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Compose objects into tree structures to represent part-whole hierarchies.
Composite lets clients treat individual objects and compositions of
objects uniformly.
Frequency of use:  medium high
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The classes and/or objects participating in this pattern are:
- Component (DrawingElement)
- declares the interface for objects in the composition.
- implements default behavior for the interface common to all classes, as
appropriate.
- declares an interface for accessing and managing its child components.
- (optional) defines an interface for accessing a component's parent in the
recursive structure, and implements it if that's appropriate.
- Leaf (PrimitiveElement)
- represents leaf objects in the composition. A leaf has no children.
- defines behavior for primitive objects in the composition.
- Composite (CompositeElement)
- defines behavior for components having children.
- stores child components.
- implements child-related operations in the Component interface.
- Client (CompositeApp)
- manipulates objects in the composition through the Component interface.
This structural code demonstrates the Composite pattern which allows the creation
of a tree structure in which individual nodes are accessed uniformly whether
they are leaf nodes or branch (composite) nodes.
Show code
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// Composite pattern -- Structural example
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using System;
using System.Collections;
namespace DoFactory.GangOfFour.Composite.Structural
{
// MainApp test application
class MainApp
{
static void Main()
{
// Create a tree structure
Composite root = new Composite("root");
root.Add(new Leaf("Leaf A"));
root.Add(new Leaf("Leaf B"));
Composite comp = new Composite("Composite X");
comp.Add(new Leaf("Leaf XA"));
comp.Add(new Leaf("Leaf XB"));
root.Add(comp);
root.Add(new Leaf("Leaf C"));
// Add and remove a leaf
Leaf leaf = new Leaf("Leaf D");
root.Add(leaf);
root.Remove(leaf);
// Recursively display tree
root.Display(1);
// Wait for user
Console.Read();
}
}
// "Component"
abstract class Component
{
protected string name;
// Constructor
public Component(string name)
{
this.name = name;
}
public abstract void Add(Component c);
public abstract void Remove(Component c);
public abstract void Display(int depth);
}
// "Composite"
class Composite : Component
{
private ArrayList children = new ArrayList();
// Constructor
public Composite(string name) : base(name)
{
}
public override void Add(Component component)
{
children.Add(component);
}
public override void Remove(Component component)
{
children.Remove(component);
}
public override void Display(int depth)
{
Console.WriteLine(new String('-', depth) + name);
// Recursively display child nodes
foreach (Component component in children)
{
component.Display(depth + 2);
}
}
}
// "Leaf"
class Leaf : Component
{
// Constructor
public Leaf(string name) : base(name)
{
}
public override void Add(Component c)
{
Console.WriteLine("Cannot add to a leaf");
}
public override void Remove(Component c)
{
Console.WriteLine("Cannot remove from a leaf");
}
public override void Display(int depth)
{
Console.WriteLine(new String('-', depth) + name);
}
}
}
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Output
-root
---Leaf A
---Leaf B
---Composite X
-----Leaf XA
-----Leaf XB
---Leaf C
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This real-world code demonstrates the Composite pattern used in building a graphical
tree structure made up of primitive nodes
(lines, circles, etc) and composite nodes (groups of
drawing elements that make up more complex elements).
Show code
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// Composite pattern -- Real World example
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using System;
using System.Collections;
namespace DoFactory.GangOfFour.Composite.RealWorld
{
// Mainapp test application
class MainApp
{
static void Main()
{
// Create a tree structure
CompositeElement root =
new CompositeElement("Picture");
root.Add(new PrimitiveElement("Red Line"));
root.Add(new PrimitiveElement("Blue Circle"));
root.Add(new PrimitiveElement("Green Box"));
CompositeElement comp =
new CompositeElement("Two Circles");
comp.Add(new PrimitiveElement("Black Circle"));
comp.Add(new PrimitiveElement("White Circle"));
root.Add(comp);
// Add and remove a PrimitiveElement
PrimitiveElement pe =
new PrimitiveElement("Yellow Line");
root.Add(pe);
root.Remove(pe);
// Recursively display nodes
root.Display(1);
// Wait for user
Console.Read();
}
}
// "Component" Treenode
abstract class DrawingElement
{
protected string name;
// Constructor
public DrawingElement(string name)
{
this.name = name;
}
public abstract void Add(DrawingElement d);
public abstract void Remove(DrawingElement d);
public abstract void Display(int indent);
}
// "Leaf"
class PrimitiveElement : DrawingElement
{
// Constructor
public PrimitiveElement(string name) : base(name)
{
}
public override void Add(DrawingElement c)
{
Console.WriteLine(
"Cannot add to a PrimitiveElement");
}
public override void Remove(DrawingElement c)
{
Console.WriteLine(
"Cannot remove from a PrimitiveElement");
}
public override void Display(int indent)
{
Console.WriteLine(
new String('-', indent) + " " + name);
}
}
// "Composite"
class CompositeElement : DrawingElement
{
private ArrayList elements = new ArrayList();
// Constructor
public CompositeElement(string name) : base(name)
{
}
public override void Add(DrawingElement d)
{
elements.Add(d);
}
public override void Remove(DrawingElement d)
{
elements.Remove(d);
}
public override void Display(int indent)
{
Console.WriteLine(new String('-', indent) +
"+ " + name);
// Display each child element on this node
foreach (DrawingElement c in elements)
{
c.Display(indent + 2);
}
}
}
}
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Output
-+ Picture
--- Red Line
--- Blue Circle
--- Green Box
---+ Two Circles
----- Black Circle
----- White Circle
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This .NET optimized code demonstrates the
same real-world situation as above but uses modern, built-in .NET features.
Show code
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// Composite pattern -- .NET optimized
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See our Singleton page for a .NET optimized code sample.
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