Why would parsing with simpledateformat throw a nullpointerexception?

Question

I am trying to validate the format of a date before assigning it to an instance variable in a constructor. Each time, it throws a null pointer exception even though I provide a value for the field each time I try to instantiate an object. I think the problem must be with the parse statement, but I cannot understand why there would be a null value there.

Moreover, I was wondering if it is proper practice to validate a date in the constructor in the first place. Should I have a method doing this instead?

Thank you so much for your help. This is my first exposure to java.

import java.text.*;

public class Photograph {
    private String caption;
    private final String filename;
    private String dateTaken;
    private int rating; //declares fields 

    public Photograph(String caption, String filename) {
        this.caption = caption;
        this.filename = filename;
        this.dateTaken = "1901-01-01";
        this.rating = 0;    //constructor 
    }

    public Photograph(String caption, String filename, String dateTaken,  
int rating) {
        this.caption = caption;
        this.filename = filename;

        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd");
        sdf.setLenient(false);

        try {
            sdf.parse(this.dateTaken);
            this.dateTaken = dateTaken;
        } catch (ParseException e) {
            this.dateTaken = "1901-01-01";
        }
    }

}

public static void main(String[] args) {
    // Testing


    //valid date
    Photograph test_photo1 = new Photograph("cap1", "pic1", "2017-09-30", 3);
    System.out.println(test_photo1.toString());
    Photograph test_photo2 = new Photograph("cap2", "pic2", "2017-12-25", 0);
    System.out.println(test_photo2.toString());
    Photograph test_photo3 = new Photograph("cap3", "pic3", "2018-14-25", 5);
    System.out.println(test_photo3.toString());
    Photograph test_photo4 = new Photograph("cap4", "pic4", "2018-03-27", 4);
    System.out.println(test_photo4.toString());
    Photograph test_photo5 = new Photograph("cap5", "pic5", "2018-03-29", 2);
    System.out.println(test_photo5.toString());
    Photograph test_photo6 = new Photograph("cap6", "pic6", "2018-10-21", 9);
    System.out.println(test_photo6.toString());
}

This is the resulting error message: Exception in thread "main" java.lang.NullPointerException at java.base/java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1470) at java.base/java.text.DateFormat.parse(DateFormat.java:393) at Photograph.(Photograph.java:25) at Photograph.main(Photograph.java:100) — werhgsd, Sep 30 '19 at 20:49
Because you're parsing a null. The field isn't initialized until the next line. Fix the order. — user207421, Sep 30 '19 at 21:25
And you should let the parse exception be thrown. The object isn't much use to anybody with that default date. — user207421, Sep 30 '19 at 21:48
I recommend you don’t use `SimpleDateFormat`. That class is notoriously troublesome and long outdated. Instead for validation just use `LocalDate.parse(dateTaken)` and catch `DateTimeParseException`. `LocalDate` and `DateTimeParseException` are from [java.time, the modern Java date and time API](https://docs.oracle.com/javase/tutorial/datetime/). — Ole V.V., Oct 01 '19 at 05:42

NoDataFound · Answer 1 · 2019-09-30T21:06:18.877

For your error, it is "quite" simple:

public Photograph(String caption, String filename, String dateTaken,  
int rating) {
  ..
        try {
            sdf.parse(this.dateTaken);
...
        } catch (ParseException e) {
   ...
        }
    }

The this.dateTaken expression resolve to null because you wanted to use dateTaken (the parameter, not the field).

On a side note, I would not use a String to represent a date, but rather a LocalDate: the parse method does exactly what you are trying to do and LocalDate replace java.util.Date and is immutable.

This example below use such LocalDate, providing three constructors based on your code. Field are made final and only one constructor (the last) matters.
You could probably avoid storing an empty date (the NO_DATE_TAKEN) either by using null logic in the class, or rather Optional. Using one or the other is another question in itself.

    public class Photograph {
        private static final LocalDate NO_DATE_TAKEN = LocalDate.of(1901, Month.JANUARY, 01);

        private final String caption;
        private final final String filename;
        private final LocalDate dateTaken;
        private final int rating; //declares fields 

        public Photograph(String caption, String filename) {
          this(caption, filename, NO_DATE_TAKEN, 0);
        }

        public Photograph(String caption, String filename, String dateTaken,  
    int rating) {
          this(caption, filename, dateTaken == null ? NO_DATE_TAKEN:LocalDate.parse(dateTaken), rating);
        }

        public Photograph(String caption, String filename, LocalDate dateTaken,  
    int rating) {
          this.caption = caption;
          this.filename = filename;
          this.dateTaken = dateTaken == null ? NO_DATE_TAKEN:dateTaken;
          this.rating = rating;
        }

    }

I would personally not use a String to store Date, and I would not use java.util.Date and SimpleDateFormat to parse Date now that we have LocalDate and LocalDate.parse doing the exact thing you are trying to do.

Why would parsing with simpledateformat throw a nullpointerexception?

1 Answers1