Method (computer science)
From Wikipedia, the free encyclopedia
 |
This article or section may be confusing or unclear for most readers. Please improve the article or discuss this issue on the talk page. This article has been tagged since December 2006. |
In object-oriented programming, the term method refers to a subroutine that is exclusively associated either with a class (called class methods, static methods, or factory methods) or with an object (called instance methods). Like a procedure in procedural programming languages, a method usually consists of a sequence of statements to perform an action, a set of input parameters to customize those actions, and possibly an output value (called return value) of some kind. The purpose of methods is to provide a mechanism for accessing (for both reading and writing) the private data stored in an object or a class.
Contents |
[edit] Kinds of methods
As stated above, instance methods are associated with a particular object, while class or static methods (also called shared methods) are instead associated with a class. In typical implementations, instance methods are passed a hidden reference (e.g. this or self or Me) to the object (whether a class or class instance) they belong to, so that they can access the data associated with it. A typical example of a class method would be one that keeps count of the number of created objects within a given class.
Constructors are similar to methods, but in truth they are blocks of statements that are called automatically upon the creation of an instance of a class. In Java, C++, C# and PHP they are distinguished by having the same name as the class of the object they're associated with; in Visual Basic the constructor is called New, and in Object Pascal constructors can have user-defined names (but are mostly called Create). Constructors have some similarities with methods, but plenty of differences, so they aren't usually considered methods.
Destructors are special instance methods that are called automatically upon the destruction of an instance of a class. In C++, they are distinguished by having the same name as the class of the object they're associated with, but with the addition of a tilde (~) in front. In Object Pascal destructors can have user-defined names (but are mostly called Destroy).
An abstract method is a dummy code method which has no implementation. It is often used as a placeholder to be overridden later by a subclass of or an object prototyped from the one that implements the abstract method. In this way, abstract methods help to partially specify a framework.
An accessor method is a method that is usually small, simple and provides the means for the state of an object to be accessed from other parts of a program. Although it introduces a new dependency, use of the methods are preferred to directly accessing state data because they provide an abstraction layer. For example, if a bank-account class provides a getBalance() accessor method to retrieve the current balance (rather than directly accessing the balance data fields), then later revisions of the same code can implement a more complex mechanism balance retrieval (say, a database fetch) without the dependent code needing to be changed. An accessor method that changes the state of an object is often called an update method or, sometimes, mutator method. Objects that provide such methods are considered mutable objects.
[edit] Isolation levels
Whereas a C programmer might push a value onto a stack data-structure by calling:
stackPush(&myStack, value);
a C++ programmer would write:
myStack.push(value);
The difference is the required level of isolation. In C, the stackPush procedure could be in the same source file as the rest of the program and if it was, any other pieces of the program in that source file could see and modify all of the low level details of how the stack was implemented, completely bypassing the intended interface. In C++, regardless of where the class is placed, only the functions which are part of myStack will be able to get access to those low-level details without going through the formal interface functions. Languages such as C can provide comparable levels of protection by using different source files and not providing external linkage to the private parts of the stack implementation but this is less neat and systematic than the more cohesive and enforced isolation of the C++ approach.
[edit] Some recommended usages
A method should preserve the class invariants of the object it is associated with, and should always assume that they are valid when it commences execution. To this effect, preconditions are used to constrain the method's parameters, and postconditions to constrain method's output, if it has one. If any one of either the preconditions or postconditions is not met, a method may raise an exception. If the object's state does not satisfy its class invariants on entry to or exit from any method, the program is considered to have a bug.
The difference between a procedure and a method is that the latter, being associated with a particular object, may access or modify the data private to that object in a way consistent with the intended behavior of the object. Consequently, rather than thinking "a method is just a sequence of commands", a programmer using an object-oriented language will consider a method to be "an object's way of providing a service" (its "method of doing the job", hence the name); a method call is thus considered to be a request to an object to perform some task.
Consequently, method calls are often modeled as a means of passing a message to an object. Rather than directly performing an operation on an object, a message (most of the time accompanied by parameters) is sent to the object telling it what it should do. The object either complies or raises an exception describing why it cannot do so. Applied to our stack example, rather than pushing a value onto the stack, a value is sent to the stack, along with the message "Push!".
[edit] Static (Shared/Class) methods
As mentioned above, a method may be declared as static (or shared in Visual Basic and class in Object Pascal), meaning that it acts at the class level rather than at the instance level. A static method cannot refer to a specific instance of the class (i.e. it cannot refer to this, self, Me, etc.) An example of a static member and its consumption in C# code:
public class ExampleClass
{
public static void StaticExample()
{
// static method code
}
public void InstanceExample()
{
// instance method code here
// can use THIS
}
}
/// Consumer of the above class:
// Static method is called -- no instance is involved ExampleClass.StaticExample();
// Instance method is called ExampleClass objMyExample = new ExampleClass(); objMyExample.InstanceExample();
