用Java写入/读取文件

Question

I am working on an assignment and cannot get this method to produce the correct output to the file. I am supposed to get the mean and write it to the file. Their is the StatsDemo class and the StatsFile class. I am kind of a beginner at Java so I'd like just a little help. My method in the StatsFile class is currently like this:

//returns the calculated arithmetic average
public double calculateMean(String filename) throws FileNotFoundException
{
    // declare variables step 5
    double accumulator = 0.0;
    int counter =0;
    String line;
    try{
    File input = new File(filename);
    //create a Scanner object passing it the File object
    Scanner keyboard = new Scanner(input);
    //for (int i = 0; i < input.length(); i++){
         // Read a double from the file.
    while(keyboard.hasNextDouble()){
         accumulator += keyboard.nextDouble();

         // Add to counter
         counter++;
    }
    keyboard.close();


}catch(FileNotFoundException e){
    }
    return (accumulator/counter);
}

The demo is as such:

import java.util.Scanner;
import java.text.DecimalFormat;
import java.io.*;
public class StatsDemo {

    public static void main(String[] args) throws FileNotFoundException  {
        // TODO Auto-generated method stub

        DecimalFormat threeDec  = new DecimalFormat("0.000");
        Scanner keyboard = new Scanner(System.in);

        String filename; // the user input file name

        System.out.print("Enter the file name: ");
        filename = keyboard.nextLine();


        FileStats fileObj = new FileStats(filename); 

        try{
            PrintWriter name = new PrintWriter("Results.txt");
            name.println("mean = " + threeDec.format(fileObj.getMean()));
            name.println("Standard Deviation = " + threeDec.format(fileObj.getStdDev()));

            name.close();
        }catch(IOException e){
            System.out.println("Error");
        }
    }
}

The catches and throws still kind of confuse me. My issue is that it currently gives me a question mark instead of the mean when i open the file. Any help will be appreciated.

Answer 1

If an exception that occurs within the try block, control jumps to the catch block. You should think about what should happen in that case. You may be able to correct the problem and continue; you may wish to look at the problem and rethrow and exception, or you may wish just to let your caller handle the problem. In the last case, you don't need a catch at all, you can just need a throws on your method declaration.

Catching an exception and doing nothing is rarely a good idea as the problem is just being ignored. Remember that code flow will carry on after the catch clause unless another exception is thrown, so if the file does not exist, you will still process the return (accumulator/counter) line, which is not what you want.

Looking at your code, your method already throws a FileNotFoundException, so just remove the try and the catch.