format java date from string MM/dd/yyyy HH:mm:ss a

String datetimeString = "6/7/2013 2:55:44 PM";  // A String representing your date
java.util.Date result = new Date();
DateFormat formatter = new SimpleDateFormat("MM/dd/yyyy hh:mm:ss a");

// Below Will Throw Exception in thread "main" java.text.ParseException: Unparseable date:
// SimpleDateFormat formatter = new SimpleDateFormat("MM-dd-yyyy"); 

result = formatter.parse(datetimeString); // Throws ParseException
System.out.println("Parse Java Util Date  = "+result);
System.out.println("formatted: " + formatter.format(result));

Output is:

Parse Java Util Date  = Fri Jun 07 14:55:44 GMT+05:30 2013

formatted: 06/07/2013 02:55:44 PM

Refer below posts for more examples:
http://viralpatel.net/blogs/check-string-is-valid-date-java/
http://docs.oracle.com/javase/1.5.0/docs/api/java/text/SimpleDateFormat.html