Determine if a String is a valid date before parsing

Question

I have this situation where I am reading about 130K records containing dates stored as String fields. Some records contain blanks (nulls), some contain strings like this: 'dd-MMM-yy' and some contain this 'dd/MM/yyyy'.

I have written a method like this:

public Date parsedate(String date){

   if(date !== null){
      try{
        1. create a SimpleDateFormat object using 'dd-MMM-yy' as the pattern
        2. parse the date
        3. return the parsed date
      }catch(ParseException e){
          try{
              1. create a SimpleDateFormat object using 'dd/MM/yyy' as the pattern
              2. parse the date
              3. return parsed date
           }catch(ParseException e){
              return null
           }
      }
   }else{
      return null
   }

} 

So you may have already spotted the problem. I am using the try .. catch as part of my logic. It would be better is I can determine before hand that the String actually contains a parseable date in some format then attempt to parse it.

So, is there some API or library that can help with this? I do not mind writing several different Parse classes to handle the different formats and then creating a factory to select the correct6 one, but, how do I determine which one?

Thanks.

Answer 1

See Lazy Error Handling in Java for an overview of how to eliminate try/catch blocks using an Option type.

Functional Java is your friend.

In essence, what you want to do is to wrap the date parsing in a function that doesn't throw anything, but indicates in its return type whether parsing was successful or not. For example:

import fj.F; import fj.F2;
import fj.data.Option;
import java.text.SimpleDateFormat;
import java.text.ParseException;
import static fj.Function.curry;
import static fj.Option.some;
import static fj.Option.none;
...

F<String, F<String, Option<Date>>> parseDate =
  curry(new F2<String, String, Option<Date>>() {
    public Option<Date> f(String pattern, String s) {
      try {
        return some(new SimpleDateFormat(pattern).parse(s));
      }
      catch (ParseException e) {
        return none();
      }
    }
  });

OK, now you've a reusable date parser that doesn't throw anything, but indicates failure by returning a value of type Option.None. Here's how you use it:

import fj.data.List;
import static fj.data.Stream.stream;
import static fj.data.Option.isSome_;
....
public Option<Date> parseWithPatterns(String s, Stream<String> patterns) { 
  return stream(s).apply(patterns.map(parseDate)).find(isSome_()); 
}

That will give you the date parsed with the first pattern that matches, or a value of type Option.None, which is type-safe whereas null isn't.

If you're wondering what Stream is... it's a lazy list. This ensures that you ignore patterns after the first successful one. No need to do too much work.

Call your function like this:

for (Date d: parseWithPatterns(someString, stream("dd/MM/yyyy", "dd-MM-yyyy")) {
  // Do something with the date here.
}

Or...

Option<Date> d = parseWithPatterns(someString,
                                   stream("dd/MM/yyyy", "dd-MM-yyyy"));
if (d.isNone()) {
  // Handle the case where neither pattern matches.
} 
else {
  // Do something with d.some()
}