線程“ AWT-EventQueue-0”中的異常（Java）

Question

我正在嘗試構建一個簡單的聊天平台，但是當我嘗試從JTextField獲取文本以將其放置在JTextArea中時遇到了問題。

readTextBox.setRows(37);
readTextBox.setColumns(45);
JTextArea readTextBox = new JTextArea();
JTextField typeTextBox = new JTextField(30);
JButton send = new JButton("Send");
send.addActionListener(new sendMessageListener());

public class sendMessageListener implements ActionListener{

    @Override
    public void actionPerformed(ActionEvent e) {
        String message = typeTextBox.getText();
        System.out.println(message);
        readTextBox.setText(readTextBox.getText()+"\n"+message);
    }
}

這是我正在使用的代碼的基礎。 問題是，當我在JTextField上鍵入並按“發送”按鈕時，出現此錯誤：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at pt.iscte.pcd.QuequeApp.Janela$sendMessageListener.actionPerformed(Janela.java:97)
    at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
    at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
    at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
    at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
    at java.awt.Component.processMouseEvent(Unknown Source)
    at javax.swing.JComponent.processMouseEvent(Unknown Source)
    at java.awt.Component.processEvent(Unknown Source)
    at java.awt.Container.processEvent(Unknown Source)
    at java.awt.Component.dispatchEventImpl(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
    at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
    at java.awt.Container.dispatchEventImpl(Unknown Source)
    at java.awt.Window.dispatchEventImpl(Unknown Source)
    at java.awt.Component.dispatchEvent(Unknown Source)
    at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
    at java.awt.EventQueue.access$500(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.awt.EventQueue$3.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.awt.EventQueue$4.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

我知道錯誤是一個nullpointerexception，但我似乎找不到原因，我使用的那個Syso甚至沒有打印文本，所以我猜問題是從JTextField獲取文本，但是我一直在那兒鍵入，因此它不是null。

有人可以幫幫我嗎？

謝謝！

Answer 1

因為您已經定義了兩次變量，所以一次定義為實例變量，一次定義為局部變量。 擺脫掉或局部變量：

//JTextArea readTextBox = new JTextArea();
//JTextField typeTextBox = new JTextField(30);
readTextBox = new JTextArea();
typeTextBox = new JTextField(30);