In object-oriented programming, the iterator pattern is a design pattern in which an iterator is used to traverse a container and access the container's elements. The iterator pattern decouples algorithms from containers; in some cases, algorithms are necessarily container-specific and thus cannot be decoupled.
For example, the hypothetical algorithm SearchForElement can be implemented generally using a specified type of iterator rather than implementing it as a container-specific algorithm. This allows SearchForElement to be used on any container that supports the required type of iterator.
Video Iterator pattern
Overview
The Iterator design pattern is one of the twenty-three well-known GoF design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software, that is, objects that are easier to implement, change, test, and reuse.
What problems can the Iterator design pattern solve?
- The elements of an aggregate object should be accessed and traversed without exposing its representation (data structures).
- New traversal operations should be defined for an aggregate object without changing its interface.
Defining access and traversal operations in the aggregate interface is inflexible because it commits the aggregate to particular access and traversal operations and makes it impossible to add new operations later without having to change the aggregate interface.
What solution does the Iterator design pattern describe?
- Define a separate (iterator) object that encapsulates accessing and traversing an aggregate object.
- Clients use an iterator to access and traverse an aggregate without knowing its representation (data structures).
Different iterators can be used to access and traverse an aggregate in different ways.
New access and traversal operations can be defined independently by defining new iterators.
See also the UML class and sequence diagram below.
Maps Iterator pattern
Definition
The essence of the Iterator Pattern is to "Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.".
Structure
UML class and sequence diagram
In the above UML class diagram, the Client
class refers (1) to the Aggregate
interface for creating an Iterator
object (createIterator()
) and (2) to the Iterator
interface for traversing an Aggregate
object (next(),hasNext()
). The Iterator1
class implements the Iterator
interface by accessing the Aggregate1
class.
The UML sequence diagram shows the run-time interactions: The Client
object calls createIterator()
on an Aggregate1
object, which creates an Iterator1
object and returns it to the Client
. The Client
uses then Iterator1
to traverse the elements of the Aggregate1
object.
UML class diagram
Language-specific implementation
Some languages standardize syntax. C++ and Python are notable examples.
C#
.NET Framework has special interfaces that support a simple iteration: System.Collections.IEnumerator
over a non-generic collection and System.Collections.Generic.IEnumerator<T>
over a generic collection.
C# statement foreach
is designed to easily iterate through the collection that implements System.Collections.IEnumerator
and/or System.Collections.Generic.IEnumerator<T>
interface. Since C# v2, foreach
is also able to iterate through types that implement System.Collections.Generic.IEnumerable<T>
and System.Collections.Generic.IEnumerator<T>
Example of using foreach
statement:
C++
C++ implements iterators with the semantics of pointers in that language. In C++, a class can overload all of the pointer operations, so an iterator can be implemented that acts more or less like a pointer, complete with dereference, increment, and decrement. This has the advantage that C++ algorithms such as std::sort
can immediately be applied to plain old memory buffers, and that there is no new syntax to learn. However, it requires an "end" iterator to test for equality, rather than allowing an iterator to know that it has reached the end. In C++ language, we say that an iterator models the iterator concept.
Java
Java has the Iterator
interface.
As of Java 5, objects implementing the Iterable
interface, which returns an Iterator
from its only method, can be traversed using Java's foreach loop syntax. The Collection
interface from the Java collections framework extends Iterable
.
JavaScript
JavaScript, as part of ECMAScript 6, supports the iterator pattern with any object that provides a next()
method, which returns an object with two specific properties: done
and value
. Here's an example that shows a reverse array iterator:
Most of the time, though, what you want is to provide Iterator semantics on objects so that they can be iterated automatically via for...of
loops. Some of JavaScript's built-in types such as Array
, Map
, or Set
already define their own iteration behavior. You can achieve the same effect by defining an object's meta @@iterator
method, also referred to by Symbol.iterator
. This creates an Iterable object.
Here's an example of a range function that generates a list of values starting from start
to end
, exclusive, using a regular for
loop to generate the numbers:
The iteration mechanism of built-in types, like strings, can also be manipulated:
PHP
PHP supports the iterator pattern via the Iterator interface, as part of the standard distribution. Objects that implement the interface can be iterated over with the foreach
language construct.
Example of patterns using PHP:
OUTPUT
string(15) "Design Patterns" string(16) "PHP7 is the best" string(13) "Laravel Rules" string(9) "DHH Rules"
Python
Python prescribes a syntax for iterators as part of the language itself, so that language keywords such as for
work with what Python calls sequences. A sequence has an __iter__()
method that returns an iterator object. The "iterator protocol" requires next()
return the next element or raise a StopIteration
exception upon reaching the end of the sequence. Iterators also provide an __iter__()
method returning themselves so that they can also be iterated over e.g., using a for
loop. Generators are available since 2.2.
In Python 3, next()
was renamed __next__()
.
See also
- Composite pattern
- Container (data structure)
- Design pattern (computer science)
- Iterator
- Observer pattern
References
External links
- Object iteration in PHP
- Iterator Pattern in C#
- Iterator pattern in UML and in LePUS3 (a formal modelling language)
- SourceMaking tutorial
- Design Patterns implementation examples tutorial
- Iterator Pattern
Source of article : Wikipedia