Classes and Objects: A Deeper Look - Java: How to Program Early Objects - page 322

MemberAccessTest.java:9: hour has private access in Time1
time.hour = 7; // error: hour has private access in Time1
^
MemberAccessTest.java:10: minute has private access in Time1
time.minute = 15; // error: minute has private access in Time1
^
MemberAccessTest.java:11: second has private access in Time1
time.second = 30; // error: second has private access in Time1
^
3 errors

Fig. 8.3 | Private members of class Time1 are not accessible. (Part 2 of 2.)

Common Programming Error 8.1
An attempt by a method that's not a member of a class to access a private member of that
class generates a compilation error.

8.4 Referring to the Current Object's Members with the
this Reference
Every object can access a reference to itself with keyword this (sometimes called the this
reference ). When an instance method is called for a particular object, the method's body
implicitly uses keyword this to refer to the object's instance variables and other methods.
This enables the class's code to know which object should be manipulated. As you'll see in
Fig. 8.4, you can also use keyword this explicitly in an instance method's body.
Section 8.5 shows another interesting use of keyword this . Section 8.11 explains why
keyword this cannot be used in a static method.
We now demonstrate implicit and explicit use of the this reference (Fig. 8.4). This
example is the first in which we declare two classes in one file—class ThisTest is declared
in lines 4-11, and class SimpleTime in lines 14-47. We do this to demonstrate that when
you compile a .java file containing more than one class, the compiler produces a separate
class file with the .class extension for every compiled class. In this case, two separate files
are produced— SimpleTime.class and ThisTest.class . When one source-code ( .java )
file contains multiple class declarations, the compiler places both class files for those classes
in the same directory. Note also in Fig. 8.4 that only class ThisTest is declared public . A
source-code file can contain only one public class—otherwise, a compilation error occurs.
Non- public classes can be used only by other classes in the same package . So, in this
example, class SimpleTime can be used only by class ThisTest .

1 // Fig. 8.4: ThisTest.java

2 // this used implicitly and explicitly to refer to members of an object.

3
4 public class ThisTest

5 {

6 public static void main(String[] args)

7 {

Fig. 8.4 | this used implicitly and explicitly to refer to members of an object. (Part 1 of 2.)