通过向else-if逻辑添加6个以上的分支来编辑动作侦听器。每
分支会将actionCommand与6个子菜单项进行比较：Metal，Motif，
窗口，从不，始终，并根据需要。
一种。每个外观子菜单项都将使用try-catch语句进行设置
外观合适的人，如果显示此错误信息
没有完成。
b。每个“滚动条”子菜单项都将设置水平和垂直滚动
将政策限制为适当的值。
C。已经创建的所有组件都需要更新。这个
可以通过调用SwingUtilities.updateComponentTreeUI方法，将对要更新的组件的引用作为参数传递来完成。具体来说，您需要添加行
SwingUtilities.updateComponentTreeUIgetContentPane());
您刚刚添加到逻辑结构的每个分支。
我的代码是this ..而且它崩溃了，我完全陷入其中。任何帮助表示赞赏。
package src.javaapplication5;
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class NoteTaker extends JFrame {
//constants for set up of note taking area
public static final int WIDTH = 600;
public static final int HEIGHT = 300;
public static final int LINES = 13;
public static final int CHAR_PER_LINE = 45;
//objects in GUI
private JTextArea theText; //area to take notes
private JMenuBar mBar; //horizontal menu bar
private JPanel textPanel; //panel to hold scrolling text area
private JMenu notesMenu; //vertical menu with choices for notes
//****THESE ITEMS ARE NOT YET USED. YOU WILL BE CREATING THEM IN THIS LAB
private JMenu viewMenu; //vertical menu with choices for views
private JMenu lafMenu; //vertical menu with look and feel
private JMenu sbMenu; //vertical menu with scroll bar option
private JScrollPane scrolledText; //scroll bars
//default notes
private String note1 = "No Note 1.";
private String note2 = "No Note 2.";
/**
* constructor
*/
public NoteTaker() {
//create a closeable JFrame with a specific size
super("Note Taker");
setSize(WIDTH, HEIGHT);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//get contentPane and set layout of the window
Container contentPane = getContentPane();
contentPane.setLayout(new BorderLayout());
//creates the vertical menus
createNotes();
createViews();
//creates horizontal menu bar and
//adds vertical menus to it
mBar = new JMenuBar();
mBar.add(notesMenu);
mBar.add(viewMenu);
//****ADD THE viewMenu TO THE MENU BAR HERE
setJMenuBar(mBar);
//creates a panel to take notes on
textPanel = new JPanel();
textPanel.setBackground(Color.blue);
JTextArea theText = new JTextArea(LINES, CHAR_PER_LINE);
theText.setBackground(Color.white);
JScrollPane scrolledText = new JScrollPane(theText);
//****CREATE A JScrollPane OBJECT HERE CALLED scrolledText
//****AND PASS IN theText, THEN
//****CHANGE THE LINE BELOW BY PASSING IN scrolledText
textPanel.add(scrolledText);
contentPane.add(textPanel, BorderLayout.CENTER);
}
/**
* creates vertical menu associated with Notes menu item on menu bar
*/
public void createNotes() {
notesMenu = new JMenu("Notes");
JMenuItem item;
item = new JMenuItem("Save Note 1");
item.addActionListener(new MenuListener());
notesMenu.add(item);
item = new JMenuItem("Save Note 2");
item.addActionListener(new MenuListener());
notesMenu.add(item);
item = new JMenuItem("Open Note 1");
item.addActionListener(new MenuListener());
notesMenu.add(item);
item = new JMenuItem("Open Note 2");
item.addActionListener(new MenuListener());
notesMenu.add(item);
item = new JMenuItem("Clear");
item.addActionListener(new MenuListener());
notesMenu.add(item);
item = new JMenuItem("Exit");
item.addActionListener(new MenuListener());
notesMenu.add(item);
}
/**
* creates vertical menu associated with Views menu item on the menu bar
*/
public void createViews() {
viewMenu = new JMenu("Views");
viewMenu.setMnemonic(KeyEvent.VK_V);
createLookAndFeel();
createScrollBars();
lafMenu.addActionListener(new MenuListener());
sbMenu.addActionListener(new MenuListener());
viewMenu.add(lafMenu);
viewMenu.add(sbMenu);
}
/**
* creates the look and feel submenu
*/
public void createLookAndFeel() {
lafMenu = new JMenu("Look and Feel");
lafMenu.setMnemonic(KeyEvent.VK_L);
JMenuItem metalItem;
JMenuItem motifItem;
JMenuItem windowsItem;
metalItem = new JMenuItem("Metal");
metalItem.addActionListener(new MenuListener());
motifItem = new JMenuItem("Motif");
motifItem.addActionListener(new MenuListener());
windowsItem = new JMenuItem("Windows");
windowsItem.addActionListener(new MenuListener());
lafMenu.add(metalItem);
lafMenu.add(motifItem);
lafMenu.add(windowsItem);
}
/**
* creates the scroll bars submenu
*/
public void createScrollBars() {
sbMenu = new JMenu("Scroll Bars");
sbMenu.setMnemonic(KeyEvent.VK_S);
JMenuItem neverItem;
JMenuItem alwaysItem;
JMenuItem asneededItem;
neverItem = new JMenuItem("Never");
neverItem.addActionListener(new MenuListener());
alwaysItem = new JMenuItem("Always");
alwaysItem.addActionListener(new MenuListener());
asneededItem = new JMenuItem("As Needed");
asneededItem.addActionListener(new MenuListener());
sbMenu.add(neverItem);
sbMenu.add(alwaysItem);
sbMenu.add(asneededItem);
}
private class MenuListener implements ActionListener {
public void actionPerformed(ActionEvent e) {
String actionCommand = e.getActionCommand();
if (actionCommand.equals("Save Note 1")) {
note1 = theText.getText();
} else if (actionCommand.equals("Save Note 2")) {
note2 = theText.getText();
} else if (actionCommand.equals("Clear")) {
theText.setText("");
} else if (actionCommand.equals("Open Note 1")) {
theText.setText(note1);
} else if (actionCommand.equals("Open Note 2")) {
theText.setText(note2);
} else if (actionCommand.equals("Exit")) {
System.exit(0);
} else if (actionCommand.equals("Metal")) {
try {
UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
SwingUtilities.updateComponentTreeUI(getContentPane());
} catch (Exception ex) {
JOptionPane.showMessageDialog(null, "Error setting "
+ "the look and feel");
System.exit(0);
}
} else if (actionCommand.equals("Motif")) {
try {
UIManager.setLookAndFeel("com.sun.java.swing.plaf.motif.MotifLookAndFeel");
SwingUtilities.updateComponentTreeUI(getContentPane());
} catch (Exception ex) {
JOptionPane.showMessageDialog(null, "Error setting "
+ "the look and feel");
System.exit(0);
}
} else if (actionCommand.equals("Windows")) {
try {
UIManager.setLookAndFeel("com.sun.java.swing.plaf.windows.WindowsLookAndFeel");
SwingUtilities.updateComponentTreeUI(getContentPane());
} catch (Exception ex) {
JOptionPane.showMessageDialog(null, "Error setting "
+ "the look and feel");
System.exit(0);
}
} else if (actionCommand.equals("Never")) {
scrolledText.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);
SwingUtilities.updateComponentTreeUI(getContentPane());
scrolledText.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_NEVER);
SwingUtilities.updateComponentTreeUI(getContentPane());
} else if (actionCommand.equals("Always")) {
scrolledText.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
SwingUtilities.updateComponentTreeUI(getContentPane());
scrolledText.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_ALWAYS);
SwingUtilities.updateComponentTreeUI(getContentPane());
} else if (actionCommand.equals("As Needed")) {
scrolledText.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
SwingUtilities.updateComponentTreeUI(getContentPane());
scrolledText.setVerticalScrollBarPolicy(JScrollPane.VERTICAL_SCROLLBAR_AS_NEEDED);
SwingUtilities.updateComponentTreeUI(getContentPane());
} //****ADD 6 BRANCHES TO THE ELSE-IF STRUCTURE
//****TO ALLOW ACTION TO BE PERFORMED FOR EACH
//****MENU ITEM YOU HAVE CREATED
else {
theText.setText("Error in memo interface");
}
}
}
public static void main(String[] args) {
NoteTaker gui = new NoteTaker();
gui.setVisible(true);
}
}
最佳答案
您会收到一个异常，因为scrollText为null。
当创建scrollText时，它将在构造函数中创建为局部变量。
JScrollPane scrolledText = new JScrollPane(theText);
它也被声明为字段，但由于在构造函数中也被声明为局部变量，因此该字段永远不会初始化。
相反，只需执行以下操作：
scrolledText = new JScrollPane(theText);