Question

我正处于学校项目的早期阶段，并且已经遇到了一个错误。如果我只使用for循环导出数组列表内容而没有while循环一切正常，但是当我想使用while循环以便我可以提供选项时，输出文本文件不包含任何内容，就像程序一样除了创建一个空文本文件之外没有做任何事情。我很困惑。

import java.io.File;
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Scanner;
import java.io.PrintStream;
import javax.swing.JOptionPane;
import java.util.Arrays;

public class studentProcessor
{
   public static void main(String[] args) throws FileNotFoundException
   {  
      PrintStream pr=System.out;
      String inputFileName = JOptionPane.showInputDialog("Input file:");
      String outputFileName = JOptionPane.showInputDialog("Output file:");
      File inputFile = new File(inputFileName);
      Scanner in = new Scanner(inputFile);
      PrintWriter out = new PrintWriter(outputFileName);
      ArrayList<Student>student = new ArrayList<Student>();
      while (in.hasNextLine())
      {
          student.add(new Student(in.nextInt(), in.next(), in.nextDouble()));
      }
      in.close();
      boolean done=false;
      while (!done)
        {
            String m=JOptionPane.showInputDialog("Please enter the number for the action which you would like to perform." +
                "\n1. Add new student(s)\n2. Remove student(s)\n3. Update student's data\n4. Search for student" +
                "\n5. Sort students\n6. Create GPA report\n7. Output information to a .txt file\n0. Exit");
            if (m.equals("1"))
            {
                JOptionPane.showMessageDialog(null, "Add new student(s)");
            }
            else if (m.equals("2"))
            {
                JOptionPane.showMessageDialog(null, "Remove student(s)");
            }
            else if (m.equals("3"))
            {
                JOptionPane.showMessageDialog(null, "Update student's data");
            }
            else if (m.equals("4"))
            {
                JOptionPane.showMessageDialog(null, "Search for a student");
            }
            else if (m.equals("5"))
            {
                JOptionPane.showMessageDialog(null, "Sort students");
            }
            else if (m.equals("6"))
            {
                JOptionPane.showMessageDialog(null, "Create GPA report");
            }
            else if (m.equals("7"))
            {
                JOptionPane.showMessageDialog(null, "Output information to a .txt file");
                out.println("The students' information is listed below:");
                for (Student Stu:student)
                {
                    out.println(Stu.getId()+" "+Stu.getLName()+" "+Stu.getGpa());
                }
            }
            else if (m.equals("0"))
            {
                System.exit(0);
                done=true;
            }
            else 
            {
                JOptionPane.showMessageDialog(null, "You have not entered a valid command, try again.");
            }
        }
      out.close();
   }
}

这是我正在使用的学生班。

class Student
{ 
 private int id;
 private String lName;
 private double gpa;

 public Student (int studentId, String studentLName, double studentGpa)
   { 
     id=studentId;
     lName=studentLName;
     gpa=studentGpa;
   }

 public int getId()
   { return id; }

 public String getLName()
   { return lName; }

 public double getGpa()
   { return gpa; }

 public void setId(int id)
   { this.id=id; }

 public void setLName(String lName)
   { this.lName=lName; }

 public void setGpa(double gpa)
   { this.gpa=gpa; }

}

感谢您的帮助=]

编辑：这是现在与printwriter一起使用的代码的新部分。感谢所有的帮助！

else if (m.equals("7"))
        {
            String outputFileName = JOptionPane.showInputDialog("Designate an output file for student information:");
            PrintWriter out = new PrintWriter(outputFileName);
            out.println("The students are listed below:");
            for (Student Stu:student)
            {
                out.println(Stu.getId()+" "+Stu.getLName()+" "+Stu.getGpa());
            }
            out.close();
        }

Answer 1

问题是out.close的行永远不会被调用。

System.exit方法在关闭打印编写器之前终止应用程序。

您应该使用try finally块来关闭资源。

如果System.exit调用位于方法的末尾而不是while循环中，那么您的代码会更清晰。您已经使用布尔值来终止循环，所以让它终止，并继续使用其余的代码。

Answer 2

您是否在关闭PrintWriter之前尝试了flush（）？

Answer 3

您需要在写入后刷新输出流：

out.flush();

此外，您还需要在用户选择写入选项时创建输出流（7）。否则，您将重复将记录写入同一文件。所以更像是这样的事情：

     } else if (m.equals("7")) {
        JOptionPane.showMessageDialog(null,
              "Output information to a .txt file");
        final PrintWriter out = new PrintWriter(outputFileName);
        out.println("The students' information is listed below:");
        for (Student Stu : student) {
           out.println(Stu.getId() + " " + Stu.getLName() + " "
                 + Stu.getGpa());
        }
        out.close();
     }

添加while循环后，Printwriter似乎没有输出到指定的文件

3 个答案: