java - SimpleDateFormat leniency leads to unexpected behavior

Question

I have found that SimpleDateFormat::parse(String source)'s behavior is (unfortunatelly) defaultly set as lenient: setLenient(true).

By default, parsing is lenient: If the input is not in the form used by this object's format method but can still be parsed as a date, then the parse succeeds.

If I set the leniency to false, the documentation said that with strict parsing, inputs must match this object's format. I have used paring with SimpleDateFormat without the lenient mode and by mistake, I had a typo in the date (letter o instead of number 0). (Here is the brief working code:)

// PASSED (year 199)
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd.mm.yyyy");
System.out.println(simpleDateFormat.parse("03.12.199o"));
simpleDateFormat.setLenient(false);
System.out.println(simpleDateFormat.parse("03.12.199o"));        //WTF?

In my surprise, this has passed and no ParseException has been thrown. I'd go further:

// PASSED (year 1990)
String string = "just a String to mess with SimpleDateFormat";

SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd.mm.yyyy");
System.out.println(simpleDateFormat.parse("03.12.1990" + string));
simpleDateFormat.setLenient(false);
System.out.println(simpleDateFormat.parse("03.12.1990" + string));

Let's go on:

// FAILED on the 2nd line
SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd.mm.yyyy");
System.out.println(simpleDateFormat.parse("o3.12.1990"));
simpleDateFormat.setLenient(false);
System.out.println(simpleDateFormat.parse("o3.12.1990"));

Finally, the exception is thrown: Unparseable date: "o3.12.1990". I wonder where is the difference in the leniency and why the last line of my first code snippet has not thrown an exception? The documentation says:

With strict parsing, inputs must match this object's format.

My input clearly doesn't strictly match the format - I expect this parsing to be really strict. Why does this (not) happen?

Answer 1

Leniency is not about whether the entire input matches but whether the format matches. Your input can still be 3.12.1990somecrap and it would work.

The actual parsing is done in parse(String, ParsePosition) which you could use as well. Basically parse(String) will pass a ParsePosition that is set up to start at index 0 and when the parsing is done the current index of that position is checked.

If it's still 0 the start of the input didn't match the format, not even in lenient mode.

However, to the parser 03.12.199 is a valid date and hence it stops at index 8 - which isn't 0 and thus the parsing succeeded. If you want to check whether everything was parsed you'd have to pass your own ParsePosition and check whether the index is matches to the length of the input.