Java Homework #6 Model Solution

1. How do you create a Date for the current time?  How do you display the
current time?

ANSWER
Date now = new Date();
System.out.println( "Date is " + now.toString() );  // Don't need ".toString()"

2. How do you do the following?
a. Create an ArrayList for storing double values?
   ANSWER - ArrayList<Double> d = new ArrayList<Double>();

b. Append an object to a list?
   ANSWER - list.add(obj)

c. Insert an object at the beginning of a list?
   ANSWER - list.add(0,obj)

d. Find the number of objects in a list?
   ANSWER = list.size();

e. Remove a given object from a list?
   ANSWER - list.remove(obj)

f. Remove the last object from the list?
   ANSWER - list.remove(list.size() - 1)

g. Check whether a given object is in a list?
   ANSWER - list.contains(obj)

h. Retrieve an object at a specified index from a list?
   ANSWER - list.get(i)

3. Identify the errors in the following code.

Array.list<String> list = new ArrayList<String> ();
list.add("Denver");
list.add("Austin");
list.add(new java.util.Date());   <ERROR - cannot add Date to String>
String city = list.get(0);
list.set(3, "Dallas");            <ERROR - no element at index 3>
System.out.println(list.get(3));  <ERROR - no element at index 3>

4. Suppose the ArrayList list contains duplicate elements. Does the
following code correctly remove the element from the array list?  If
not, correct the code.

      for ( int i = 0; i < list.size(); i++ )
          list.remove(element);

      NO.  This attempts to remove the same element list.size() times.
      To remove duplicates in a list, one way is like so:

      int pos = list.indexOf(element);  // Position of 1st copy of element
      for ( int i = 0; i < list.size(); i++ ) {  // Remove all copies of element
        if ( element.equals( list.get(i) )
            list.remove(i);
      }
      list.add(pos, element);  // Add back the first one, in the right position.

      There are several other possible ways: you could use an iterator in
      a while-loop.  You could use a loop to remove all copies except the
      last one (I like this approach the best):

      if ( list.indexOf(element) != -1 ) {
         while ( list.indexOf(element) != list.lastIndexOf(element ) )
           list.remove(element);
      }

      Finally, if the order of the elements doesn't matter, you should have
      used a Set instead of a List, which automatically discards duplicates.
      You can try this advanced technique to de-duplicate a list (stolen from
      a post on StackExchange):


      Set deDupSet = new LinkedHashSet(list);
      list.clear();
      list.addAll( deDupSet );

5. Explain why the following code displays [1, 3] rather than [2, 3].

      ArrayList<Integer> list = new ArrayList<Integer>();
      list.add(1);
      list.add(2);
      list.add(3);
      list.remove(1);
      System.out.println(list);

    ANSWER - list.remove(1) removed the index, not the value.  Needed to
    do this: list.remove(new Integer(1))

6. How do you create an object of the java.util.Calendar class, for the current time?

    ANSWER - Calendar calendar = Calendar.newInstance();
    This is also okay:
    Calendar calendar = new GregorianCalendar();

7. For a Calendar object c, how do you get its year, month, date, hour,
minute, and second?

    ANSWER - Calendar calendar = Calendar.getInstance();
             System.out.println ("year is " + calendar.get(Calendar.YEAR));
             System.out.println ("month is " + calendar.get(Calendar.MONTH));
             System.out.println ("date is " + calendar.get(Calendar.DAY_OF_MONTH));
             System.out.println ("hour is " + calendar.get(Calendar.HOUR));
             System.out.println ("minute is " + calendar.get(Calendar.MINUTE));
             System.out.println ("second is " + calendar.get(Calendar.SECOND));

    (Note how month is shown as 0..11, not 1..12!)

8. Using the java.time API, how can you add one month to a LocalDate object?
How can you create a LocalDate object that represents the last Friday of
the current month?

ANSWER -
    import java.time.*;
    import static java.time.DayOfWeek.*;
    import static java.time.format.DateTimeFormatter.ofLocalizedDate;
    import static java.time.format.FormatStyle.FULL;
    import static java.time.temporal.ChronoUnit.*;  // MONTHS
    import static java.time.temporal.TemporalAdjusters.lastInMonth;

    class Question8 {
       public static void main ( String [] args ) {
          LocalDate today = LocalDate.now();

          // Add one month to today:
          LocalDate nextMonth = today.plus( 1, MONTHS );
// OR:    LocalDate nextMonth = today.plusMonths( 1 );
          System.out.println( "One month from now is: "
             + nextMonth.format( ofLocalizedDate(FULL) ) );

          // Last Friday of current month:
          System.out.println( "Last Friday of current month is: "
             + today.with( lastInMonth(FRIDAY) )
                    .format( ofLocalizedDate(FULL) ) );
       }
    }