Answer:
Line of code:
Correct or Not?
String line = "The Sky was like a WaterDrop" ;
correct
String a = line.toLowerCase();
correct
String b = toLowerCase( line );
NOT correct
String c = toLowerCase( "IN THE SHADOW OF A THORN");
NOT correct
String d = "Clear, Tranquil, Beautiful".toLowerCase();
correct
System.out.println( "Dark, forlorn...".toLowerCase() );
correct

The "correct" answer for the last two lines might surprise you, but those 
lines are correct (although perhaps not very sensible.) Here is why:

Temporary Objects

String d     =  "Clear, Tranquil, Beautiful".toLowerCase();
                 ---------------+-----------     ---+---
   |                            |                   |
   |                            |                   |
   |             First:  a temporary String         |
   |                     object is created          |
   |                     containing these           |
   |                     these characters.          |
   |                                                |
   |                                               Next: the toLowerCase() method of
   |                                                     the temporary object is
Finally:  the reference to the second                    called. It creates a second 
          object is assigned to the                      object, containing all lower
          reference variable, d.                         case characters.
           

The temporary object is used to construct a second object. The reference to the second object is assigned to d. Now look at the last statement:

System.out.println( "Dark, forlorn...".toLowerCase() );

A String is constructed. Then a second String is constructed (by the toLowerCase() method). A reference to the second String is used as a parameter for println(). Both String objects are temporary. After println() finishes, both Strings are garbage.

(There is nothing special about temporary objects. What makes them temporary is how the program uses them. The objects in the above statement are temporary because the program saves no reference to them. The garbage collector will soon recycle them.)

Question 23:
Review:
Can an object reference variable exist without referring to an object?
Can an object exist without an object reference variable that refers to it?