java，将计算值打印到文件时不是数组？

Question

This program calculates the surface gravities of 9 planets, prints the data to the screen in a formatted table, and is supposed to print only the surfaceGravity values to a file. I'm not sure why my code won't print the calculated surfaceGravity value to the file called gravities2.txt. Can someone please provide some enlightenment? Is it maybe because the gravity data is not put into an array- it's just values that get looped through?

import java.io.IOException;
import java.io.PrintWriter;
import java.io.File;

public class GravityV1
{   
   //print the data to screen
   public static void printResults(String[] names, double[] diameter, double[] mass, int x)
   {
     System.out.printf("%-10s%21.2f%20.4e",names[x],diameter[x],mass[x]);   
   }

   //calculate the surface gravity
   public static double calcGravity(double m, double d)throws IOException
   { 
      double gravity = 0.0;    

      gravity = (((6.67E-11) * (m)) / (Math.pow((d) / 2, 2)));

      printToFile(gravity);     

      return gravity;        
   }

   //writes gravity data to a text file
   public static void printToFile(double gravity)throws IOException
   { 
       File gravities = new File("gravity2.txt"); 

       PrintWriter outFile = new PrintWriter(gravities);

       outFile.printf("%2.2f\n", gravity);
   }

   //main method
   public static void main (String [ ] args) throws IOException
   {        
      double[] mass = { 3.30E23, 4.869E24, 5.972E24, 6.4219E23, 1.900E27, 5.68E26, 8.683E25, 1.0247E26, 1.27E22 };

      double[] diameter = { 4880000, 12103000.6, 12756000.3, 6794000, 142984000, 120536000, 51118000, 49532000, 2274000 };

      String[] names = { "Mercury", "Venus", "Earth", "Mars", "Jupiter", "Saturn", "Uranus", "Neptune", "Pluto" };

      System.out.printf("%-7s%20s%20s%20s\n","Planet Name","Diameter","Mass(kg)", "g (m/s^2)");    
      System.out.println("=============================================================================");

      for (int x = 0; x < 9; x++) {
         printResults(names, diameter, mass, x);

         double surfaceGravity = calcGravity(mass[x], diameter[x]);

         System.out.printf("%17.2f\n", surfaceGravity);         
      }  

   System.out.println();
  }  
}

The output to the screen is correct, but unfortunately the file contains nothing. The file does get created, so I suppose that's one good thing... Any help is very greatly appreciated!!

Answer 1

You need to close and flush the PrintWriter . This is because the PrintWriter "buffers" your output, or saves it, and only writes once there is a fair amount saved up (this helps your program go faster, because it causes fewer big writes rather than a lot of small ones). Closing or flushing a file forces it to write.

Adding outFile.close() at the end of the function will help, although you should try to refactor you code so that you don't have to constantly create new writers, and can simply close the one writer when you are done!

Answer 2

Two issues:

1 - You're never closing the PrintWriter . You need to invoke outFile.close() after you're done writing.

2 - Since you're calling the printToFile method from within the loop in your main method, and that method creates a new PrintWriter object each time it is called, what you will effectively end up with is a file containing only the LAST planet in the loop.

To solve the last problem, I would create a PrintWriter before the loop is created, and pass it in to the printToFile method each time you call it. Remember to close() it after the loop.

Answer 3

尝试在调用printf方法时添加冲洗。

outFile.flush();