使用'if'语句检查后出现空指针异常

Question

我收到一个非常恼人的错误，说我得到一个空指针异常但是有一个if语句在继续之前检查文本是否为null：

    public String[] getFileData() throws IOException
{
    String file_name = "C:/Users/Liloka/Source/textfiles/Lines.txt";

    try {
        ReadFile file = new ReadFile(file_name);
        aryLines = file.OpenFile();

        for(int i =0; i<aryLines.length; i++)
        {
            System.out.println(aryLines[i]);
        }
    }

    catch(IOException e)
    {   
        System.out.println(e.getMessage());
    }
    return aryLines;
}

public void actionPerformed(ActionEvent evt)
{
    if(evt.getSource() == enterBtn)
    {
        String Text = textToAdd.getText();
        if(!(Text.equals(null)))
        {
            RF.addNewElement(Text);
            System.out.println(Text);

            try
            {
                RF.writeToFile();
                getFileData();
            }
        catch(Exception e)
            {

            }
        }
        else    JOptionPane.showMessageDialog(null, "Please enter a word!");
    }

}

唯一一次它甚至认为'其他'是通过这个：

    if(Text.equals(null));

我也尝试过：

   if(Text != null));

这在过去对我有用但现在不行！ 其他课程是：

public String[] OpenFile() throws IOException
{
    FileReader fr = new FileReader(path);
    BufferedReader br = new BufferedReader(fr);

    int numberOfLines = readLines();
    textData = new String[numberOfLines];
    int i;
        for(i=0; i<numberOfLines; i++)
        {
                textData[i] = br.readLine();
        }

    br.close();
    return textData;
}

int readLines() throws IOException
{
    FileReader file_to_read = new FileReader(path);
    BufferedReader bf = new BufferedReader(file_to_read);

    String aLine;
    numberOfLines=0;

    while((aLine = bf.readLine()) != null)
    {
        numberOfLines++;
    }
    //numberOfLines++;
    bf.close();
    return numberOfLines;
}

public void addNewElement(String newElement)
{   
    String texticles = newElement;
    numberOfLines = numberOfLines++;
    textData[numberOfLines] = texticles;
    //numberOfLines++; //Increments numberOfLines for the next element to be added
}

public void writeToFile() throws IOException
{
    FileWriter fstream = new FileWriter(path);
    BufferedWriter outFile = new BufferedWriter(fstream);
    //numberOfLines++;

        outFile.write(textData[numberOfLines]);
        //outFile.write(",");

        outFile.write("\r\n");

    outFile.close();
}

再次感谢你！

错误：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at textfiles.JListExample.actionPerformed(JListExample.java:115)
at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)

当我键入或未键入内容并按下回车键时，我收到错误。

这是错误

if(Text != null)

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at textfiles.JListExample.actionPerformed(JListExample.java:115)
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$000(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.awt.EventQueue$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.awt.EventQueue$2.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$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)

Answer 1

 if(Text.equals(null));

每次Text为null时，上面都会抛出NullPointerException。 任何时候你使用“。” operator on null你得到一个NullPointerException。

如果你在if之后得到一个NPE （Text！= null） ，请发布堆栈跟踪。

Answer 2

我的猜测是textToAdd或RF为null。

如果textToAdd是一个JTextComponent （或子类，但我在这里猜测），那么它的getText()方法不能返回null。 因此，正如此测试所示， Text不能为null。

package test;

import javax.swing.JFrame;
import javax.swing.JTextField;

public class TextComponentTest extends JFrame {
    JTextField tf;

    public TextComponentTest() {
        super();
        tf = new JTextField();
        getContentPane().add(tf);
    }

    public static void main(String[] args) {
        TextComponentTest test = new TextComponentTest();
        test.setVisible(true);

        String s = test.tf.getText();
        System.out.println(">" + s + "<");
        System.out.println(">" + s.length() + "<");

        test.tf.setText(null);
        s = test.tf.getText();
        System.out.println(">" + s + "<");
        System.out.println(">" + s.length() + "<");
    }
}

输出是：

><
>0<
><
>0<