Design Pattern # Composite Design Pattern

 The Composite Design Pattern is a structural design pattern used to represent part-whole hierarchies. It allows you to treat individual objects and groups of objects uniformly.

In simple words:
👉 It lets you build tree-like structures where leaf nodes and composite nodes are handled in the same way.

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📌 Why do we need Composite Pattern?

Imagine you are designing a file explorer like Windows Explorer:

• A file is a simple object.

• A folder is a complex object containing files and other folders.

But you still want to perform common operations like:

• open

• delete

• show details

Without worrying whether it's a file or folder.

Composite pattern helps you treat both the same way.

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📌 Structure of Composite Pattern

1. Component (interface or abstract class)

Defines common operations for both leaf & composite.

2. Leaf

Represents the basic object (no children).

3. Composite

Contains leaf or composite children and implements child-related operations.

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📌 Real-Life Examples

Example	Leaf	Composite
File System	File	Folder
Company Structure	Employee	Manager with team
UI Components	Button/Textbox	Panel with child components

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📌 Java Example (Simple and Clean)

// Component
interface Employee {
    void showDetails();
}

// Leaf
class Developer implements Employee {
    private String name;
    public Developer(String name) {
        this.name = name;
    }

    @Override
    public void showDetails() {
        System.out.println("Developer: " + name);
    }
}

// Composite
class Manager implements Employee {
    private String name;
    private List<Employee> team = new ArrayList<>();

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

    public void add(Employee emp) {
        team.add(emp);
    }

    public void remove(Employee emp) {
        team.remove(emp);
    }

    @Override
    public void showDetails() {
        System.out.println("Manager: " + name);
        for (Employee e : team) {
            e.showDetails();
        }
    }
}

// Client
public class CompositeDemo {
    public static void main(String[] args) {
        Developer dev1 = new Developer("Amit");
        Developer dev2 = new Developer("Riya");

        Manager mgr = new Manager("Karan");
        mgr.add(dev1);
        mgr.add(dev2);

        mgr.showDetails();
    }
}

Output

Manager: Karan
Developer: Amit
Developer: Riya

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📌 When to Use Composite Pattern

Use it when:

• Your model forms a tree structure.

• You want to treat single and grouped objects the same way.

• You want uniform operations across all elements.

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📌 Interview One-Liner Answer

Composite Pattern allows you to treat individual objects and compositions of objects uniformly by representing them as a tree structure.

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