Question

我正在尝试从用户那里获取字符串输入并将其存储在一个文件中但是出现错误说：我不知道问题是什么，在我实际运行代码之前它不会弹出错误。我只想要存储在一个文件中的用户输入

Exception in thread "main" java.lang.NullPointerException
    at BingoHelper.<init>(BingoHelper.java:53)
    at BINGO.main(BINGO.java:8)

主类代码：

import javax.swing.*;
import java.io.*;

    public class BINGO {
        public static void main(String[] args) throws IOException{
            JLabel bg = new JLabel();
            //JButton b = new JButton("Click to enter name");
            BingoHelper EnterFaze = new BingoHelper();
            EnterFaze.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            EnterFaze.setSize(500,500);
            EnterFaze.setVisible(true);
            EnterFaze.setLocationRelativeTo(null);
            EnterFaze.setLayout(null);
            EnterFaze.add(bg);
        }
    }

第二课

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.io.*;

public class BingoHelper extends JFrame implements WindowListener, ActionListener{
    JTextField text = new JTextField();

JLabel bg = new JLabel();

private JButton b; {
        b = new JButton("Click to enter name");
        }

JPanel pnlButton = new JPanel();


public static String fn;
public static String sn;

public void actionPerformed (ActionEvent e) {
    //BingoHelper.fn;
    //BingoHelper.sn;
    fn = JOptionPane.showInputDialog("What is your first name?");
    sn = JOptionPane.showInputDialog("What is your second name(Optional)");
    //JOptionPane.showMessageDialog(null, "Welcome " + fn + " " + sn + ".", "", JOptionPane.INFORMATION_MESSAGE);
    text.setText("Welcome " + fn + " " + sn + ".");
    b.setVisible(false);
    text.setVisible(true);
    text.setBounds(140,0,220,20);
    text.setHorizontalAlignment(JLabel.CENTER);
    text.setEditable(false);
    text.setBackground(Color.YELLOW);
} 

public BingoHelper() throws IOException{
    super("BINGO");
    add(text);
    text.setVisible(false);
    add(b);
    this.add(pnlButton);
    pnlButton.setBackground(Color.BLUE);
    //pnlButton.add(b);+
    b.setVisible(true);
    b.setBounds(145,145,145,20);
    //b.setPreferredSize(new Dimension(150,40));
    b.addActionListener(this);
    b.setBackground(Color.GREEN);
    rootPane.setDefaultButton(b);
    File f = new File("test.txt");

    String nameToWrite = fn;
    OutputStream outStream = new FileOutputStream(f);
    outStream.write(nameToWrite.getBytes());
    outStream.close();
}

public void windowClosing(WindowEvent e) {
    dispose();
    System.exit(0);

}
public void windowOpened(WindowEvent e) {}
public void windowActivated(WindowEvent e) {}
public void windowIconified(WindowEvent e) {}
public void windowDeiconified(WindowEvent e) {}
public void windowDeactivated(WindowEvent e) {}
public void windowClosed(WindowEvent e) {}

}

Answer 1

这是问题所在：

String nameToWrite = fn;
...
outStream.write(nameToWrite.getBytes());

在fn被调用至少一次后，

actionPerformed将只有一个非空值。因此，当您调用构造函数时，fn为空，因此nameToWrite为空，因此nameToWrite.getBytes()会抛出NullPointerException。你期望得到什么名字？为什么在构造函数中写入文件？这似乎是一件非常奇怪的事情。它不应该是为了响应用户操作而编写的吗？

（另外，不要在未指定字符集的情况下使用getBytes() - 并且更喜欢在OutputStreamWriter周围创建OutputStream，而不是自己执行文本到二进制转换。并使用finally块或try-with-resources语句关闭流。）

Answer 2

将您的帮助代码更改为以下内容以获取您要查找的内容。我所做的改变是，

删除了在构造函数中写入文件的代码。
添加了一种新方法来写入文件。
从actionPerformed。


public class BingoHelper extends JFrame implements WindowListener, ActionListener {
    JTextField text = new JTextField();
    JLabel bg = new JLabel();
    private JButton b;
    {
        b = new JButton("Click to enter name");
    }
        JPanel pnlButton = new JPanel();
        public static String fn;
        public static String sn;

public void actionPerformed(ActionEvent e) {
    // BingoHelper.fn;
    // BingoHelper.sn;
    fn = JOptionPane.showInputDialog("What is your first name?");
    sn = JOptionPane.showInputDialog("What is your second name(Optional)");
    // JOptionPane.showMessageDialog(null, "Welcome " + fn + " " + sn + ".", "",
    // JOptionPane.INFORMATION_MESSAGE);
    text.setText("Welcome " + fn + " " + sn + ".");
    b.setVisible(false);
    text.setVisible(true);
    text.setBounds(140, 0, 220, 20);
    text.setHorizontalAlignment(JLabel.CENTER);
    text.setEditable(false);
    text.setBackground(Color.YELLOW);
    writeToFile();
}

private void writeToFile() {
    File f = new File("test.txt");

    String nameToWrite = fn;
    OutputStream outStream = null;
    try {
        outStream = new FileOutputStream(f);
        outStream.write(nameToWrite.getBytes());
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } finally {
        if (null != outStream) {
            try {
                outStream.close();
            } catch (IOException e) {
                // do nothing
            }
        }
    }
}

public BingoHelper() throws IOException {
    super("BINGO");
    add(text);
    text.setVisible(false);
    add(b);
    this.add(pnlButton);
    pnlButton.setBackground(Color.BLUE);
    // pnlButton.add(b);+
    b.setVisible(true);
    b.setBounds(145, 145, 145, 20);
    // b.setPreferredSize(new Dimension(150,40));
    b.addActionListener(this);
    b.setBackground(Color.GREEN);
    rootPane.setDefaultButton(b);
}

public void windowClosing(WindowEvent e) {
    dispose();
    System.exit(0);

}

public void windowOpened(WindowEvent e) {
}

public void windowActivated(WindowEvent e) {
}

public void windowIconified(WindowEvent e) {
}

public void windowDeiconified(WindowEvent e) {
}

public void windowDeactivated(WindowEvent e) {
}

public void windowClosed(WindowEvent e) {
}}

Answer 3

你有一个NullPointerException，因为fn为null。 NullPointerException在Java编程中非常常见，您应该查看引发异常的行号中的所有变量。为了更好地理解NullPointerException，请参考http://javapapers.com/core-java/nullpointerexception-null-bad-good-and-ugly/

你真的想从构造函数中编写文件吗？

输出流NullPointerException

3 个答案: