Java Reference
In-Depth Information
The focus of this chapter is on class design and explores the differences between
procedural programming and object-oriented programming.
Key
Point
The preceding two chapters introduced objects and classes. You learned how to define classes,
create objects, and use objects from several classes in the Java API (e.g.,
Date
,
Random
,
String
,
StringBuilder
, and
Scanner
). This topic's approach is to teach problem solving
and fundamental programming techniques before object-oriented programming. This chapter
shows how procedural and object-oriented programming differ. You will see the benefits of
object-oriented programming and learn to use it effectively.
We will use several examples to illustrate the advantages of the object-oriented approach.
The examples involve designing new classes and using them in applications. We first intro-
duce some language features supporting these examples.
You can define immutable classes to create immutable objects. The contents of
immutable objects cannot be changed.
Key
Point
Normally, you create an object and allow its contents to be changed later. However, occasion-
ally it is desirable to create an object whose contents cannot be changed once the object has
been created. We call such an object an
immutable object
and its class an
immutable class
.
The
String
class, for example, is immutable. If you deleted the
set
method in the
CircleWithPrivateDataFields
class in Listing 8.9, the class would be immutable,
because radius is private and cannot be changed without a
set
method.
If a class is immutable, then all its data fields must be private and it cannot contain public
set
methods for any data fields. A class with all private data fields and no mutators is not
necessarily immutable. For example, the following
Student
class has all private data fields
and no
set
methods, but it is not an immutable class.
VideoNote
Immutable objects and
this
keyword
immutable object
immutable class
1
2
public class
Student {
Student
class
private int
id;
3
4
5
6
public
Student(
int
ssn, String newName) {
7 id = ssn;
8
9 dateCreated =
new
java.util.Date();
10 }
11
12
private
String name;
private
java.util.Date dateCreated;
name = newName;
public int
getId() {
13
return
id;
14 }
15
16
public
String getName() {
17
return
name;
18 }
19
20
21
public
java.util.Date getDateCreated() {
return
dateCreated;
22 }
23 }
As shown in the following code, the data field
dateCreated
is returned using the
getDateCreated()
method. This is a reference to a
Date
object. Through this reference,
the content for
dateCreated
can be changed.
