输出流空指针异常

Question

我想从用户获取字符串输入并将其存储在文件中，但出现错误说： 我不知道是什么问题，它不会弹出作为错误，直到我实际运行代码。我只是想从用户输入存储在由

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

主类代码的文件：

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); 
     } 
    } 

二等

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

更改你的帮手代码到下面来获取你要找的东西。我所做的更改是，

1. 删除要在构造函数中写入文件的代码。
2. 添加了写入文件的新方法。
3. 从actionPerformed调用新方法。

 

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 2

这是问题：

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

fn只有在调用actionPerformed至少一次后才会有非空值。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。你叫什么名字预计得到这一点？为什么在构造函数中写入文件？这似乎是一件很奇怪的事情。它不应该被写入以响应用户操作吗？

（顺便说一句，请不要在未指定字符集反正使用getBytes() - 和喜欢OutputStream围绕创建一个OutputStreamWriter，而不是执行文本到二进制转换自己，要么finally块关闭流，或。试用资源声明）。

Answer 3

由于fn为空，您得到了NullPointerException。 NullPointerException在Java编程中非常普遍，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，请参阅http://javapapers.com/core-java/nullpointerexception-null-bad-good-and-ugly/

您确实想从构造函数中编写文件吗？