Method (computer science)

In object-oriented programming, a method is a subroutine that is exclusively associated either with a class (in which case it is called a class method or a static method) or with an object (in which case it is an instance method). Like a subroutine in procedural programming languages, a method usually consists of a sequence of programming statements to perform an action, a set of input parameters to customize those actions, and possibly an output value (called the return value) of some kind. Methods provide a mechanism for accessing and manipulating the encapsulated data stored in an object.

Types of methods
As stated above, instance methods are associated with a particular object, while class or static methods are associated with a class. In a typical implementation, instance methods are passed a hidden reference (e.g.,   or  ) to the object (whether a class or class instance) they belong to, so that they can access the data associated with it. For class/static methods this may or may not happen according to the language; A typical example of a class method would be one that keeps count of the number of created objects within a given class.

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  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. A method that changes the state of an object is no longer called an accessor method, but rather an update method, a modifier method, or a mutator method. Objects that provide such methods are considered mutable objects.

Many languages support methods, called constructors, that are called automatically upon the creation of an instance of a class. Some languages have a special syntax for constructors. In Java, C++, C#, ActionScript, and PHP they have the same name as the class of which they are a member (PHP 5 also allows  as a constructor); in Visual Basic .NET the constructor is called. Object Pascal constructors are signified by the keyword  and can have user-defined names (but are mostly called  ). Under Objective-C the constructor method is split between two methods,  and , with the   method setting aside memory for an instance of the class and the   method handling the bulk of initializing the instance; a call to the   method invokes both the   and the   method for the class instance.

Likewise, some languages have special destructor methods, i.e. 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 (or  in PHP 5). In Object Pascal destructors have the keyword  and can have user-defined names (but are mostly called Destroy). Under Objective-C the destructor method is named.

CLASS METHOD-Sometime you need to access program code when you do not have an instance of a particular object available.A method that is marked using the keyword static can be used in this way and it is sometime called a class method.

Isolation levels
Whereas a C programmer might push a value onto a stack data-structure by calling:

a C++ programmer would write:

The difference is the required level of isolation. In C, the  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 methods which are part of  will be able to get access to those low-level details without going through the formal interface methods. 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.

Some recommended usages
A public 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 (private methods do not necessarily need to follow this recommendation). 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".

Static methods
As mentioned above, a method may be declared as static, meaning that it acts at the class level rather than at the instance level. Therefore, a static method cannot refer to a specific instance of the class (i.e. it cannot refer to,  ,  , etc.), unless such references are made through a parameter referencing an instance of the class, although in such cases they must be accessed through the parameter's identifier instead of. An example of a static member and its consumption in C# code:

Confusingly, methods marked as  in Object Pascal also cannot refer to a class object, as can class methods in Python or Smalltalk. For example, this Python method can create an instance of  or of any subclass of it, because it receives a reference to a class object as  :