Composite pattern is one of the Structural design pattern. Composite design pattern is used when we have to represent a part-whole hierarchy.
When we need to create a structure in a way that the objects in the structure has to be treated the same way, we can apply composite design pattern. Lets understand it with a real life example - A diagram is a structure that consists of Objects such as Circle, Lines, Triangle etc. When we fill the drawing with color (say Red), the same color also gets applied to the Objects in the drawing. Here drawing is made up of different parts and they all have same operations. Composite Pattern consists of following objects.
Here I am applying composite design pattern for the drawing scenario.
Composite pattern base component defines the common methods for leaf and composites. We can create a class Shape
with a method draw(String fillColor)
to draw the shape with given color. Shape.java
package com.journaldev.design.composite;
public interface Shape {
public void draw(String fillColor);
}
Composite design pattern leaf implements base component and these are the building block for the composite. We can create multiple leaf objects such as Triangle, Circle etc. Triangle.java
package com.journaldev.design.composite;
public class Triangle implements Shape {
@Override
public void draw(String fillColor) {
System.out.println("Drawing Triangle with color "+fillColor);
}
}
Circle.java
package com.journaldev.design.composite;
public class Circle implements Shape {
@Override
public void draw(String fillColor) {
System.out.println("Drawing Circle with color "+fillColor);
}
}
A composite object contains group of leaf objects and we should provide some helper methods to add or delete leafs from the group. We can also provide a method to remove all the elements from the group. Drawing.java
package com.journaldev.design.composite;
import java.util.ArrayList;
import java.util.List;
public class Drawing implements Shape{
//collection of Shapes
private List<Shape> shapes = new ArrayList<Shape>();
@Override
public void draw(String fillColor) {
for(Shape sh : shapes)
{
sh.draw(fillColor);
}
}
//adding shape to drawing
public void add(Shape s){
this.shapes.add(s);
}
//removing shape from drawing
public void remove(Shape s){
shapes.remove(s);
}
//removing all the shapes
public void clear(){
System.out.println("Clearing all the shapes from drawing");
this.shapes.clear();
}
}
Notice that composite also implements component and behaves similar to leaf except that it can contain group of leaf elements. Our composite pattern implementation is ready and we can test it with a client program.
TestCompositePattern.java
package com.journaldev.design.test;
import com.journaldev.design.composite.Circle;
import com.journaldev.design.composite.Drawing;
import com.journaldev.design.composite.Shape;
import com.journaldev.design.composite.Triangle;
public class TestCompositePattern {
public static void main(String[] args) {
Shape tri = new Triangle();
Shape tri1 = new Triangle();
Shape cir = new Circle();
Drawing drawing = new Drawing();
drawing.add(tri1);
drawing.add(tri1);
drawing.add(cir);
drawing.draw("Red");
drawing.clear();
drawing.add(tri);
drawing.add(cir);
drawing.draw("Green");
}
}
Output of the above composite pattern client program is:
Drawing Triangle with color Red
Drawing Triangle with color Red
Drawing Circle with color Red
Clearing all the shapes from drawing
Drawing Triangle with color Green
Drawing Circle with color Green
java.awt.Container#add(Component)
is a great example of Composite pattern in java and used a lot in Swing. Earlier structural design pattern articles:
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Great example. The Shape interface name should be Drawable to follow naming conventions and to describe what the interface adds to a class. Basically a drawing is a composition of drawable elements such as shapes and lines. The class name Shape would be better suited for an Abstract Class.
- Stephane Tremblay
Hi Pankaj your blog is really helpful to me. I just want to say thanks to you.
- taotao
This is not composite pattern. check this for right explanation https://www.dofactory.com/net/composite-design-pattern. in composite pattern there have to be nodes and leaves to form a tree. Just aggregating objects into another object is not a composite pattern.
- ravi
Nice article pankaj, you are the champ.
- Manoj Singh
Thanks a lot ! Your tutorials were so useful for me !!! and i’m sure, i’m not the only one ! Design Patterns especially, That got me a good mark in my exams :)))) Thanks; another time !!! Good Continuation !
- Nabil Amrouche
Really nice post. help me a lot to get the concept.
- perminder singh
It really help me a lot… Can you give an example of composite pattern that has a GUI Thank you!!!
- Zhacantal
I like the example you have provided.
- Amit
Hi Pankaj, Thanks for sharing best core java tutorial for freshers and experience. Its really helpful for java developer. kindly request to you please share Spring MVC+Hibernate application as per industry stranded and useful for better performance. Thanks and Regards Yogesh Kharode
- Yogesh Kharode