Adding The Date and Time to the File name

Question

Hello I am trying to add the date and time to a file name in JAVA. I can get the date and time printed within the file, which I also want done, but when I place the toString in the FileWriter I get a Null Pointer.

package com.mkyong;
import java.util.*;
import java.io.*;
import java.*;
import java.util.Date;
import java.text.DateFormat;
import java.text.SimpleDateFormat;

    public class Simplex {

        private static PrintWriter outFile;

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



            // Instantiate a Date object
             Date date = new Date();

             // display time and date using toString()
             outFile.println(date.toString());
             outFile.println();
            //creates the new file to be saved


            outFile = new PrintWriter(new FileWriter("simplex" + (date.toString()) + ".txt"));

Solution 1

If using java 8

DateTimeFormatter timeStampPattern = DateTimeFormatter.ofPattern("yyyyMMddHHmmss");
        System.out.println(timeStampPattern.format(java.time.LocalDateTime.now()));

Solution 2

The line outFile = new PrintWriter(..) should occur before first usage of outFile.

Basically you are using outFile before its initialized.

Solution 3

I'd suggest you to use YYYY-MM-dd_hh-mm-ss formatting pattern in file name that allows you to sort out files in a more convinient way. Take a look at SimpleDateFormat class.

    ...
    Format formatter = new SimpleDateFormat("YYYY-MM-dd_hh-mm-ss");
    outFile = new PrintWriter(new FileWriter("simplex_" + formatter.format(date) + ".txt"))
    ...

Comments

[Paul]

Ah Thank you. But when I reorder it, the compiler still does not like this line: outFile = new PrintWriter(new FileWriter("simplex" +(date.toString()) + ".txt"));

[Suraj Chandran]

Try putting that line just after the date=new Date() statement

[Basil Bourque]

I suggest always passing a ZoneId object to the now() method. If omitted, the JVM‘s current default time zone is applied. That default can vary. Better to specify explicitly the expected/desired zone.