Question

我正在制作任务计划员，其中一人在JTextArea中输入当天的作业。我使JTextAreas静态，所以我可以在课外访问它们。为了清理我的代码，我决定使用for循环并将JTextAreas放入静态实例变量数组中。但是，当我尝试通过数组访问JTextArea时，我得到一个NullPointerException。以下是JTextArea声明的示例：

static JTextArea MondayEng;

数组（每个主题一个）声明如下：

static JTextArea[] Eng =  new JTextArea[]{
    MondayEng,
    TuesdayEng,
    WednesdayEng,
    ThursdayEng,
    FridayEng
};

然后有一个JTextArea[][]包含数组：

static JTextArea[][] subjs = new JTextArea[][] {Eng, Hist, Math, Sci, Lang};

JTextAreas在JFrame（Called PWin）中实例化，如下所示：

MondayEng = new JTextArea();
MondayEng.setBounds(664, 68, 153, 100);
contentPane.add(MondayEng);

我会尝试像这样获取JTextAreas的内容：

for (int i = 0; i < PWin.subjs.length; i++) { // Iterate through subjects
        String sub = subjects[i]; // Store the current subject in a variable
        for (int y = 0; y < days.length; y++) { // Iterate through days in week             
            String day = days[y]; // Store the current day in a variable                
            String str = PWin.subjs[i][y].getText(); // Get the text of the JTextArea currently selected, plop it in a string

保存文件时出现此异常：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at PSave.<init>(PSave.java:19)
    at PWin$1.actionPerformed(PWin.java:262)
    at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2028)
    at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2351)
    at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
    at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:236)
    at java.awt.Component.processMouseEvent(Component.java:6373)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
    at java.awt.Component.processEvent(Component.java:6138)
    at java.awt.Container.processEvent(Container.java:2085)
    at java.awt.Component.dispatchEventImpl(Component.java:4735)
    at java.awt.Container.dispatchEventImpl(Container.java:2143)
    at java.awt.Component.dispatchEvent(Component.java:4565)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4621)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4282)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4212)
    at java.awt.Container.dispatchEventImpl(Container.java:2129)
    at java.awt.Window.dispatchEventImpl(Window.java:2478)
    at java.awt.Component.dispatchEvent(Component.java:4565)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:679)
    at java.awt.EventQueue.access$000(EventQueue.java:85)
    at java.awt.EventQueue$1.run(EventQueue.java:638)
    at java.awt.EventQueue$1.run(EventQueue.java:636)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
    at java.awt.EventQueue$2.run(EventQueue.java:652)
    at java.awt.EventQueue$2.run(EventQueue.java:650)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:649)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

任何帮助都会受到很多赞赏！

Answer 1

您的问题似乎是在创建数组时，尚未构建JTextAreas。 Java是按值传递的，因此当您传入稍后要构造的JTextAreas的名称时，它与传递null相同。

Answer 2

你需要替换它：

MondayEng = new JTextArea();
MondayEng.setBounds(664, 68, 153, 100);
contentPane.add(MondayEng);

使用：

Eng[0] = new JTextArea();
Eng[0].setBounds(664, 68, 153, 100);
contentPane.add(Eng[0]);

否则，数组Eng仍包含null，您只是创建一个新的JTextArea对象，并使MondayEng引用该对象而不是指向什么Eng[0]指向。

然而，更正确的答案是在将JTextArea传递给数组之前构建它们。类似的东西：

MondayEng = new JTextArea();
MondayEng.setBounds(664, 68, 153, 100);
contentPane.add(MondayEng);

// ...

static JTextArea[] Eng =  new JTextArea[]{
    MondayEng,
    TuesdayEng,
    WednesdayEng,
    ThursdayEng,
    FridayEng
};

Answer 3

由于JTextArea [] [] subjs是一个二维数组，因此需要注意正确生成数组。您创建的方式是单维数组。理想情况下，您可能需要创建类似

JTextArea[][] subs = new JTextArea[][]{ {Eng, Hist, Math, Sci, Lang} };

访问JTextAreas数组时出现NullPointerException

3 个答案: