Classes and Objects: A Deeper Look - Java: How to Program Early Objects - page 324

Error-Prevention Tip 8.1
Most IDEs will issue a warning if you say x = x; instead of this.x = x; . The statement
x = x; is often called a no-op (no operation).

Method buildString (lines 31-36) returns a String created by a statement that uses
the this reference explicitly and implicitly. Line 34 uses it explicitly to call method toUni-
versalString . Line 35 uses it implicitly to call the same method. Both lines perform the
same task. You typically will not use this explicitly to reference other methods within the
current object. Also, line 45 in method toUniversalString explicitly uses the this refer-
ence to access each instance variable. This is not necessary here, because the method does
not have any local variables that shadow the instance variables of the class.

Performance Tip 8.1
Java conserves storage by maintaining only one copy of each method per class—this method
is invoked by every object of the class. Each object, on the other hand, has its own copy of
the class's instance variables. Each method of the class implicitly uses this to determine
the specific object of the class to manipulate.

Class ThisTest 's main method (lines 6-10) demonstrates class SimpleTime . Line 8
creates an instance of class SimpleTime and invokes its constructor. Line 9 invokes the
object's buildString method, then displays the results.

8.5 Time Class Case Study: Overloaded Constructors
As you know, you can declare your own constructor to specify how objects of a class should
be initialized. Next, we demonstrate a class with several overloaded constructors that en-
able objects of that class to be initialized in different ways. To overload constructors, sim-
ply provide multiple constructor declarations with different signatures.

Class Time2 with Overloaded Constructors
The Time1 class's default constructor in Fig. 8.1 initialized hour , minute and second to
their default 0 values (i.e., midnight in universal time). The default constructor does not
enable the class's clients to initialize the time with nonzero values. Class Time2 (Fig. 8.5)
contains five overloaded constructors that provide convenient ways to initialize objects. In
this program, four of the constructors invoke a fifth, which in turn ensures that the value
supplied for hour is in the range 0 to 23, and the values for minute and second are each
in the range 0 to 59. The compiler invokes the appropriate constructor by matching the
number, types and order of the types of the arguments specified in the constructor call
with the number, types and order of the types of the parameters specified in each construc-
tor declaration. Class Time2 also provides set and get methods for each instance variable.

1 // Fig. 8.5: Time2.java

2 // Time2 class declaration with overloaded constructors.

3
4 public class Time2

5 {

Fig. 8.5 | Time2 class with overloaded constructors. (Part 1 of 4.)