How to convert String to Date without knowing the format?

You cant!

If you have the date 2010-08-05 then it can be either 5th August 2010, or 8th May 2010 - you need to know the date format (or at least prioritise one format over the over) to tell them apart.

I agree with Kragen that in the general case there is no correct solution. However, if the following conditions hold, you may use the solution below:

1. You have a set of all possible formats

2. There is no ambiguity between the formats; no date expression can be successfully parsed by two of them.

Consider the following solution which iterates over a list of possible formats. This solution makes use of ThreadLocal, in order to make date parsing efficient in a multi-threaded environment (remember that SimpleDateFormat isn't thread safe):

public class FlexibleDateParser {    private List<ThreadLocal<SimpleDateFormat>> threadLocals = new  ArrayList<ThreadLocal<SimpleDateFormat>>();    public FlexibleDateParser(List<String> formats, final TimeZone tz){        threadLocals.clear();        for (final String format : formats) {            ThreadLocal<SimpleDateFormat> dateFormatTL = new ThreadLocal<SimpleDateFormat>() {                protected SimpleDateFormat initialValue() {                    SimpleDateFormat sdf = new SimpleDateFormat(format);                    sdf.setTimeZone(tz);                     sdf.setLenient(false);                    return sdf;                }            };            threadLocals.add(dateFormatTL);        }           }    public Date parseDate(String dateStr) throws ParseException {        for (ThreadLocal<SimpleDateFormat> tl : threadLocals) {            SimpleDateFormat sdf = tl.get();            try {                return sdf.parse(dateStr);            } catch (ParseException e) {                // Ignore and try next date parser            }        }        // All parsers failed        return null;    }       }

As noted before, you need to at least have an ordered list of pattern candidates. Once you have that, Apache DateUtils has a parseDate(String dateString, String[] patterns) method that lets you easily try out a list of patterns on your date string, and parse it by the first one that matches:

public static Date parseDate(String str,                         String[] parsePatterns)                  throws ParseException

Parses a string representing a date by trying a variety of different parsers.

The parse will try each parse pattern in turn. A parse is only deemed successful if it parses the whole of the input string. If no parse patterns match, a ParseException is thrown.

The parser will be lenient toward the parsed date.