Java Reference
In-Depth Information
public static int
getValue(
int
setNo,
int
i,
int
j) {
29
30
static method
return
dates[setNo][i][j];
31 }
32 }
This class uses a three-dimensional array to store dates (lines 2-22). You could use a dif-
ferent data structure (i.e., five two-dimensional arrays for representing five sets of numbers).
The implementation of the
getValue
method would change, but the program that uses
GuessDate
wouldn't need to change as long as the contract of the public method
getValue
remains unchanged. This shows the benefit of data encapsulation.
The class defines a private no-arg constructor (line 25) to prevent the user from creating
objects for this class. Since all methods are static in this class, there is no need to create
objects from this class.
benefit of data encapsulation
private constructor
10.12
Why is the no-arg constructor in the
Math
class defined private?
✓
✓
Check
Point
Class design guidelines are helpful for designing sound classes.
Key
Point
You have learned how to design classes from the preceding two examples and from many
other examples in the preceding chapters. This section summarizes some of the guidelines.
10.11.1 Cohesion
A class should describe a single entity, and all the class operations should logically fit
together to support a coherent purpose. You can use a class for students, for example, but you
should not combine students and staff in the same class, because students and staff are differ-
ent entities.
A single entity with many responsibilities can be broken into several classes to separate the
responsibilities. The classes
String
,
StringBuilder
, and
StringBuffer
all deal with
strings, for example, but have different responsibilities. The
String
class deals with
immutable strings, the
StringBuilder
class is for creating mutable strings, and the
StringBuffer
class is similar to
StringBuilder
except that
StringBuffer
contains
synchronized methods for updating strings.
coherent purpose
separating responsibilities
10.11.2 Consistency
Follow standard Java programming style and naming conventions. Choose informative names
for classes, data fields, and methods. A popular style is to place the data declaration before the
constructor and place constructors before methods.
Make the names consistent. It is not a good practice to choose different names for similar
operations. For example, the
length()
method returns the size of a
String
, a
StringBuilder
, and a
StringBuffer
. It would be inconsistent if different names were
used for this method in these classes.
In general, you should consistently provide a public no-arg constructor for constructing a
default instance. If a class does not support a no-arg constructor, document the reason. If no con-
structors are defined explicitly, a public default no-arg constructor with an empty body is assumed.
If you want to prevent users from creating an object for a class, you can declare a private
constructor in the class, as is the case for the
Math
class and the
GuessDate
class.
naming conventions
naming consistency
no-arg constructor
10.11.3 Encapsulation
A class should use the
private
modifier to hide its data from direct access by clients. This
makes the class easy to maintain.
encapsulating data fields
