Why SimpleDateFormat does not throw exception for invalid format?

Question

import java.text.ParseException;

public class Hello {

    public static void main(String[] args) throws ParseException {
        System.out.println(new java.text.SimpleDateFormat("yyyy-MM-dd").parse("23-06-2015"));
    }
}

why this returns Sun Dec 05 00:00:00 GMT 28 I am expecting an exception.

Answer 1

The Javadoc for SimpleDateFormat has this to say about repeated pattern letters:

Number: For formatting, the number of pattern letters is the minimum number of digits, and shorter numbers are zero-padded to this amount. For parsing, the number of pattern letters is ignored unless it's needed to separate two adjacent fields

(Emphasis mine)

So for parsing, "yyyy-MM-dd" is equivalent to "y-M-d".

With this pattern, "23-06-2015" is parsed as year = 23, month = 6, dayOfMonth = 2015.

By default, this gets resolved by starting at 1st June 0023, and counting 2015 days forward, taking you to 5th December 0028.

You can change this behaviour with SimpleDateFormat.setLenient(false) -- with leniency disabled, it will throw an exception for out-of-range numbers. This is properly documented in Calendar.setLenient()

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Note, for new code in Java 8, it's a good idea to avoid the old Date and Calendar classes. Use LocalDateTime.parse(CharSequence text, DateTimeFormatter formatter) if you can.