DataInputStream for input text files?

I am learning to read and write in Java and am stuck with a simple exercise. The program reads from 2 txt files that each contain numbers in rows. It writes to an output file the result of the multiplication of each row of numbers. eg. file 1 row 1 : 10, file 2 row 1: 2 , the program should write 20 to the output file. My code seems to have something missing somewhere. The output file is created but nothing is written to it. Any ideas?

import java.io.*;
import java.util.*;

class ReadWriteData
{
    public static void main(String[] args) throws Exception 
    {

        //create ouput file
        PrintWriter output = new PrintWriter("output2.txt");

        DataInputStream file1 = new DataInputStream(new FileInputStream(args[0]));
        DataInputStream file2 = new DataInputStream(new FileInputStream(args[1]));  

        try
        {

        // read data from file
        while (true)
        {
            double number1 = file1.readDouble();
            double number2 = file2.readDouble();
            double result = number1 * number2 ;
            output.println(result);

        }


        }

        ca开发者_开发百科tch (IOException e)
        {
            System.err.println("Error");
            System.exit(1);
        }

        output.close() ;

    }
}

Here is an implementation with a BufferedReader that works.

public static void main(String[] args) throws Exception {
    //create ouput file
    PrintWriter output = new PrintWriter("output2.txt");
    BufferedReader file1 = new BufferedReader(new FileReader("numbers1.txt"));
    BufferedReader file2 = new BufferedReader(new FileReader("numbers2.txt"));

    try {
        // read data from file
        while (true) {
            String number1AsString = file1.readLine();
            String number2AsString = file2.readLine();
            if (number1AsString == null || number2AsString == null) {
                break;
            }
            double number1 = Double.parseDouble(number1AsString);
            double number2 = Double.parseDouble(number2AsString);
            double result = number1 * number2;
            System.out.println("result:" + result);
            output.println(result);
        }
    } catch (IOException e) {
        System.out.println(e.getMessage());
    } finally {
        output.close();
        file1.close();
        file2.close();
    }
}

Edit: Also you may want to modularize your code for instance creating a method that help reduce duplicated code. Also you may be interested to look for NumberFormatException in case any number is not properly formatted or includes letters for example.

private double readDoubleFromFile(BufferedReader file) throws IOException {
    String numberAsString = file.readLine();
    if (numberAsString == null) {
        throw new IOException();
    }
    double number = Double.parseDouble(numberAsString);
    return number;
}

The DataInputStream class is not for reading text files. it can only be used to read what DataOutput writes. If you have rows of human-readable numbers, you need to use an InputStreamReader and then parse the resulting streams with things like Double.parseDouble

Maybe you want to use a BufferedReader for this.

  BufferedReader in = new BufferedReader(
                          new FileReader(args[0]));

Then:

  String num = null;
  while((num = in.readLine()) != null){
        double d = Double.parseDouble(num);
        //now you have a double value
  }

This way you do not depend on the exception to indicate the end of file.

You need to call output.flush just before closing the stream. Also, you should close the streams to the files in a finally block, this will make sure that the close command wil always be executed.

The DataInputStream class reads from a binary file (or other source such as socket). This means that it is going to be completely misinterpreting those input text files, with possibly amusing (or very irritating) results. To read numbers from a text file, you should use a BufferedReader wrapping an InputStreamReader to read lines and then convert those to numbers with suitable parsing methods (e.g., Double.parseDouble if you're wanting to produce a floating-point number).

When testing these things, it's often helpful to put in some debugging output inside the loop that prints out each value as you read it. Like that, you can see if things have got stuck in some unexpected way.

With this while (true) without a break your code is basically running in an infinite loop and never stopping unless there's an exception.

If it did terminate but you didn't see an exception, then it might be caused by calling System.exit(1) in the catch. It might be too late then to print "Error" anyway (the stdout might have been abrupted too early) and the file will never be flushed/closed. Remove that System.exit(1) line.

Also, closing is supposed to happen in finally block. And best is to not print some nothing-saying message on exception but just let them go. Since you already have a throws Exception on the method, just remove the entire catch. Only use it when you can handle exceptions in a sensible manner.

PrintWriter output = new PrintWriter("output2.txt");
try {
    output.println("something");
} finally {
    output.close();
}

After

output.println(result);

add

output.flush();