How to show/write result in log file?

Question

I am supposed to write/show input of my "result"(calculator) to a log file... for example: results.txt But If I do more then 1 calculation... only the last one gets printed in results.txt.

public class Calculator {
    public static void main(String[] args) throws FileNotFoundException {
        String filename = "results.txt";
        //PrintWriter output = new PrintWriter(filename);
        int number1;
        int number2;
        char mathchoice;
        int result;
        char userchoice;
        do {
            PrintWriter output = new PrintWriter(filename);

            Scanner choice = new Scanner(System.in);
            System.out.println("Enter first number...");
            number1 = choice.nextInt();
            System.out.println("Math choice...");
            mathchoice = choice.next().charAt(0);
            System.out.println("Enter second number...");
            number2 = choice.nextInt();

            if (mathchoice == '+') {
                result = number1 + number2;
                System.out.println("Result is: " + result);
                output.print("Result is: " + result);
            } else if (mathchoice == '-') {
                result = number1 - number2;
                System.out.println("Result is: " + result);
                output.print("Result is: " + result);
            } else if (mathchoice == '*') {
                result = number1 * number2;
                output.print("Result is: " + result);
                System.out.println("Result is: " + result);
            } else if (mathchoice == '/') {
                if (number2 == 0) {
                    throw new java.lang.ArithmeticException("Devided by zero not aloud");
                } else {
                    result = number1 / number2;
                    output.print("Result is: " + result);
                    System.out.println("Result is: " + result);

                }

            } else if (mathchoice == '%') {
                result = number1 % number2;
                output.print("Result is: " + result);
                System.out.println("Result is: " + result);
            } else if (mathchoice == '^') {
                result = (int) Math.pow(number1, number2);
                output.print("Result is: " + result);
                System.out.println("Result is: " + result);
            } else {
                System.out.println("Wrong choice");

            }
            output.close();
            System.out.println("Again?");
            userchoice = choice.next().charAt(0);

        }while (userchoice == 'j');
    }
    
}

I added the output.close(), at the end of the lus. But I only get the last calculation.

Answer 1

If you want to get the exact same output as printed in the console, so System.out + System.in, one way to go about this is by creating a custom InputStreamReader to give to the Scanner, and a custom PrintStream to set System.out as. These custom implementations can then write to the file whenever they read/write data.

Here is an example:

import java.io.*;
import java.nio.file.Paths;
import java.util.Scanner;

public class ScannerTest {

    static boolean alive = true;

    public static void main(String[] args){

        int number1;
        int number2;
        char mathchoice;
        int result;
        char userchoice;

        final File file = Paths.get("result.txt").toFile();

        FileOutputStream fileOutputStream = null;
        try {
            fileOutputStream = new FileOutputStream(file);
            file.createNewFile();
            System.setOut(new PrintStreamFileForwarder(System.out, fileOutputStream));
        } catch (IOException e) {
            e.printStackTrace();
        }

        Scanner choice = new Scanner(new InputStreamFileForwarder(System.in, fileOutputStream));

        do {

            System.out.println("Enter first number...");
            number1 = choice.nextInt();
            System.out.println("Math choice...");
            mathchoice = choice.next().charAt(0);
            System.out.println("Enter second number...");
            number2 = choice.nextInt();

            if (mathchoice == '+') {
                result = number1 + number2;
                System.out.println("Result is: " + result);
            } else if (mathchoice == '-') {
                result = number1 - number2;
                System.out.println("Result is: " + result);
            } else if (mathchoice == '*') {
                result = number1 * number2;
                System.out.println("Result is: " + result);
            } else if (mathchoice == '/') {
                if (number2 == 0) {
                    throw new java.lang.ArithmeticException("Devided by zero not aloud");
                } else {
                    result = number1 / number2;
                    System.out.println("Result is: " + result);
                }

            } else if (mathchoice == '%') {
                result = number1 % number2;
                System.out.println("Result is: " + result);
            } else if (mathchoice == '^') {
                result = (int) Math.pow(number1, number2);
                System.out.println("Result is: " + result);
            } else {
                System.out.println("Wrong choice");

            }
            System.out.println("Again?");
            userchoice = choice.next().charAt(0);

        } while (userchoice == 'j');
        alive = false;
    }

    public static class InputStreamFileForwarder extends InputStreamReader {

        private final FileOutputStream fileOutputStream;

        public InputStreamFileForwarder(InputStream console, FileOutputStream fileOutputStream) {
            super(console);
            this.fileOutputStream = fileOutputStream;
        }

        @Override
        public int read(char[] cbuf, int offset, int length) throws IOException {
            int read = super.read(cbuf, offset, length);
            if(read > 0) {
                char[] allRead = new char[read];
                System.arraycopy(cbuf, offset, allRead, 0, read);
                fileOutputStream.write(new String(allRead).getBytes());
            }
            return read;
        }
    }

    public static class PrintStreamFileForwarder extends PrintStream {

        private final FileOutputStream fileOutputStream;

        public PrintStreamFileForwarder(PrintStream console, FileOutputStream fileOutputStream) {
            super(console);
            this.fileOutputStream = fileOutputStream;
        }

        @Override
        public void write(byte[] buf, int off, int len) {
            super.write(buf, off, len);
            try {
                fileOutputStream.write(buf, off, len);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

Answer 2

You can create a file and write directly to it, see the "java.io" classes, such as OutputStream, FileWriter and etc. You basically need to create a reference to the file and use the "write" method to write to it.

// on Windows use "c:\temp\my-file.log"
FileOutputStream fos = new FileOutputStream("/tmp/my-file.log");
fos.write("My log message".getBytes());

But if possible, do not reinvent the wheel and use LOG4J to record your logs. LOG4J is a widely used library and it makes logging work much easier.

And to manage your project's dependencies, use Maven. Create a maven project and add the dependencies in POM.XML, it will greatly simplify your life.