Classes & Objects Computer Science I Last updated 9/30/10

Object-Oriented Programming OOP: A programming paradigm that uses "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs Programming techniques may include … Data abstraction Encapsulation Modularity Polymorphism Inheritance

Background Object-oriented programming has roots that can be traced to the 1960s As hardware and software became increasingly complex, manageability often became a concern Researchers studied ways to maintain software quality and developed object-oriented programming in part to address common problems by strongly emphasizing discrete, reusable units of programming logic

Background OOP focuses on data rather than processes Programs are composed of self-sufficient modules ("classes") Each instance of which ("objects") contains all the information needed to manipulate its own data structure ("members") Modular programming focuses on the function of a module, rather than specifically the data MP provides for code reuse, and self-sufficient reusable units of programming logic, enabling collaboration through the use of linked modules (subroutines)

Background An object-oriented program may be viewed as a collection of interacting objects Each object is capable of … receiving messages, processing data, and sending messages to other objects

Background Each object can be viewed as an independent 'machine' with a distinct role or responsibility Actions (methods) on these objects are closely associated with the object For example … OOP data structures tend to carry their own operators around with them Or at least inherit them from a similar object or class In the conventional model, the data and operations on the data don't have a tight, formal association

What is a class? A template for an object A user-defined datatype that contains the variables, properties and methods in it Defines the abstract characteristics of a thing (object), including its characteristics and the thing's behaviors For example … The class Dog would consist of traits shared by all dogs, such as breed and fur color (characteristics), and the ability to bark and sit (behaviors)

What is a class? Provides modularity and structure in an object-oriented computer program Should typically be recognizable to a non-programmer familiar with the problem domain, meaning that the characteristics of the class should make sense in context The code should be relatively self-contained Collectively, the defined properties and methods are called members

What is a class? One can have an instance of a class; the instance is the actual object created at run-time For example … The Lassie object is an instance of the Dog class The set of attribute values for a particular object is called its state The object consists of state and the behavior that's defined in the object's classes

What is an object? A discrete bundle of functions and procedures, often relating to a particular real-world concept such as a bank account holder or hockey player Other pieces of software can access the object only by calling its functions and procedures that have been allowed to be called by outsiders Some agree that isolating objects in this way makes their software easier to manage and keep track of Others feel that software becomes more complex to maintain and document, or even to engineer from the start

OOP Features Dynamic Encapsulation (or multi-methods, in which case the state is kept separate) Polymorphism Inheritance (or delegation) Open recursion

What is a method? "The process by which an object sends data to another object or asks the other object to invoke a method.“ * Also known as interfacing For example… The object called Breeder may tell the Lassie object to sit by passing a sit message that invokes Lassie's sit() method The syntax varies between languages * Armstrong, The Quarks of Object-Oriented Development. In descending order of popularity, the "quarks" are: Inheritance, Object, Class, Encapsulation, Method, Message Passing, Polymorphism, Abstraction

Dynamic Dispatch When a method is invoked on an object, the object itself determines what code gets executed by looking up the method at run time in a table associated with the object This feature distinguishes an object from an abstract data type (or module), which has a fixed (static) implementation of the operations for all instances It is a programming methodology that gives modular component development while at the same time being very efficient

Encapsulation Conceals the functional details of a class from objects that send messages to it For example … The Dog class has a bark() method variable The code for the bark() method defines exactly how a bark happens Timmy, Lassie's friend, however, does not need to know exactly how she barks Encapsulation is achieved by specifying which classes may use the members of an object The result is that each object exposes to any class a certain interface — those members accessible to that class

Encapsulation Rationale: Prevents clients of an interface from depending on those parts of the implementation that are likely to change in the future Thereby allowing those changes to be made more easily without changes to clients For example… An interface can ensure that puppies can only be added to an object of the class Dog by code in that class Members are often specified as public, protected or private, determining whether they are available to all classes, sub-classes or only the defining class

Polymorphism Allows the programmer to treat derived class members just like their parent class's members More precisely, the ability of objects belonging to different data types to respond to calls of methods of the same name, each one according to an appropriate type-specific behavior One method, or an operator such as +, -, or *, can be abstractly applied in many different situations For example … If a Dog is commanded to speak(), this may elicit a bark() If a Pig is commanded to speak(), this may elicit an oink()

Inheritance A process in which a class inherits all the state and behavior of another class This is called child-parent or is-a relationship Subclasses are more specialized versions of a class, which inherit attributes and behaviors from their parent classes, and can introduce their own

Inheritance For example … The class Dog might have sub-classes called Collie, Chihuahua, and GoldenRetriever Lassie would be an instance (object) of the Collie subclass Assume that the Dog class defines a method called bark() and a property called furColor Each of the sub-classes will inherit these members; the programmer only needs to write the code for them once

Inheritance Each subclass can alter its inherited traits For example … The Collie subclass might specify that the default furColor for a collie is brown-and-white The Chihuahua subclass might specify that the bark() method produces a high pitch by default

Inheritance Subclasses can also add new members For example … The Chihuahua subclass could add a method called tremble() An individual Chihuahua instance would then use a high-pitched bark() from the Chihuahua subclass, which in turn inherited the usual bark() from Dog The Chihuahua object would also have the tremble() method, but Lassie would not, because she is a Collie, not a Chihuahua

Inheritance Inheritance is an "a… is a" relationship between classes, while instantiation is an "is a" relationship between an object and a class For example … a Collie is a Dog ("a… is a"), but Lassie is a Collie ("is a") Thus, the object named Lassie has the methods from both classes Collie and Dog

Open recursion A special variable (syntactically it may be a keyword), usually called this or self, that allows a method body to invoke another method body of the same object This variable is late-bound ; it allows a method defined in one class to invoke another method that is defined later, in some subclass thereof