Understanding Classes Within Object Oriented
Programming
A class is a collection of objects of similar type. Once a class is defined, any number of objects can be created which belong to that class.
To fully understand object oriented programming, you will want to be familiar with classes. When you look at the world around you, it will become obvious that
many objects are the same type. For example, the car you drive is just one of
the millions of cars that exist in the world.
A D V E R T I S E M E N T
In object-oriented programming, a class is a programming language construct that is used as a blueprint to create objects. This blueprint includes attributes and methods that the created objects all share.
Usually, a class represents a person, place, or thing - it is an abstraction of a concept within a computer program. Fundamentally, it encapsulates the state and behavior of that which it conceptually represents. It encapsulates state through data placeholders called member variables; it encapsulates behavior through reusable code called methods.
More technically, a class is a cohesive package that consists of a particular kind of metadata. It describes the rules by which objects behave; these objects are referred to as instances of that class. A class has both an interface and a structure. The interface describes how the class and its instances can be interacted with via methods, while the structure describes how the data is partitioned into attributes within an instance. A class may also have a representation (metaobject) at runtime, which provides runtime support for manipulating the class-related metadata. In object-oriented design, a class is the most specific type of an object in relation to a specific layer.
Programming languages that support classes all subtly differ in their support for various class-related features. Most support various forms of class inheritance. Many languages also support features providing encapsulation, such as access specifiers.
Classes, when used properly, can accelerate development by reducing redundant code entry, testing and bug fixing. If a class has been thoroughly tested and is known to be a solid work, it is typically the case that using or extending the well-tested class will reduce the number of bugs - as compared to the use of freshly-developed or ad hoc code - in the final output. In addition, efficient class reuse means that many bugs need to be fixed in only one place when problems are discovered.
Another reason for using classes is to simplify the relationships of interrelated data. Rather than writing code to repeatedly call a GUI window drawing subroutine on the terminal screen (as would be typical for structured programming), it is more intuitive to represent the window as an object and tell it to draw itself as necessary. With classes, GUI items that are similar to windows (such as dialog boxes) can simply inherit most of their functionality and data structures from the window class. The programmer then need only add code to the dialog class that is unique to its operation. Indeed, GUIs are a very common and useful application of classes, and GUI programming is generally much easier with a good class framework.
A class is used to create new instances (objects) by instantiating the class.
Instances of a class share the same set of attributes, yet may differ in what those attributes contain. For example, a class "Person" would describe the attributes common to all instances of the Person class. Each person is generally alike, but varies in such attributes as "height" and "weight". The class would list types of such attributes and also define the actions which a person can perform: "run", "jump", "sleep", "walk", etc. One of the benefits of programming with classes is that all instances of a particular class will follow the defined behavior of the class they instantiate.
In most languages, the structures as defined by the class determine how the memory used by its instances will be laid out. This technique is known as the cookie-cutter model. The alternative to the cookie-cutter model is the model of Python, wherein objects are structured as associative key-value containers. In such models, objects that are instances of the same class could contain different instance variables, as state can be dynamically added to the object. This may resemble prototype-based languages in some ways, but it is not equivalent.
Along with having an interface, a class contains a description of structure of data stored in the instances of the class. The data is partitioned into attributes (or properties, fields, data members). Going back to the television set example, the myriad attributes, such as size and whether it supports color, together comprise its structure. A class represents the full description of a television, including its attributes (structure) and buttons (interface).
A class also describes a set of invariants that are preserved by every method in the class. An invariant is a constraint on the state of an object that should be satisfied by every object of the class. The main purpose of the invariants is to establish what objects belong to the class. An invariant is what distinguishes data types and classes from each other; that is, a class does not allow use of all possible values for the state of the object, and instead allows only those values that are well-defined by the semantics of the intended use of the data type. The set of supported (public) methods often implicitly establishes an invariant. Some programming languages support specification of invariants as part of the definition of the class, and enforce them through the type system. Encapsulation of state is necessary for being able to enforce the invariants of the class.
Some languages allow an implementation of a class to specify constructor (or initializer) and destructor (or finalizer) methods that specify how instances of the class are created and destroyed, respectively. A constructor that takes arguments can be used to create an instance from passed-in data. The main purpose of a constructor is to establish the invariant of the class, failing if the invariant isn't valid. The main purpose of a destructor is to destroy the identity of the instance, invalidating any references in the process. Constructors and destructors are often used to reserve and release, respectively, resources associated with the object. In some languages, a destructor can return a value which can then be used to obtain a public representation (transfer encoding) of an instance of a class and simultaneously destroy the copy of the instance stored in current thread's memory.
A class may also contain static attributes or class attributes, which contain data that are specific to the class yet are common to all instances of the class. If the class itself is treated as an instance of a hypothetical metaclass, static attributes and static methods would be instance attributes and instance methods of that metaclass.
As a data type, a class is usually considered as a compile-time construct. A language may also support prototype or factory metaobjects that represent run-time information about classes, or even represent metadata that provides access to reflection facilities and ability to manipulate data structure formats at run-time. Many languages distinguish this kind of run-time type information about classes from a class on the basis that the information is not needed at run-time. Some dynamic languages do not make strict distinctions between run-time and compile-time constructs, and therefore may not distinguish between metaobjects and classes.
There are many categories of classes depending on modifiers. Note that these categories do not necessarily categorize classes into distinct partitions. For example, while it is impossible to have a class that is both abstract and concrete, a sealed class is implicitly a concrete class, and it may be possible to have an abstract partial class.
- Concrete classes:A concrete class is a class that can be instantiated.
- Abstract classes:An abstract class, or abstract base class (ABC), is a class that cannot be instantiated. Such a class is only meaningful if the language supports inheritance. An abstract class is designed only as a parent class from which child classes may be derived. Abstract classes are often used to represent abstract concepts or entities. The incomplete features of the abstract class are then shared by a group of subclasses which add different variations of the missing pieces.
Abstract classes are superclasses which contain abstract methods and are defined such that concrete subclasses are to extend them by implementing the methods. The behaviors defined by such a class are "generic" and much of the class will be undefined and unimplemented. Before a class derived from an abstract class can become concrete, i.e. a class that can be instantiated, it must implement particular methods for all the abstract methods of its parent classes.
- Sealed classes:Some languages also support sealed classes. A sealed class cannot be used as a base class. For this reason, it cannot also be an abstract class. Sealed classes are primarily used to prevent derivation. They add another level of strictness during compile-time, improve memory usage, and trigger certain optimizations that improve run-time efficiency.
- Local and inner classes:In some languages, classes can be declared in scopes other than the global scope. There are various types of such classes.
One common type is an inner class or nested class, which is a class defined within another class. Since it involves two classes, this can also be treated as another type of class association. The methods of an inner class can access static methods of the enclosing class(es). An inner class is typically not associated with instances of the enclosing class, i.e. an inner class is not instantiated along with its enclosing class. Depending on language, it may or may not be possible to refer to the class from outside the enclosing class. A related concept is inner types (a.k.a. inner data type, nested type), which is a generalization of the concept of inner classes. C++ is an example of a language that supports both inner classes and inner types.
Another type is a local class, which is a class defined within a procedure or function. This limits references to the class name to within the scope where the class is declared. Depending on the semantic rules of the language, there may be additional restrictions on local classes compared non-local ones. One common restriction is to disallow local class methods to access local variables of the enclosing function. For example, in C++, a local class may refer to static variables declared within its enclosing function, but may not access the function's automatic variables.
- Named vs. anonymous classes:In most languages, a class is bound to a name or identifier upon definition. However, some languages allow classes to be defined without names. Such a class is called an anonymous class
- Metaclasses:Metaclasses are classes whose instances are classes. A metaclass describes a common structure of a collection of classes. A metaclass can implement a design pattern or describe a shorthand for particular kinds of classes. Metaclasses are often used to describe frameworks.
- Partial classes:Partial classes are classes that can be split over multiple definitions (typically over multiple files), making it easier to deal with large quantities of code. At compile time the partial classes are grouped together, thus logically make no difference to the output. An example of the use of partial classes may be the separation of user interface logic and processing logic. A primary benefit of partial classes is allowing different programmers to work on different parts of the same class at the same time. They also make automatically generated code easier to interpret, as it is separated from other code into a partial class.
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