Printwriter似乎没有输出到指定的文件后，我添加了一个while循环

Question

我正处于学校项目的早期阶段，已经遇到了一个错误。如果我简单地使用for循环导出数组列表内容而没有使用while循环，那么一切正常，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，就像程序一样没有做任何事情，只是创建了一个空文本文件。我很困扰。

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(); 
    } 
} 

这里是我正在使用的Student类。

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：这是现在的PrintWriter工作代码的新的部分。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

你需要写它后刷新您的输出流：

out.flush(); 

此外，你将要创建的输出流，当用户选择你写选项（7）。否则，您将重复将记录写入同一文件。所以更像这样的东西：

 } 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(); 
    } 

Answer 2

您是否在关闭它之前尝试flush（）ing PrintWriter？

Answer 3

问题是，out.close的行永远不会被调用。

System.exit方法会在关闭打印作业器之前终止您的应用程序。

您应该使用finally finally块来关闭资源。

如果System.exit调用是在方法的结尾而不是在while循环中，那么您的代码也会更清晰。您已经使用布尔值来终止循环，所以让它终止，并继续执行其余的代码。