Add string constant TASK_ADDED_MESSAGE in AddHandler in order to return a success message when a task has been added. Fix a bug where the TaskBuilder doesn't set the endDate even when the parser has found that the user did specify an endDate.
Remove DEFAULT_START_HOUR_VALUE, DEFAULT_START_MINUTE_VALUE, DEFAULT_END_HOUR_VALUE, DEFAULT_END_MINUTE_VALUE, string constants. Now when the user specifies only the date, without the time, the time attributes will be set to null. Also, if user only specifies the startTime, then endTime will be set to startTime..
Remove INVALID_INPUT_MESSAGE string constant.
Change setStartDateYear, setEndDateYear, setStartTimeHour, setEndTimeHour functions to properly update hasStartDate, hasEndTime, hasStartTime, hasEndTime attributes. They will set these attributes to true only if they don't receive a null parameter.
Implement comprehensive test cases to test AddHandler and execute function in AddHandler.java. These two functions are tested in one jUnit test function. Implement comprehensive test cases to test parse function in AddParserTest.java.