Objects and classes - Data Structures and Problem Solving Using Java - page 72

Figure 3.1 illustrates a class declaration for an IntCell object. 1 The decla-
ration consists of two parts: public and private . Public members represent the
portion that is visible to the user of the object. Since we expect to hide data,
generally only methods and constants would be placed in the public section.
In our example, we have methods that read from and write to the IntCell
object. The private section contains the data; this is invisible to the user of the
object. The storedValue member must be accessed through the publicly visible
routines read and write ; it cannot be accessed directly by main . Another way of
viewing this is shown in Figure 3.2.

Public members
are visible to non-
class routines; pri-
vate members are
not.

Figure 3.3 shows how IntCell objects are used. Since read and write are
members of the IntCell class, they are accessed by using the dot member
operator. The storedValue member could also be accessed by using the dot
member operator, but since it is private , the access at line 14 would be illegal
if it were not commented out.

Members that are
declared private
are not visible to
nonclass routines.

Here is a summary of the terminology. The class defines members , which
may be either fields (data) or methods (functions). The methods can act on the
fields and may call other methods. The visibility modifier public means that the
member is accessible to anyone via the dot operator. The visibility modifier
private means that the member is accessible only by other methods of this class.

A field is a member
that stores data; a
method is a mem-
ber that performs
an action.

figure 3.1
A complete
declaration of an
IntCell class

1 // IntCell class
2 // int read( ) --> Returns the stored value
3 // void write( int x ) --> x is stored
4
5 public class IntCell
6 {
7 // Public methods
8 public int read( ) { return storedValue; }
9 public void write( int x ) { storedValue = x; }
10
11 // Private internal data representation
12 private int storedValue;
13 }

figure 3.2
IntCell members:
read and write are
accessible, but
storedValue is hidden.

read write storedValue

1. Public classes must be placed in files of the same name. Thus IntCell must be in file
IntCell.java . We will discuss the meaning of public at line 5 when we talk about packages.