Chapter 2

The Essence of Objects

So, what exactly is Object Orientation? It is a problem solving technique used to develop software systems. Object orientation is the culmination of years of experience in finding an effective way to develop software, and is certainly the dominant method used to develop major software systems today. Object  orientation is a technique of modeling a real-world system in software based on objects. The object is the core concept. An object is a software model of a real-world entity or concept. Almost anything can be modeled as an object in software. For example, you could model a temperature sensor as an object. Or, in a more abstract system, you could model color as an object. Even something as basic as a number can be considered an object that has a value and a type. Typically, each object has an associated set of attributes such as value, state, or whatever else is needed to model the object. Sensor attributes might include a state such as active or inactive, an attribute indicating its current value, and information about its physical location. Objects usually provide the ability to modify their state as well. In addition to keeping track of the current temperature, for example, a temperature sensor object might provide

a way to turn the sensor on or off. The attributes, or data, used inside an object are really only a tiny part of what an object is and does. An object also has a set of. responsibilities that it carries out by providing services to other

While a program is running, individual objects usually don't stand alone. They belong to a collection of other similar objects that all are members of the same group, or class. A program will be made up of many different classes, each class made up of similar objects.

class A class is a description of a set of objects. The set of objects share common attributes and common behavior. Class is similar in concept to abstract data types found in non- OO programming languages, but is more comprehensive in that it includes both structure and behavior. A class definition describes all the attributes of member objects of that class, as well as the class methods that implement the behavior of member objects.

object The basic unit of object orientation. An object is an entity that has attributes, behavior, and identity. Objects are members of a class, and the attributes and behavior of an object are defined by the class definition.

Classes and objects are closely related, but are not the same thing. A class is a description or definition of the characteristics of objects that belong to that class. An object is a single instance or member of a class that is created and exists while the program is running. A class may have just a single object or any number of member objects existing at any given time. All members of a class have similar behavior. For example, consider a software system used to monitor various sensors in a factory. One obvious kind of object present is such a system is a sensor. A class called Sensor would be defined and used to model physical sensors. The class would define the basic characteristics of any Sensor, such as its location, value, and identification number, as well as a set of services used to carry out its responsibilities.

What Is an Object-Oriented System?

Just what is an object-oriented system? What makes an OO system different than other software systems? One way to define an objectoriented system is to use a list of properties that characterize objectoriented systems. A non-object-oriented system might share some properties such as using abstraction or encapsulation, but will not be built using objects or classes. It is also possible to use an objectoriented language to implement a system using classes or objects, but the system must have all the following properties to be considered a true object-oriented system.

Any object-oriented software system will have the following

properties:

1. Abstraction with objects

2. Encapsulated classes

3. Communication via messages

4. Object lifetime

5. Class hierarchies

6. Polymorphism

The next section gives a brief overview of each of these properties, while the following sections give more detailed explanations of each.

object orientation A method of developing software that uses abstraction with objects, encapsulated classes, communication via messages, object lifetime, class hierarchies, and polymorphism.

Fundamental Properties of an Object-Oriented System

Abstraction with objects

An abstraction is a mechanism that allows a complex, real-world situation to be represented using a simplified model. Object orientation abstracts the real world based on objects and their interactions with other objects. For example, one possible abstraction of a color is the RGB model.

abstraction A model of a real-world object or concept.

Encapsulated classes

The abstractions of related ideas are encapsulated into a single unit. The states and behaviors of the abstraction are incorporated into an encapsulated whole, called a class. The actual internal implementation of the states and behaviors is hidden from the rest of the system. While this not a new programming technique, in OO the encapsulation is an inherent and integral part of the system and design. Earlier, we described a Color class that provided a way to change it red, green, or blue values. In fact, as long as the outside world continues to see and use a Color object in a consistent way, it wouldn't matter just how color is represented internally by the Color object. It could use either the HSV (hue, saturation, value) color model or the RGB model internally, and the outside world would be unaffected. The state and behavior of objects are controlled by welldefined and restricted interfaces to the object. Encapsulation ensures that the internal details of an object are hidden from the rest of the world, that each object maintains its own unique identity and state, and that the state can only be changed by well-defined messages.

encapsulation The process of hiding all the internal details of an object from the outside world. In Java, encapsulation is enforced by having the definitions for attributes and methods inside a class definition.

Interaction via messages

In order to accomplish useful tasks, objects need to interact with other objects. The interaction can be between objects of the same class, or objects of different classes. This interaction is handled by sending messages (in Java, this is done by calling methods) to other objects to pass information or request action. For example, when a user selects a command button in a dialog box on the screen by clicking the mouse, a message is sent to the dialog object notifying it that the command button was pressed. Messages can be used to change the state of an object or to request an action by the object.

Object lifetime

All objects have a lifetime. They are created and initialized as they are needed during program execution, exist and carry out their functions, and are eventually destroyed. While objects exist, they maintain their own identity and state. Many objects that are instances of the same class can exist at any given time. Each object has a unique identity, and will have attributes that are different from other instances of objects in the same class.

Class hierarchies

In an OO design, classes of objects are arranged into hierarchies that model and describe relationships among the classes. The simplest relationship is an association. For example, there could be an employment association between a person and a company. These simple associations exist between different classes.

A second way to use a hierarchy is to define more specialized classes based on a pre-existing generalized class. For example, a dialog class can be considered a specialized case of a more general window class. The more specialized class will automatically share the attributes of the more general class (e.g., size and screen position), and will probably add new attributes and behaviors to the generalized class (e.g., associated control objects). This kind of hierarchy is called inheritance.

Polymorphism

Polymorphism is the final fundamental characteristic of objectoriented systems. When inheritance is used to extend a generalized class to a more specialized class, it will usually include extending some of the behaviors of the generalized class. The specialized class will often implement a behavior that is somewhat different than the generalized class, but the name used to define the behavior will be the same. It is important that a given instance of an object use the correct behavior, and the property of polymorphism allows this to happen automatically and seamlessly. Polymorphism is actually easier to use than it is to explain.

Abstraction with Objects

Abstraction is one of the basic tools of all programming, not just OO programming. When trying to write a program to solve some real world problem, abstraction serves as a way model the real world problem. For example, if you were trying to write an address book program, you would use abstractions such as names, addresses, phone numbers, alphabetical order, and other concepts associated with an address book.

Encapsulated Classes

Encapsulation is one of the most important aspects of OO. It is what allows each object to be independent. The exact implementation of attributes and of object behavior is hidden from the rest of the world through encapsulation. The class definition is the main programming tool that is used to implement encapsulation. A class is a description of a collection of objects with common attributes, behavior, and responsibilities. The definition or specification of a class includes the definitions of the attributes comprising the state, the methods that carry out the responsibilities of the class by implementing the behavior, and how to set the initial attribute state of an object. A class is identified by a name.

As long as you maintain a well-defined interface to the rest of the world, you can easily modify your class definition without breaking the rest of the system.

attribute Used to hold state information of an object. An attribute might be as simple as an on or off boolean variable, or it might be a complex structure such as another object. A class definition describes the attributes and operations (methods) of a class.

behavior The activity of an object that is visible to the outside world. Includes how an object responds to messages by changing its internal state or returning state information to other objects.

method An operation or service performed upon an object, defined as part of the declaration of a class. Methods are used to implement object behavior. Synonyms for method include member function, operation, and service.

state State reflects the current values of all the attributes of a given object, and is the result of the behavior of an object over time.

Java programs are defined as collections of classes. Normally each Java class is defined in a separate file. The attributes of a class are defined by the declaration of variables of various types such as int or boolean. A Java class includes the definitions of the methods used to implement the behaviors of the class. The method definitions are integral parts of the class definition.

Communication via messages

Messages are how objects communicate with each other. Any object may send a message to other objects, and it may receive messages from other objects. In practical programming terms, sending a message is accomplished by invoking or calling some class method, and receiving a message is accomplished by having a class method called by a different object. Usually, a message is sent by a method call as a normal part of the execution of the program logic of an object. However, messages may also originate from the operating system interface or language runtime system. Consider an object that implements a dialog interface with a user. When the user clicks on a button, a message is sent to the dialog object or button handler telling it that a specific button has been pressed (the implementation specifics aren't important).

Object orientation is a natural for this kind of programming. In an OO program, what you often end up with is a set of objects that can respond to a set of messages originating from a variety of sources such as a mouse click, a sensor value change, or a data base transaction. Individual encapsulated objects can respond to messages and send their own messages to others objects in response. Objects in the system interact via well-defined messages with other objects in the system.

Object Lifetime

Objects are dynamic entities. They are created on an as-needed basis during program execution. When an object is created, it is instantiated, or bound to the class definition. An instantiated member of a class is called an object, or equivalently an instance. When a new object first comes into existence, a special method called the constructor is automatically invoked by the run-time system. The constructor is responsible for giving an instance its initial state. Once an object has been created, it can receive and send messages. To get an idea of object lifetime, consider graphical user interface (GUI) classes such as the Swing library provided with Java. One type of object included in a GUI is a dialog interface. Upon some action by the user, say selecting a menu item, a given dialog object will be created. As part of the creation process, its constructor will be called.

When the user closes the dialog, the dialog object will no longer be needed. Once it no longer has any references to it, it can be garbage collected by the Java run-time.

constructor An operation that creates an object and defines its initial state. For complex objects, construction can be a significant activity, and cause the constructors of other objects to be invoked as well.

garbage collection The automatic detection and freeing of memory that is no longer in use. An object becomes available for garbage collection when it is no longer referred to by any other object. Java uses garbage collection rather than explicit destructors found in other OO languages such as C++.

reference A data element whose value is an address. In Java, all objects are accessed by reference. Any object variable will be a reference to an actual object, and not the object itself.

identity The characteristics or state of an object that allows it to be distinguished from other objects.

instance A specific object that is an instantiation of a class. An instance has specific attributes and behaviors, and a unique identity. Instance and object are often used interchangeably.

instantiation Creating an instance of an object of a given class. Instantiating an instance brings it into existence.

object lifetime The time an object exists - from its instantiation in its constructor until the object no longer exists and has been finalized by the Java garbage collector. The creation point of an object is under program control, but the exact moment when an object ceases to exist cannot be determined because of the way the Java garbage collector works. Basic UML Class Notation The basic UML notation for a class is a rectangle with three horizontal parts. The top part is used to hold the name of the class.  Associations are shown by lines between classes, and are usually labeled with the name of the association. Inheritance is shown by a line with a triangular arrow pointing to the more general class (the superclass). Aggregation is shown by a line with a hollow diamond pointing to the whole class, while composition uses a solid diamond instead.

Figure 2-4.

An animal Generalization/Specialization Hierarchy

Note that the UML uses an open arrowhead pointing to the more general class to show inheritance. Thus, in Figure 2-4 of the animal hierarchy, a Mammal inherits from Animal, and so on.

UML: You can show more or less detail in UML diagrams depending which information you want to concentrate on. For example, since Figure 2-4 is mainly focused on the inheritance relationships, and it doesn't show the attribute or operation boxes. We will use this same diagram in Figure 2-5, but show more detail, including some selected operations.

Many discussions of object orientation will give special emphasis to programming with inheritance, but in fact, both aggregation and inheritance are important parts of object-oriented programming. Both kinds of hierarchies are used to design programs that reflect the characteristics of the problem being modeled. Any class can be defined using a combination of both kinds of hierarchy.

default behaviors In an inheritance hierarchy, the class behaviors defined by superclasses that will be used by default unless they are overridden by some subclass.

derived In an inheritance hierarchy, a subclass is derived from a superclass. The derived subclass inherits the attributes and methods of the parent superclass.

generalization/specialization An inheritance hierarchy. Each subclass is a specialization of a more generalized superclass.

implements In Java, a specification that the class will implement the code required by an interface.

inheritance A mechanism that allows one class (subclass) to share the attributes and behaviors of another class (superclass). Inheritance defines an is-a relationship between classes. The subclass or derived class inherits the attributes and behaviors of the superclass, and will usually extend or modify those attributes and behaviors.

interface In Java, an interface is a specification of methods a class using the interface must implement. An interface is a specification, and does not define any code. It provides an alternative to multiple inheritance.

is-a A term used in inheritance hierarchies. In general, a

subclass is-a specialized kind of a more general superclass.

Primate

Rodent

Carnivore

Mammal

is-a test A simple test to check for proper inheritance design. If

you cannot say a subclass is-a kind of the superclass, then

inheritance is probably not appropriate.

multiple inheritance When a subclass is derived from multiple superclasses, it is said to have single inheritance. Java does not allow multiple inheritance, but provides interfaces as an alternative.

overriding When a subclass specifies an alternative definition for an attribute or method of its superclass, it is overriding the definition in the superclass. Also called overloading. Java can only overload methods.

root class The top most or most generalized user class of an inheritance hierarchy. In Java, all classes are at least implicitly derived from the Java Object class, which make it the most primitive root class. Most applications will have many hierarchies with different non-Object root classes.

single inheritance When a subclass is derived from a single superclass, it is said to have single inheritance.

subclass In an inheritance hierarchy, a subclass is derived from an associated superclass. A subclass is a specialization of the generalized superclass.

superclass In an inheritance hierarchy, a superclass is a more generalized class. A subclass will be derived from a superclass. (A superclass is also known as a parent class or a base class.)