我意识到之前已经问过这个问题，但我尝试过的解决方案都没有。我试图根据用户输入交换JPanel中的内容。 例如，我想删除两个按钮并添加几个JTextField。
无论我做什么，我都会得到一个空指针异常。我尝试在窗体上创建元素并简单地隐藏它（setVisible（false））并将其设置为在按钮单击时可见而不是创建它。那也返回了一个nullptr。
以下是该按钮的动作事件监听器：
importOnlineButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent event) {
mainContentPanel.remove(importOfflineButton);
mainContentPanel.remove(importOnlineButton);
JTextField test = new JTextField(1);
mainContentPanel.add(test);
//pack();
revalidate();
repaint();
}
});
这是我的完整代码：
package com.arcologydesigns.Views;
import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import java.awt.event.KeyEvent;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.net.URL;
import static java.awt.event.ActionEvent.*;
/**
* EducatorMainForm created by Borislav S. on 7/11/2015 @ 9:57 PM.
*/
public class EducatorMainForm extends JFrame {
private ImageIcon img;
private JPanel educatorMainPanel;
private JLabel userIdLabel;
private JButton importOnlineButton;
private JButton importOfflineButton;
private JPanel mainContentPanel;
private JPanel userIdPanel;
private JLabel helpText1;
private JLabel helpText2;
private String imgUrl;
private JPopupMenu pmenu;
private class MenuItemAction extends AbstractAction {
public MenuItemAction(String text, Icon icon,
Integer mnemonic) {
super(text);
putValue(SMALL_ICON, icon);
putValue(MNEMONIC_KEY, mnemonic);
}
@Override
public void actionPerformed(ActionEvent e) {
System.out.println(e.getActionCommand());
}
}
public EducatorMainForm(char userType) {
educatorMainPanel.setSize(300, 300);
//setLocationRelativeTo(null);
switch (userType) {
case 'S':
imgUrl = "images/educator_logo_student.jpg";
break;
case 'I':
imgUrl = "images/educator_logo_instructor.jpg";
break;
case 'G':
imgUrl = "images/educator_logo_guest.jpg";
break;
default:
imgUrl = "images/educator_logo_guest.jpg";
System.out.print("Invalid user type.");
break;
}
createAndShowGUI();
createPopupMenu();
createMenuBar();
importOnlineButton.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent event) {
mainContentPanel.remove(importOfflineButton);
mainContentPanel.remove(importOnlineButton);
JTextField test = new JTextField(1);
mainContentPanel.add(test);
pack();
revalidate();
repaint();
}
});
}
private void createAndShowGUI() {
// Set title and default close operation
setTitle("Educator Productivity Toolkit - EPT v0.1");
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// Set a background for JFrame
ClassLoader cldr = this.getClass().getClassLoader();
URL imageURL = cldr.getResource(imgUrl);
if (imageURL != null) {
img = new ImageIcon(imageURL);
}
setContentPane(new JLabel(img));
// Set some layout, say FlowLayout
setLayout(new FlowLayout());
// Set the background, black with 125 as alpha value. This is less transparent
if(educatorMainPanel != null) {
educatorMainPanel.setBackground(new Color(33, 71, 116, 165));
// Set some size to the panels
educatorMainPanel.setPreferredSize(new Dimension(1920, 1080));
//p2.setPreferredSize(new Dimension(250,150));
// Add the panels to the JFrame
add(educatorMainPanel);
}
// Set the size of the JFrame and make it visible
setExtendedState(JFrame.MAXIMIZED_BOTH); // set to full screen
setVisible(true);
} // end createAndShowGUI()
private void createPopupMenu() {
pmenu = new JPopupMenu();
JMenuItem maxMi = new JMenuItem("Maximize");
maxMi.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
if (getExtendedState() != JFrame.MAXIMIZED_BOTH) {
setExtendedState(JFrame.MAXIMIZED_BOTH);
}
}
});
pmenu.add(maxMi);
JMenuItem quitMi = new JMenuItem("Quit");
quitMi.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
System.exit(0);
}
});
/**
* Lambda expression alternative to above function:
* quitMi.addActionListener((e) -> { System.exit(0); });
* Only available in Java 8
* */
pmenu.add(quitMi);
addMouseListener(new MouseAdapter() {
@Override
public void mouseReleased(MouseEvent e) {
if (e.getButton() == MouseEvent.BUTTON3) {
pmenu.show(e.getComponent(), e.getX(), e.getY());
}
}
});
}
private void createMenuBar() {
Icon iconNew = UIManager.getIcon("FileChooser.newFolderIcon");
Icon iconSave = UIManager.getIcon("FileView.floppyDriveIcon");
Icon iconOpen = UIManager.getIcon("Tree.timeFactor");
Icon iconExit = UIManager.getIcon("InternalFrame.closeIcon");
Icon iconOptions = UIManager.getIcon("ToolBar.handleIcon");
JMenuBar menubar = new JMenuBar();
JMenu fileMenu = new JMenu("File");
fileMenu.setMnemonic(KeyEvent.VK_F);
JMenuItem newMi = new JMenuItem(new MenuItemAction("New", iconNew, KeyEvent.VK_N));
JMenuItem openMi = new JMenuItem(new MenuItemAction("Open", iconOpen, KeyEvent.VK_O));
JMenuItem saveMi = new JMenuItem(new MenuItemAction("Save", iconSave, KeyEvent.VK_S));
JMenuItem optionsMi = new JMenuItem(new MenuItemAction("Options", iconOptions, KeyEvent.VK_O));
JMenuItem exitMi = new JMenuItem("Exit", iconExit);
exitMi.setMnemonic(KeyEvent.VK_E);
exitMi.setToolTipText("Exit application");
exitMi.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_W, CTRL_MASK));
exitMi.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent event) {
System.exit(0);
}
});
fileMenu.add(newMi);
fileMenu.add(openMi);
fileMenu.add(saveMi);
fileMenu.addSeparator();
fileMenu.add(exitMi);
JMenu viewMenu = new JMenu("View");
viewMenu.setMnemonic(KeyEvent.VK_V);
JMenu toolsMenu = new JMenu("Tools");
toolsMenu.add(optionsMi);
optionsMi.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent event) {
OptionsForm o = new OptionsForm();
}
});
JMenu helpMenu = new JMenu("Help");
JCheckBoxMenuItem sbarMi = new JCheckBoxMenuItem("Show statubar");
sbarMi.setMnemonic(KeyEvent.VK_S);
sbarMi.setDisplayedMnemonicIndex(5);
sbarMi.setSelected(true);
sbarMi.addItemListener(new ItemListener() {
@Override
public void itemStateChanged(ItemEvent e) {
if (e.getStateChange() == ItemEvent.DESELECTED) {
helpText1.setVisible(false);
helpText2.setVisible(false);
} else {
helpText1.setVisible(true);
helpText2.setVisible(true);
}
}
});
viewMenu.add(sbarMi);
menubar.add(fileMenu);
menubar.add(viewMenu);
menubar.add(toolsMenu);
menubar.add(Box.createHorizontalGlue());
menubar.add(helpMenu);
setJMenuBar(menubar);
}
public void setUserIdLabel(String _userIdLabel) {
this.userIdLabel.setText(_userIdLabel);
}
public void setImageURL(String _imageURL) {
imgUrl = _imageURL;
}
} // end class EducatorMainForm
这是例外（空指针）：
Exception in thread "AWT-EventQueue-0" java.lang.ClassCastException: java.lang.String cannot be cast to com.intellij.uiDesigner.core.GridConstraints
at com.intellij.uiDesigner.core.GridLayoutManager.addLayoutComponent(GridLayoutManager.java:133)
at java.awt.Container.addImpl(Container.java:1120)
at java.awt.Container.add(Container.java:966)
at com.arcologydesigns.Views.EducatorMainForm$1.actionPerformed(EducatorMainForm.java:82)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
at java.awt.Component.processMouseEvent(Component.java:6505)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
at java.awt.Component.processEvent(Component.java:6270)
at java.awt.Container.processEvent(Container.java:2229)
at java.awt.Component.dispatchEventImpl(Component.java:4861)
at java.awt.Container.dispatchEventImpl(Container.java:2287)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
at java.awt.Container.dispatchEventImpl(Container.java:2273)
at java.awt.Window.dispatchEventImpl(Window.java:2719)
at java.awt.Component.dispatchEvent(Component.java:4687)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
at java.awt.EventQueue$4.run(EventQueue.java:708)
at java.awt.EventQueue$4.run(EventQueue.java:706)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)