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"));

score 10 · Answer 1 · answered Apr 18 '16 at 07:57

10

If using java 8

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

answered Apr 18 '16 at 07:57

Kumar Abhishek

3,004
33
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3

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. – Basil Bourque Apr 18 '16 at 19:42

score 5 · Answer 2 · answered Jun 18 '12 at 17:42

5

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

Basically you are using outFile before its initialized.

answered Jun 18 '12 at 17:42

Suraj Chandran

24,433
12
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94

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")); – Paul Jun 18 '12 at 17:51
Try putting that line just after the date=new Date() statement – Suraj Chandran Jun 18 '12 at 17:57

Viktor Stolbin · Answer 3 · 2012-06-18T17:52:46.217

5

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"))
    ...

edited Jun 18 '12 at 17:52

answered Jun 18 '12 at 17:47

Viktor Stolbin

2,899
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score 2 · Answer 4 · answered Apr 20 '22 at 12:36

2

LocalDateTime current = LocalDateTime.now();
DateTimeFormatter format = DateTimeFormatter.ofPattern("ddMMyyyyHHmmss");
String formatedDateTime = current.format(format);
outFile = new PrintWriter(new FileWriter("simplex" + formatedDateTime  + ".txt"));

answered Apr 20 '22 at 12:36

Vamsi Jayavarapu

105
2
11

score 0 · Answer 5 · answered Jun 18 '12 at 17:44

0

// display time and date using toString()
outFile.println(date.toString());

With the code, You use outFile before initialize it.

answered Jun 18 '12 at 17:44

plucury

1,120
1
8
16

score 0 · Answer 6 · edited Aug 14 '15 at 00:27

Just save it in a variable. You should use the new Date(long) constructor, btw.

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(System.currentTimeMillis());
         String dateString = date.toString();


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


         outFile.println(dateString);
         outFile.println();
        //creates the new file to be saved

score 0 · Answer 7 · answered Jun 18 '12 at 17:47

Problem is that outFile is declared as a static, but never initialized until after you already used it.

You need to first initialize/instantiate outFile first before actually using it:

 private static PrintWriter outFile;

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

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

        //creates the new file to be saved
        outFile = new PrintWriter(new FileWriter("simplex" + (date.toString()) + .txt"));
        // display time and date using toString()
         outFile.println(date.toString());
         outFile.println();

Although I'm not entirely sure why you are even creating outFile as a static object, and not just a local variable.

score 0 · Answer 8 · answered Jun 05 '17 at 09:58

0

The below mentioned snippet can be used

 String logFileName = new SimpleDateFormat("yyyyMMddHHmm'.txt'").format(new Date());

 logFileName = "loggerFile_" + logFileName;

answered Jun 05 '17 at 09:58

Akshay Prabhakar

11
3

Adding The Date and Time to the File name

8 Answers8