并且具有我的自定义类别的JList。 JList本身按预期显示，但没有出现滚动条。 （我的数据大小也大于JList的大小。）我检查了许多相关链接，所有链接都提到了这两行，但对我而言不起作用。

JScrollPane scrollPane = new JScrollPane(list); 

add(scrollPane);

完整代码：

import java.awt.Button;
import java.awt.Component;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Label;
import javax.swing.*;

public class UserPart extends JFrame {
    Label messageL = new Label("Confirm an item for more options.");
    Button confirmB = new Button("Confirm");
    Button backB = new Button("Back");
    DefaultListModel<User2> model;
    public UserPart() {

        JList<User2> list = new JList<User2>();
        model = new DefaultListModel<User2>();
        for (int i = 0; i < MainActivity2.arr.size(); i++) {
            User2 u = new User2();
            u.setForShow(i);
            model.addElement(u);
        }
        // SETTING THE APPEARANCE OF THE JLIST
        list.setModel(model);
        list.setCellRenderer(new MyListCellRenderer());
        list.setLayoutOrientation(JList.VERTICAL);
        list.setPreferredSize(new Dimension(500, 750));
        //////////////////////////////////////////////    
        GridBagLayout grid = new GridBagLayout();
        GridBagConstraints gbc = new GridBagConstraints();
        setLayout(grid);
        setTitle("List of users");
        GridBagLayout layout = new GridBagLayout();
        this.setLayout(layout);

        gbc.gridx = 0;
        gbc.gridy = 0;
        this.add(messageL, gbc);

        gbc.gridx = 0;
        gbc.gridy = 1;
        this.add(list, gbc); //ADDED JLIST TO LAYOUT HERE
        JScrollPane scrollPane = new JScrollPane(list); // DEFINED SCROLL BAR HERE
        add(scrollPane); // ADDED SCROLL BAR TO LAYOUT HERE
        setSize(1000, 1500);
        setPreferredSize(getSize());
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }
}

Answer 1

将约束权重（weightx，weighty）设置为大于零的值-要使用所有布局-还将fill设置为BOTH。不要将列表直接添加到框架中，只需添加滚动窗格with约束集即可，如上所述。请勿将AWT组件（Label）与Swing组件混合使用，而应使用JLabel。这里是总结：

JLabel label = new JLabel("Confirm an item for more options.");

JScrollPane scroll = new JScrollPane(list);

GridBagConstraints gbc = new GridBagConstraints();

JFrame frame = new JFrame();
frame.setLayout(new GridBagLayout());
gbc.gridx = 0;
gbc.gridy = 0;
frame.add(label, gbc);
gbc.gridx = 0;
gbc.gridy = 1;
gbc.weightx = 1;
gbc.weighty = 1;
gbc.fill = GridBagConstraints.BOTH;
frame.add(scroll, gbc);