The Composite Design Pattern

The Composite Pattern is designed to build a class hierarchy from primitive and composite objects, that may represent complex structures, but have similar behavior.

What problems does it solve?

The pattern is used to implement a tree-like, hierarchical structure of elements, that have same behavior. The elements can be primitive (leafs) or composite objects. Composite objects have children, which might be leafs or composite objects.

Glossary:

• Composition - the abstraction of the composites and the leafs, their common functionality
• Leaf - primitive objects in the composition - they implement the composition, but doesn’t have children
• Composite - composite objects in the composition - they implement the child behavior, bu also have children of type Composition
• Client - The client operates with the compositions in a general manner. In fact the client doesn’t even know if it’s a leaf or composite.

Pros:

• Client knows only about the general behavior off all components and treats them the same way.
• to decouple client
• to achieve good level of abstraction

Cons:

• It is hard to restrict the components of the composite

How to recognize it?

When you have behavioral methods, that take an instance of same abstract/interface type into and create or process a tree structure.

Folder homeDir = new Folder("Home");

Folder documentsDir = new Folder("Personal Documents");
homeDir.addChild(documentsDir);

documentsDir.addChild(new File("CV.doc"));

Examples from Java API

java.awt.Container#add(Component) (practically all over Swing thus)
javax.faces.component.UIComponent#getChildren() (practically all over JSF UI thus)

Scenarios

• Use it for a tree-like structure, when all elements have some general behavior

• Like a FileSystem, where you have tree of folders and files and you can move them, delete the etc (general behavior).

Example 1

We have hierarchical folder structure, that may contain other folders in files.

Source Code

1). Entry Interface (our Composition)

public interface Entry {

    String getName();

    void setName(String name);

    void ls(boolean recursive);
}

2). File Class (our Leaf or Primitive object)

public class File implements Entry {

    @Getter
    @Setter
    private String name;

    public File(String name) {
        this.name = name;
    }

    @Override
    public void ls(boolean recursive) {
        System.out.println(getName());
    }

}

3). Folder Class (our Composite object, that extends the primitive behavior, but also has children)

public class Folder extends File {

    private List<Entry> children = new ArrayList<>();

    public Folder(String name) {
        super(name);
    }

    public void addChild(Entry child) {
        this.children.add(child);
        child.setName(getName() + "\\" + child.getName());
    }

    @Override
    public void ls(boolean recursive) {
        System.out.println(getName());
        for (Entry c : this.children) {
            if(recursive){
                c.ls(true);
            }else{
                System.out.println(c.getName());
            }
        }
    }
}

4). FileSystemClient class (our client)

public class FSClient {

    public static void ls(Entry entry){
        entry.ls(false);
    }

    public static void lsDeep(Entry entry){
        entry.ls(true);
    }
}

1. Main class

   public class _Main {
   
    public static void main(String[] args) {
   
        Folder homeDir = new Folder("Home");
   
        Folder documentsDir = new Folder("Personal Documents");
        homeDir.addChild(documentsDir);
   
        documentsDir.addChild(new File("CV.doc"));
        documentsDir.addChild(new File("Diplom.pdf"));
        documentsDir.addChild(new File("ID_Photo.png"));
   
        Folder examsDir = new Folder("Exams");
        documentsDir.addChild(examsDir);
   
        Folder examAEDir = new Folder("AE");
        examsDir.addChild(examAEDir);
        examAEDir.addChild(new File("AE_Book.pdf"));
        examAEDir.addChild(new File("AE_Sample_Tests.pdf"));
   
        Folder examDEDir = new Folder("DE");
        examsDir.addChild(examDEDir);
        examDEDir.addChild(new File("DE_Book.pdf"));
        examDEDir.addChild(new File("DE_Sample_Tests.pdf"));
   
   
        FSClient.lsDeep(homeDir);
    }
   }
   

   Output:

   Home
   Home\Personal Documents
   Home\Personal Documents\CV.doc
   Home\Personal Documents\Diplom.pdf
   Home\Personal Documents\ID_Photo.png
   Home\Personal Documents\Exams
   Home\Personal Documents\Exams\AE
   Home\Personal Documents\Exams\AE\AE_Book.pdf
   Home\Personal Documents\Exams\AE\AE_Sample_Tests.pdf
   Home\Personal Documents\Exams\DE
   Home\Personal Documents\Exams\DE\DE_Book.pdf
   Home\Personal Documents\Exams\DE\DE_Sample_Tests.pdf