object-oriented programming

In this chapter, we

• Introduce you to object-oriented programming;

• Show you how you can create objects that belong to classes in the standard Java library; and

• Show you how to write your own classes.
If you do not have a background in object-oriented programming, you will want to read this chapter carefully. 

Thinking about object-oriented programming requires a different way of thinking than for procedural languages. 

The transition is not always easy, but you do need some familiarity with object concepts to go further with Java.

For experienced C++ programmers, this chapter, like the previous chapter, presents familiar information; 

however, there are enough differences between the two languages that you should read the later sections of this chapter carefully. 

You’ll find the C++ notes helpful for making the transition. 

Introduction to Object-Oriented Programming

Object-oriented programming (or OOP for short) is the dominant programming paradigm these days, having replaced the “structured,” 

procedural programming techniques that were developed in the 1970s. 

Java is totally object oriented, and you have to be familiar with OOP to become productive with Java.

An object-oriented program is made of objects. Each object has a specific functionality that is exposed to its users, and a hidden implementation. 

Many objects in your programs will be taken “off-the-shelf” from a library; others are custom designed. 

Whether you build an object or buy it might depend on your budget or on time. 

But, basically, as long as objects satisfy your specifications, you don’t care how the functionality was implemented. 

In OOP, you don’t care how an object is implemented as long as it does what you want.

Traditional structured programming consists of designing a set of procedures (or algorithms) to solve a problem. 

After the procedures were determined, the traditional next step was to find appropriate ways to store the data. 

This is why the designer of the Pascal language, Niklaus Wirth, called his famous book on programming Algorithms + Data Structures = Programs (Prentice Hall, 1975). Notice that in Wirth’s title, algorithms come first, and data structures come second. This
mimics the way programmers worked at that time. 

First, they decided the procedures for manipulating the data; then, they decided what structure to impose on the data to make the manipulations easier. 

OOP reverses the order and puts data first, then looks at the algorithms that operate on the data.

For small problems, the breakdown into procedures works very well. But objects are more appropriate for larger problems. 

Consider a simple web browser. It might require 2,000 procedures for its implementation, all of which manipulate a set of global data. 

In the object-oriented style, there might be 100 classes with an average of 20 methods per class (see Figure 4–1). 

The latter structure is much easier for a programmer to grasp. It is also much easier to find bugs. Suppose the data of a particular object is in an incorrect state. 

It is far easier to search for the culprit among the 20 methods that had access to that data item than among 2,000 procedures.

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Figure 4–1 Procedural vs. OO programming

Classes
A class is the template or blueprint from which objects are made. Thinking about classes as cookie cutters. Objects are the cookies themselves. 

When you construct an object from a class, you are said to have created an instance of the class.

As you have seen, all code that you write in Java is inside a class. 

The standard Java library supplies several thousand classes for such diverse purposes as user interface design, dates and calendars, and network programming. Nonetheless, you still have to create your own classes in Java to describe the objects of the problem
domains of your applications.

Encapsulation (sometimes called information hiding) is a key concept in working with objects. 

Formally, encapsulation is nothing more than combining data and behavior in one package and hiding the implementation details from the user of the object. 

The data in an object are called its instance fields, and the procedures that operate on the data are called its methods. 

A specific object that is an instance of a class will have specific values for its instance fields. The set of those values is the current state of the object. 

Whenever you invoke a method on an object, its state may change.

The key to making encapsulation work is to have methods never directly access instance fields in a class other than their own. 

Programs should interact with object data only through the object’s methods. Encapsulation is the way to give the object its “black box” behavior, 

which is the key to reuse and reliability. This means a class may totally change how it stores its data, 

but as long as it continues to use the same methods to manipulate the data, no other object will know or care.

When you do start writing your own classes in Java, another tenet of OOP makes this easier: classes can be built by extending other classes. 

Java, infact, comes with a “cosmic procedure procedure method Object data

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superclass” called  Object . 

All other classes extend this class. You will see more about the Object class in the next chapter.

When you extend an existing class, the new class has all the properties and methods of the class that you extend. 

You supply new methods and data fields that apply to your new class only. The concept of extending a class to obtain another class is called inheritance. 

See the next chapter for details on inheritance.

Objects

To work with OOP, you should be able to identify three key characteristics of objects:

• The object’s behavior—What can you do with this object, or what methods can you apply to it?

• The object’s state—How does the object react when you apply those methods?

• The object’s identity—How is the object distinguished from others that may have the same behavior and state?
All objects that are instances of the same class share a family resemblance by supporting the same behavior. 

The behavior of an object is defined by the methods that you can call.

Next, each object stores information about what it currently looks like. This is the object’s state. An object’s state may change over time, but not spontaneously. 

A change in the state of an object must be a consequence of method calls. (If the object state changed without a method call on that object, someone broke encapsulation.)

However, the state of an object does not completely describe it, because each object has a distinct identity. 

For example, in an order-processing system, two orders are distinct even if they request identical items. 

Notice that the individual objects that are instances of a class always differ in their identity and usually differ in their state.

These key characteristics can influence each other. For example, the state of an object can influence its behavior. 

(If an order is “shipped” or “paid,” it may reject a method call that asks it to add or remove items. 

Conversely, if an order is “empty,” that is, no items have yet been ordered, it should not allow itself to be shipped.)

Identifying Classes
In a traditional procedural program, you start the process at the top, with the  main function. 

When designing an object-oriented system, there is no “top,” and newcomers to OOP often wonder where to begin. 

The answer is, you first find classes and then you add methods to each class.

A simple rule of thumb in identifying classes is to look for nouns in the problem analysis. Methods, on the other hand, correspond to verbs.

For example, in an order-processing system, some of these nouns are

• Item

• Order

• Shipping address

• Payment

• Account

These nouns may lead to the classes  Item , Order , and so on.

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Next, look for verbs. Items are added to orders. Orders are shipped or canceled. 

Payments are applied to orders. With each verb, such as “add,” “ship,” “cancel,” and “apply,” you identify the one object that has the major responsibility for carrying it out. For example,

when a new item is added to an order, the order object should be the one in charge because it knows how it stores and sorts items. 

That is,  add should be a method of the Order class that takes an Item object as a parameter.

Of course, the “noun and verb” rule is only a rule of thumb, 

and only experience can help you decide which nouns and verbs are the important ones when building your classes.

Relationships between Classes

The most common relationships between classes are

• Dependence (“uses–a”)

• Aggregation (“has–a”)

• Inheritance (“is–a”)

The dependence, or “uses–a” relationship, is the most obvious and also the most general.
For example, the  Order class uses the  Account class because  Order objects need to access Account objects to check for credit status. 

But the  Item class does not depend on the  Account class, because  Item objects never need to worry about customer accounts. 

Thus, a class depends on another class if its methods use or manipulate objects of that class.

Try to minimize the number of classes that depend on each other. The point is, if a class A is unaware of the existence of a class  B , 

it is also unconcerned about any changes to  B !

(And this means that changes to  B do not introduce bugs into  A .) In software engineering terminology
b07b
, you want to minimize the  coupling between classes.
The aggregation, or “has–a” relationship, is easy to understand because it is concrete; 

for example, an  Order object contains  Item objects. Containment means that objects of class  A contain objects of class  B .

NOTE: Some methodologists view the concept of aggregation with disdain and prefer to use a more general “association” relationship. 

From the point of view of modeling, that is understandable. But for programmers, the “has–a” relationship makes a lot of sense. 

We like to use aggregation for a second reason—the standard notation for associations is less clear.

See Table 4–1.
The inheritance, or “is–a” relationship, expresses a relationship between a more special and a more general class. 

For example, a  RushOrder class inherits from an  Order class. 

The specialized  RushOrder class has special methods for priority handling and a different method for computing shipping charges, but its other methods, such as adding items and billing, are inherited from the  Order class. 

In general, if class  A extends class  B , class  A inherits methods from class  B but has more capabilities. 

(We describe inheritance more fully in the next chapter, in which we discuss this important notion at some length.) 

Many programmers use the UML (Unified Modeling Language) notation to draw class diagrams that describe the relationships between classes. 

You can see an example of such a diagram in Figure 4–2. You draw classes as rectangles, and relationships as arrows with various adornments. 

Table 4–1 shows the most common UML arrow styles.

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Figure 4–2 A class diagram
NOTE: A number of tools are available for drawing UML diagrams. Several vendors offer high-powered (and high-priced) tools that aim to be the focal point of your development process. 

Among them are Rational Rose (http://www.ibm.com/software/awdtools/developer/rose) and Together (http://www.borland.com/us/products/together). 

Another choice is the open source program ArgoUML (http://argouml.tigris.org). A commercially supported version is available from GentleWare (http://gentleware.com). 

If you just want to draw a simple diagrams with a minimum of fuss, try out Violet (http://violet.sourceforge.net).

Table 4–1 UML Notation for Class Relationships

Relationship UML Connector

Inheritance

Interface inheritance

Dependency

Aggregation

Association

Directed association

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Using Predefined Classes 111

Using Predefined Classes
Because you can’t do anything in Java without classes, you have already seen several classes at work. 

However, not all of these show off the typical features of object orientation. Take, for example, the  Math class. 

You have seen that you can use methods of the  Math class, such as  Math.random , without needing to know how they are implemented—all you need to know is the name and parameters (if any). That is the point of encapsulation and will certainly be true of
all classes. But the  Math class only encapsulates functionality;

it neither needs nor hides data. Because there is no data, you do not need to worry about making objects and initializing their instance fields—there aren’t any!

In the next section, we look at a more typical class, the  Date class. You will see how to construct objects and call methods of this class.

Objects and Object Variables

To work with objects, you first construct them and specify their initial state. Then you apply methods to the objects.

In the Java programming language, you use constructors to construct new instances. A constructor is a special method whose purpose is to construct and initialize objects. Let us look at an example. The standard Java library contains a  Date class. Its objects
describe points in time, such as “December 31, 1999, 23:59:59 GMT”.
NOTE: You may be wondering: Why use classes to represent dates rather than (as in some languages) a built-in type? 

For example, Visual Basic has a built-in date type and programmers can specify dates in the format #6/1/1995#. 

On the surface, this sounds convenient—programmers can simply use the built-in date type rather than worrying about classes. 

But actually, how suitable is the Visual Basic design? In some locales, dates are specified as month/day/year, in others as day/month/year. 

Are the language designers really equipped to foresee these kinds of issues? 

If they do a poor job, the language becomes an unpleas-ant muddle, but unhappy programmers are powerless to do anything about it.

With classes,the design task is offloaded to a library designer. If the class is not perfect, other programmers can easily write their own classes to enhance or replace the system classes. 

(To prove the point: The Java date library is a bit muddled, and a major redesign is underway;

see http://jcp.org/en/jsr/detail?id=310.)
Constructors always have the same name as the class name. Thus, the constructor for

the Date class is called  Date . To construct a  Date object, you combine the constructor with

the new operator, as follows:

new Date()

This expression constructs a new object. The object is initialized to the current date and

time.

If you like, you can pass the object to a method:

System.out.println(new Date());
Alternatively, you can apply a method to the object that you just constructed. 

One of the methods of the  Date class is the  toString method. That method yields a string representation of the date. 

Here is how you would apply the  toString method to a newly constructed  Date object:

String s = new Date().toString();