程序有问题。如何纠正它？

-2

以下程序用于从文本文件中输入整数并绘制XY图形。我使用JFileChooser来选择文件。我正在尝试获取文件的完整路径。我为按钮创建了一个ActionListener方法。当我运行它，我得到以下信息：程序有问题。如何纠正它？

Exception in thread "main" java.lang.NullPointerException 
at java.io.File.<init>(Unknown Source) 
at pia.main(pia.java:34)

import java.awt.event.ActionEvent; 
import java.awt.event.*; 
import java.awt.event.ActionListener; 
import java.io.BufferedReader; 
import java.io.File; 
import java.io.FileReader; 
import java.io.IOException; 
import javax.swing.JButton; 
import javax.swing.JFileChooser; 
import javax.swing.JPanel; 
import org.jfree.chart.*; 
import org.jfree.chart.plot.*; 
import org.jfree.data.xy.*; 

public class Pio { 

    public static void main(String[] args) throws IOException { 
     JButton but = new JButton("Open file"); 
     String URL = null; 
     but.addActionListener(new ActionListerner()       

    { 

      public void actionPerformed(ActionEvent e) { 

      JFileChooser chooser = new JFileChooser(); 

      int ret = chooser.showDialog(null, "Open file"); 

      if (ret == JFileChooser.APPROVE_OPTION) { 

       File file = chooser.getSelectedFile(); 

      String URL = file.getPath(); 

      } 

    }); 

     File f = new File(URL); 
     FileReader inputF = new FileReader(f); 
     BufferedReader in = new BufferedReader(inputF); 

     int[] a = new int[100]; 
     int[] b = new int[100]; 
     String s = in.readLine(); 

     int i = 0; 

     while (s != null && i < 100) { // your arrays can only hold 1000 ints 
      String pair[] = s.split(" "); 

      a[i] = Integer.parseInt(pair[0]); 
      b[i] = Integer.parseInt(pair[1]); 
      i++; // don't forget to increment 

      s = in.readLine(); 
     } 

     in.close(); 
     System.out.println("the output of the file is " + f); 

     XYSeries data = new XYSeries("Line Graph"); 
     int n = a.length; 

     for (int j = 0; j < n; j++) { 
      data.add(a[j], b[j]); 
     } 

     XYDataset xY = new XYSeriesCollection(data); 
     JFreeChart chart = ChartFactory.createXYLineChart("line graph", "Cycles", "Temperature", xY, PlotOrientation.VERTICAL, true, true, true); 
     ChartFrame frame = new ChartFrame("Example", chart); 

     JPanel panel = new JPanel(); 
     panel.add(but); 
     frame.add(panel); 
     frame.setSize(1000, 500); 
     frame.setVisible(true); 
    } 
}

来源

2012-06-08 User1234

定义一个类字段，以便范围将横跨整个班级，是这样的：

private String filePath; 

// ... 

public void actionPerformed(ActionEvent e) 
{ 
    JFileChooser chooser=new JFileChooser(); 
    int ret = chooser.showDialog(null, "Open file"); 
    if(ret == JFileChooser.APPROVE_OPTION) 
    { 
     File file = chooser.getSelectedFile(); 
     filePath = file.getPath(); 
    } 
}

现在，你可以使用它像这样：

if(filePath != null) System.out.println("The file path: " + filePath); 
else System.out.println("The file path is not set yet!");

来源

2012-06-08 12:31:26

我已经编辑了代码，并且我预感到这个URL在** ActionListener **中没有被使用。 – User1234

您需要声明String URL的ActionListener的范围之内。

来源

2012-06-08 12:29:49 bluevector

我宣布**网址**作为** ActionListerner **之外的字符串。 – User1234

程序有问题。如何纠正它？

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