How to format and save console output to file in Java?

Question

Background: Hello! First of all, please have in mind that I'm fairly new to Java, thus, forgive me if I have something wrong. I have been trying to format my console and save its output to a text file for 2 days now. Here's my code so far:

I'm trying to set the format of the console to

[12/20/2017 23:55:30] program(message here)

and I'm doing it like this:

public class Format {
    private static final DateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");

    PrintStream console = new PrintStream(System.out) {

        public void println(String x) {
            Date date = new Date();
            super.println("[" + sdf.format(date) + "] " + "program(" + x + ")");
        }
    };

Along with this:

    public void progStream() throws FileNotFoundException {
        System.setOut(console);
        System.out.println("This is a test.");
    }

Up to here it works just fine and I can see all the system outputs show up on the eclipse's console with the correct format. However, when I try to save it a text file it saves only the message. Without the format.

    public void progStream() throws FileNotFoundException {
        File log = new File("log" + System.currentTimeMillis() + ".txt");
        FileOutputStream fos = new FileOutputStream(log);
        PrintStream console = new PrintStream(fos);
        System.setOut(console);
        System.out.println("This is a test.");
    }

I have tried replacing System.out.println("message"); with console.print("message"); but I get the same issue. Unfortunately, I have run out of tutorials and forums to look out and I still can't find a solution to this. Thank you for your help.

Use a logging API, there's one built in to the core API or you could use something Log4j — MadProgrammer
– MadProgrammer, Commented Dec 20, 2017 at 7:05
Can you try posting a simple entire code as partial codes are quite confusing to understand — swithen colaco
– swithen colaco, Commented Dec 20, 2017 at 7:12

m. vokhm · Accepted Answer · 2017-12-20 08:28:05Z

In your code, you never use the Format class you created to prepend your output with the date. You should have created an instance of this class and call the method println() of the console field of the newly created instance. Below is your code, slightly modified, that I suppose does what you want:

  import java.io.File;
  import java.io.FileNotFoundException;
  import java.io.FileOutputStream;
  import java.io.PrintStream;
  import java.text.DateFormat;
  import java.text.SimpleDateFormat;
  import java.util.Date;

  public class So_47900381 {

    static public class Format {
      private static final DateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");
      PrintStream console = new PrintStream(System.out) {;
        public void println(String x) {
          Date date = new Date();
          super.println("[" + sdf.format(date) + "] " + "program(" + x + ")");
        }
      };
    }

    public static void progStream() throws FileNotFoundException {
      File log = new File("log" + System.currentTimeMillis() + ".txt");
      PrintStream console = new PrintStream(new FileOutputStream(log));
      System.out.println("Writing to " + log.getPath());
      System.setOut(console);
      Format fmt = new Format();
      fmt.console.println("This is a test.");
    }

    public static void main(String[] args) {
      try {
       progStream();
      } catch (Exception x) { x.printStackTrace(); }
    }

  }

But I think there's no need to have a separate field, that is a descendant of the PrintStream and used to print to, inside the Format class. The class itself could descend from the PrintStream as well. Look at the code below:

  import java.io.File;
  import java.io.FileNotFoundException;
  import java.io.PrintStream;
  import java.text.DateFormat;
  import java.text.SimpleDateFormat;
  import java.util.Date;

  public class So_47900381_1{

    static public class Format extends PrintStream {
      private static final DateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");

      public Format(File file) throws FileNotFoundException {
        super(file);
        System.setOut(this);
      }

      public void println(String x) {
        Date date = new Date();
        super.println("[" + sdf.format(date) + "] " + "program(" + x + ")");
      }
    }

    public static void progStream() throws FileNotFoundException {
      File log = new File("log" + System.currentTimeMillis() + ".txt");
      System.out.println("Writing to " + log.getPath());
      Format fmt = new Format(log);          // redirects the output to the file
      System.out.println("This is a test."); // now it's written to the file 
    }

    public static void main(String[] args) {
      try {
        progStream();
      } catch (Exception x) { x.printStackTrace(); }
    }

  }

And yes, look at the log4j and/or google for java logging.

Thank you, this is really helpful. I have about a week or two working with Java so this is all new to me. Your answer is very much appreciated.
@DavidMorales :) Then mark it as accepted :) And note that I have changed it a little - System.out.println("This is a test.") in the latter snippet

Bruno Medeiros · Accepted Answer · 2017-12-20 07:17:04Z

0

I think that's what you want to achieve:

import java.lang.*;
import java.io.*;
import java.util.*;
import java.text.*;

public class HelloWorld {

  public static void main(String[] args) throws IOException {

    PrintStream console = new PrintStream(System.out);
    PrintStream file = new PrintStream(new FileOutputStream(new File("log" + System.currentTimeMillis() + ".txt")));  

    Format format = new Format();

    // console
    format.print("bla", console);

    // file
    format.print("bla", file);  
  }
}

public class Format {

  private static final DateFormat sdf = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");

  public void print(String x, PrintStream stream) {
     Date date = new Date();
     stream.println("[" + sdf.format(date) + "] " + "program(" + x + ")");
  }

}

PS.: If you really want to log things, I recommend you to consider a logging framework like log4j.

PS. 2: System.setOut(stream) does NOT seem a good idea as your standard output will be redirected to the file.

I hope my answer helps you :)

answered Dec 20, 2017 at 7:17

Bruno Medeiros

2,41725 silver badges37 bronze badges

2 Comments

David Morales Over a year ago

Thank you! It worked. The best part is that I actually understood what I was doing wrong. I will read about logging frameworks, thank you for the reference!

Bruno Medeiros Over a year ago

Nice I could help :)

Collectives™ on Stack Overflow

How to format and save console output to file in Java?

2 Answers 2

2 Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related