线程中的异常“AWT-EventQueue-0”？ [英] Exception in thread "AWT-EventQueue-0"?
问题描述
我正在使用java和javax.swing编写一个简单的计算器程序
I'm working on a simple calculator program using java and javax.swing
基本上，当你按一个按钮时，该程序应该获取函数该按钮（数字或操作）并将其显示在textArea中。计算器本身的整个逻辑并不重要。此外，还有一个清除菜单项，用于清除textArea中的所有文本。
Basically, when you press a button, the program is supposed to fetch the function of that button (number or operation) and display it in the textArea. The whole logic for the calculator itself doesn't matter. Also, there's a clear menu item that clears all the text in textArea.
但是，每按一次按钮，我都会得到以下内容错误：
However, every time a press a button, I get the following error:
线程AWT-EventQueue-0中的异常java.lang.NullPointerException
at calculator.CalculatorGUI.actionPerformed（CalculatorGUI） .java：106）
当我按清除菜单项时，我也遇到了类似的错误，当我想要更改文本区域中的文本时，似乎java不喜欢它。
I'm also getting a similar error when I press the clear menu item, it seems like java doesn't like it when I want to change the text in the text area.
这是我的代码:(第106行是actionPerfomed方法，我为方便起见标记了它。
Here's my code: (Line 106 is in the actionPerfomed method, I've labeld it for your convenience)
public class CalculatorGUI implements ActionListener
{
// The calculator for actually calculating!
// private RPNCalculator calculator;
// The main frame for the GUI
private JFrame frame;
// The menubar for the GUI
private JMenuBar menuBar;
// The textbox
private JTextArea textArea;
private JScrollPane scrollArea;
// Areas for numbers and operators
private JPanel numKeysPane;
private JPanel opKeysPane;
private String input;
final String[] numbers = { "1", "2", "3", "4", "5", "6", "7", "8", "9", "0" };
final String[] operations = { "+", "-", "*", "/" };
/**
* Constructor
*/
public CalculatorGUI() {
// Initialize the calculator
calculator = new RPNCalculator();
}
/**
* Initialize and display the calculator
*/
public void showCalculator() {
String buttonValue;
JButton button;
JMenu menu;
JMenuItem menuItem;
// Create the main GUI components
frame = new JFrame("RPN Calculator");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
numKeysPane = new JPanel(new GridLayout(4, 3));
opKeysPane = new JPanel(new GridLayout(2, 2));
initializeMenu();
initializeNumberPad();
initializeOps();
JTextArea textArea = new JTextArea();
textArea.setEditable(false);
textArea.setLineWrap(true);
textArea.setWrapStyleWord(true);
JScrollPane scrollArea = new JScrollPane(textArea);
// Create the components to go into the frame and
// stick them in a container named contents
frame.getContentPane().add(numKeysPane, BorderLayout.CENTER);
frame.getContentPane().add(scrollArea, BorderLayout.NORTH);
frame.getContentPane().add(opKeysPane, BorderLayout.LINE_END);
// Finish setting up the frame, and show it.
frame.pack();
frame.setVisible(true);
}
/*
* (non-Javadoc)
*
* @see java.awt.event.ActionListener#actionPerformed(java.awt.event.ActionEvent)
*/
public void actionPerformed(ActionEvent e) {
String s = (String)e.getActionCommand();
// calculator.performCommand(s);
textArea.append(s + " "); // <<--- THIS IS LINE 106
}
/**
* Initialize the number pad for the calculator
*/
private void initializeNumberPad() {
JButton button;
for (int i = 0; i < numbers.length; i++) {
button = new JButton(numbers[i]);
button.addActionListener(this);
numKeysPane.add(button);
}
}
private void initializeOps(){
JButton button;
for (int i = 0; i < operations.length; i++){
button = new JButton(operations[i]);
button.addActionListener(this);
opKeysPane.add(button);
}
}
/**
* Initialize the menu for the GUI
*/
private void initializeMenu() {
JMenu menu;
JMenuItem menuItem;
JMenuItem menuItem2;
// Create a menu with one item, Quit
menu = new JMenu("Calculator");
menuItem = new JMenuItem("Quit");
// When quit is selected, destroy the application
menuItem.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
// A trace message so you can see this
// is invoked.
System.err.println("Close window");
frame.setVisible(false);
frame.dispose();
}
});
menuItem2 = new JMenuItem("Clear");
menuItem2.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e){
textArea.setText("");
}
});
menu.add(menuItem2);
menu.add(menuItem);
// Create the menu bar
menuBar = new JMenuBar();
menuBar.add(menu);
frame.setJMenuBar(menuBar);
}
/**
* Helper method for displaying an error as a pop-up
* @param message The message to display
*/
private static void errorPopup(String message) {
JOptionPane.showMessageDialog(null, message);
}
}
任何帮助都将不胜感激！谢谢！
Any help would be greatly appreciated! Thanks!
推荐答案
您没有初始化 JTextArea 成员字段叫 textArea 。您正在影响声明中的成员字段。试试这个：
You don't ever initialize the JTextArea member field called textArea. You are shadowing the member field in your declaration. Try this:
textArea = new JTextArea();
textArea.setEditable(false);
textArea.setLineWrap(true);
textArea.setWrapStyleWord(true);
而不是
JTextArea textArea = new JTextArea();
textArea.setEditable(false);
textArea.setLineWrap(true);
textArea.setWrapStyleWord(true);
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