约束必须是一个字符串（或null）
我找不到拯救生命的错误。 错误是“约束必须是一个字符串（或null）”我不知道它为什么给我这个错误，我必须缺少一些简单的东西。 我尝试添加
例如：dataPane = new JPanel（new GridBagLayout（））; 我的所有面板都没有。
我正在尝试将面板（2）添加到扩展面板。 这是我的代码：
public class SearchFlight extends JPanel { //giving names to the components\\\ private JRadioButton oneWay; private JRadioButton roundTrip; private ButtonGroup buttonGroup; private JLabel fromDestdLabel; private JComboBox fromDestCb; private JLabel toDestLbl; private JComboBox toDestCb; private JLabel departLbl; private JComboBox departMonth; private JComboBox departDay; private JTextField departYear; private JLabel arriveLbl; private JComboBox arriveMonth; private JComboBox arriveDay; private JTextField arriveYear; private JLabel adultLbl; private JComboBox adultCb; private JLabel childLbl; private JComboBox childCb; private JLabel infantLbl; private JComboBox infantCb; private JButton searchBtn; private JButton canxBtn; private JPanel buttonPane; private JPanel dataPane; public SearchFlight(){ initcomp(); } public void initcomp(){ //initilizing all the componets\\ oneWay = new JRadioButton("One Way"); roundTrip = new JRadioButton("Round Trip"); buttonGroup = new ButtonGroup(); fromDestdLabel = new JLabel("From"); fromDestCb = new JComboBox(); toDestLbl = new JLabel("To"); toDestCb = new JComboBox(); departLbl = new JLabel("Depart"); departMonth = new JComboBox(); departDay = new JComboBox(); departYear = new JTextField(); arriveLbl = new JLabel("Arrive"); arriveMonth = new JComboBox(); arriveDay = new JComboBox(); arriveYear = new JTextField(); adultLbl = new JLabel("Adult"); adultCb = new JComboBox(); childLbl = new JLabel("Child"); childCb = new JComboBox(); infantLbl = new JLabel("infant"); infantCb = new JComboBox(); searchBtn = new JButton("Search"); canxBtn = new JButton("Cancel"); buttonPane = new JPanel(); dataPane = new JPanel(new GridBagLayout()); setLayout(new GridBagLayout()); GridBagConstraints c = new GridBagConstraints(); BorderLayout borderLayout = new BorderLayout(); c.fill = GridBagConstraints.HORIZONTAL; dataPane.setLayout(borderLayout); c.gridx = 1; c.gridy = 0; dataPane.add(oneWay, c); c.gridx = 2; c.gridy = 0; dataPane.add(roundTrip, c); c.gridx = 0; c.gridy = 1; dataPane.add(fromDestdLabel ,c); c.gridx = 1; c.gridy = 1; c.gridwidth = 2; dataPane.add(fromDestCb, c); c.gridx = 0; c.gridy = 2; dataPane.add(toDestLbl,c); c.gridx = 1; c.gridy = 2; dataPane.add(toDestCb, c); c.gridwidth = 1; c.gridx = 0; c.gridy = 3; dataPane.add(departLbl ,c); c.gridx = 1; c.gridy = 3; dataPane.add(departMonth,c); c.gridx = 2; c.gridy = 3; dataPane.add(departDay, c); c.gridx = 3; c.gridy = 3; dataPane.add(departYear, c); c.gridx = 0; c.gridy = 4; dataPane.add(arriveLbl, c); c.gridx = 1; c.gridy = 4; dataPane.add(arriveMonth, c); c.gridx = 2; c.gridy = 4; dataPane.add(arriveDay,c); c.gridx = 3; c.gridy = 4; dataPane.add(arriveYear,c); c.gridx = 0; c.gridy = 5; dataPane.add(adultLbl,c); c.gridx = 1; c.gridy = 5; dataPane.add(adultCb,c); c.gridx = 0; c.gridy = 6; dataPane.add(childLbl,c); c.gridx = 1; c.gridy = 6; dataPane.add(childCb,c); c.gridx = 0; c.gridy = 7; dataPane.add(infantLbl,c); c.gridx = 1; c.gridy = 7; dataPane.add(infantCb,c); buttonPane.add(searchBtn); buttonPane.add(canxBtn); add(buttonPane, BorderLayout.SOUTH); add(dataPane); } } 你给dataPane一个BorderLayout，但是在向它添加组件时尝试使用GridBagConstraints–不允许，即使允许，也没有意义。
相反，您有两种选择之一：
- 将容器的布局保持为BorderLayout，但在向此容器添加组件时使用BorderLayout常量（如BorderLayout.EAST），或者
- 将dataPane的布局管理器更改为GridBagLayout，然后确保在添加组件时继续使用GridBagConstraints。
编辑
你在评论中说：
所以我使用dataPane = new JPanel（new GridBagLayout（））; 然后通过add（dataPane）添加它;
是的，可以使用GridBagLayout，但我不确定你的第二点是什么意思，重新add(dataPane)因为它与原始问题无关。