Software Engineering is the application of a systematic, disciplined, quantifiable approach to the development, operation, and maintenance of software, and the study of these approaches; that is, the application of engineering to software...
, the composite pattern is a partitioning design pattern
Design pattern (computer science)
In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...
. The composite pattern describes that a group of objects are to be treated in the same way as a single instance of an object. The intent of a composite is to "compose" objects into tree structures to represent part-whole hierarchies. Implementing the composite pattern lets clients treat individual objects and compositions uniformly.
Motivation
When dealing with tree-structured data, programmers often have to discriminate between a leaf-node and a branch. This makes code more complex, and therefore, error prone. The solution is an interface that allows treating complex and primitive objects uniformly. In object-oriented programming
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
, a composite is an object designed as a composition of one-or-more similar objects, all exhibiting similar functionality. This is known as a "has-a
Has-a
In database design and object oriented program architecture, has-a is a relationship where one object "belongs" to another object , and behaves according to the rules of ownership. In simple words, has-a relationship in an object is called a member field of an object...
" relationship between objects. The key concept is that you can manipulate a single instance of the object just as you would manipulate a group of them. The operations you can perform on all the composite objects often have a least common denominator relationship. For example, if defining a system to portray grouped shapes on a screen, it would be useful to define resizing a group of shapes to have the same effect (in some sense) as resizing a single shape.
When to use
Composite can be used when clients should ignore the difference between compositions of objects and individual objects. If programmers find that they are using multiple objects in the same way, and often have nearly identical code to handle each of them, then composite is a good choice; it is less complex in this situation to treat primitives and composites as homogeneous.
Structure
Component
is the abstraction for all components, including composite ones
declares the interface for objects in the composition
(optional) defines an interface for accessing a component's parent in the recursive structure, and implements it if that's appropriate
Leaf
represents leaf objects in the composition .
implements all Component methods
Composite
represents a composite Component (component having children)
implements methods to manipulate children
implements all Component methods, generally by delegating them to its children
Design Patterns: Elements of Reusable Object-Oriented Software is a software engineering book describing recurring solutions to common problems in software design. The book's authors are Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides with a foreword by Grady Booch. The authors are...
, the pattern also involves including the child-manipulation methods in the main Component interface, not just the Composite subclass. More recent descriptions sometimes omit these methods.
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, implements a graphic class, which can be either an ellipse or a composition of several graphics. Every graphic can be printed. In algebraic form,
Graphic = ellipse | GraphicList
GraphicList = empty | Graphic GraphicList
It could be extended to implement several other shapes (rectangle, etc.) and methods (translate
Translation (geometry)
In Euclidean geometry, a translation moves every point a constant distance in a specified direction. A translation can be described as a rigid motion, other rigid motions include rotations and reflections. A translation can also be interpreted as the addition of a constant vector to every point, or...
/** "Composite" */
class CompositeGraphic implements Graphic {
//Collection of child graphics.
private List mChildGraphics = new ArrayList;
//Prints the graphic.
public void print {
for (Graphic graphic : mChildGraphics) {
graphic.print;
}
}
//Adds the graphic to the composition.
public void add(Graphic graphic) {
mChildGraphics.add(graphic);
}
//Removes the graphic from the composition.
public void remove(Graphic graphic) {
mChildGraphics.remove(graphic);
}
}
/** "Leaf" */
class Ellipse implements Graphic {
//Prints the graphic.
public void print {
System.out.println("Ellipse");
}
}
/** Client */
public class Program {
public static void main(String[] args) {
//Initialize four ellipses
Ellipse ellipse1 = new Ellipse;
Ellipse ellipse2 = new Ellipse;
Ellipse ellipse3 = new Ellipse;
Ellipse ellipse4 = new Ellipse;
//Initialize three composite graphics
CompositeGraphic graphic = new CompositeGraphic;
CompositeGraphic graphic1 = new CompositeGraphic;
CompositeGraphic graphic2 = new CompositeGraphic;
//Composes the graphics
graphic1.add(ellipse1);
graphic1.add(ellipse2);
graphic1.add(ellipse3);
graphic2.add(ellipse4);
graphic.add(graphic1);
graphic.add(graphic2);
//Prints the complete graphic (four times the string "Ellipse").
graphic.print;
}
}
