Intent

It is belongs to structural design patterns catalog.
Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.

Explanation

Real world example

Every sentence is composed of words which are in turn composed of characters. Each of these objects is printable and they can have something printed before or after them like sentence always ends with full stop and word always has space before it

In plain words

Composite pattern lets clients treat the individual objects in a uniform manner.

Wikipedia says

In software engineering, the composite pattern is a partitioning design pattern. The composite pattern describes that a group of objects is 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.

Structure

Participants

1. Component

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.

2. Leaf

represents leaf objects in the composition. A leaf has no children.

defines behavior for primitive objects in the composition.

3. Composite

defines behavior for components having children.

stores child components.

implements child-related operations in the Component interface.

4. Client

manipulates objects in the composition through the Component interface.

Collaborations

Clients use the Component class interface to interact with objects in the composite structure. If the recipient is a Leaf, then the request is handled directly. If the recipient is a Composite, then it usually forwards requests to its child components, possibly performing additional operations before and/or after forwarding.

Source Code (Example 1)

Taking our sentence example from above. Here we have the base class and different printable types

public abstract class LetterComposite {
  private List<LetterComposite> children = new ArrayList<>();
  public void add(LetterComposite letter) {
    children.add(letter);
  }
  public int count() {
    return children.size();
  }
  protected void printThisBefore() {}
  protected void printThisAfter() {}
  public void print() {
    printThisBefore();
    for (LetterComposite letter : children) {
      letter.print();
    }
    printThisAfter();
  }
}

public class Letter extends LetterComposite {
  private char c;
  public Letter(char c) {
    this.c = c;
  }
  @Override
  protected void printThisBefore() {
    System.out.print(c);
  }
}

public class Word extends LetterComposite {
  public Word(List<Letter> letters) {
    for (Letter l : letters) {
      this.add(l);
    }
  }
  @Override
  protected void printThisBefore() {
    System.out.print(" ");
  }
}

public class Sentence extends LetterComposite {
  public Sentence(List<Word> words) {
    for (Word w : words) {
      this.add(w);
    }
  }
  @Override
  protected void printThisAfter() {
    System.out.print(".");
  }
}

Then we have a messenger to carry messages

public class Messenger {
  LetterComposite messageFromOrcs() {
    List<Word> words = new ArrayList<>();
    words.add(new Word(Arrays.asList(new Letter('W'), new Letter('h'), new Letter('e'), new Letter('r'), new Letter('e'))));
    words.add(new Word(Arrays.asList(new Letter('t'), new Letter('h'), new Letter('e'), new Letter('r'), new Letter('e'))));
    words.add(new Word(Arrays.asList(new Letter('i'), new Letter('s'))));
    words.add(new Word(Arrays.asList(new Letter('a'))));
    words.add(new Word(Arrays.asList(new Letter('w'), new Letter('h'), new Letter('i'), new Letter('p'))));
    words.add(new Word(Arrays.asList(new Letter('t'), new Letter('h'), new Letter('e'), new Letter('r'), new Letter('e'))));
    words.add(new Word(Arrays.asList(new Letter('i'), new Letter('s'))));
    words.add(new Word(Arrays.asList(new Letter('a'))));
    words.add(new Word(Arrays.asList(new Letter('w'), new Letter('a'), new Letter('y'))));
    return new Sentence(words);
  }

  LetterComposite messageFromElves() {
    List<Word> words = new ArrayList<>();
    words.add(new Word(Arrays.asList(new Letter('M'), new Letter('u'), new Letter('c'), new Letter('h'))));
    words.add(new Word(Arrays.asList(new Letter('w'), new Letter('i'), new Letter('n'), new Letter('d'))));
    words.add(new Word(Arrays.asList(new Letter('p'), new Letter('o'), new Letter('u'), new Letter('r'), new Letter('s'))));
    words.add(new Word(Arrays.asList(new Letter('f'), new Letter('r'), new Letter('o'), new Letter('m'))));
    words.add(new Word(Arrays.asList(new Letter('y'), new Letter('o'), new Letter('u'), new Letter('r'))));
    words.add(new Word(Arrays.asList(new Letter('m'), new Letter('o'), new Letter('u'), new Letter('t'), new Letter('h'))));
    return new Sentence(words);
  }
}

And then it can be used as

LetterComposite orcMessage = new Messenger().messageFromOrcs();
orcMessage.print(); // Where there is a whip there is a way.
LetterComposite elfMessage = new Messenger().messageFromElves();
elfMessage.print(); // Much wind pours from your mouth.

Source Code (Example 2)

Step 1 : Create an Employee interface that will be treated as a component.

/ this is the Employee interface i.e. Component.  
public interface Employee {  
    public  int getId();  
    public String getName();  
    public double getSalary();  
       public void print();  
    public void add(Employee employee);  
       public void remove(Employee employee);  
       public Employee getChild(int i);  
}// End of the Employee interface.

Step 2 : Create a BankManager class that will be treated as a Composite and implements Employee interface.

// this is the BankManager class i.e. Composite.  
import java.util.ArrayList;  
import java.util.Iterator;  
import java.util.List;  
public class BankManager implements Employee {  
     private int id;  
     private String name;  
     private double salary;  
  
     public BankManager(int id,String name,double salary) {  
      this.id=id;      
      this.name = name;  
      this.salary = salary;  
     }  
         List<Employee> employees = new ArrayList<Employee>();  
     @Override  
     public void add(Employee employee) {  
        employees.add(employee);  
     }  
        @Override  
     public Employee getChild(int i) {  
      return employees.get(i);  
     }  
     @Override  
     public void remove(Employee employee) {  
      employees.remove(employee);  
     }    
     @Override  
     public int getId()  {  
      return id;  
     }  
     @Override  
     public String getName() {  
      return name;  
     }  
    @Override  
     public double getSalary() {  
      return salary;  
     }  
     @Override  
     public void print() {  
      System.out.println("==========================");  
      System.out.println("Id ="+getId());  
      System.out.println("Name ="+getName());  
      System.out.println("Salary ="+getSalary());  
      System.out.println("==========================");  
        
      Iterator<Employee> it = employees.iterator();  
        
          while(it.hasNext())  {  
            Employee employee = it.next();  
            employee.print();  
         }  
     }  
}// End of the BankManager class.

Step 3 : Create a Cashier class that will be treated as a leaf and it will implement to the Employee interface.

public  class Cashier implements Employee{  
    /* 
         In this class,there are many methods which are not applicable to cashier because 
         it is a leaf node. 
     */  
        private int id;  
            private String name;  
        private double salary;  
        public Cashier(int id,String name,double salary)  {  
            this.id=id;  
            this.name = name;  
            this.salary = salary;  
        }  
        @Override  
        public void add(Employee employee) {  
            //this is leaf node so this method is not applicable to this class.  
        }  
        @Override  
        public Employee getChild(int i) {  
            //this is leaf node so this method is not applicable to this class.  
            return null;  
        }  
        @Override  
        public int getId() {  
            // TODO Auto-generated method stub  
            return id;  
        }  
        @Override  
        public String getName() {  
            return name;  
        }  
        @Override  
        public double getSalary() {  
            return salary;  
        }  
        @Override  
        public void print() {  
            System.out.println("==========================");  
            System.out.println("Id ="+getId());  
            System.out.println("Name ="+getName());  
            System.out.println("Salary ="+getSalary());  
            System.out.println("==========================");  
        }  
        @Override  
        public void remove(Employee employee) {  
            //this is leaf node so this method is not applicable to this class.  
        }  
}

Step 4 : Create a Accountant class that will also be treated as a leaf and it will implement to the Employee interface.

public class Accountant implements Employee{  
/* 
    In this class,there are many methods which are not applicable to cashier because 
    it is a leaf node. 
*/  
    private int id;  
    private String name;  
    private double salary;  
   public Accountant(int id,String name,double salary)  {  
       this.id=id;  
       this.name = name;  
       this.salary = salary;  
   }  
   @Override  
   public void add(Employee employee) {  
       //this is leaf node so this method is not applicable to this class.  
   }  
   @Override  
   public Employee getChild(int i) {  
       //this is leaf node so this method is not applicable to this class.  
       return null;  
   }  
   @Override  
    public int getId() {  
        // TODO Auto-generated method stub  
        return id;  
   }  
   @Override  
   public String getName() {  
       return name;  
   }  
   @Override  
   public double getSalary() {  
       return salary;  
   }  
   @Override  
   public void print() {  
       System.out.println("=========================");  
       System.out.println("Id ="+getId());  
       System.out.println("Name ="+getName());  
       System.out.println("Salary ="+getSalary());  
       System.out.println("=========================");  
   }  
  @Override  
   public void remove(Employee employee) {  
       //this is leaf node so this method is not applicable to this class.  
   }  
}

Step 5 : Create a CompositePatternDemo class that will also be treated as a Client and ii will use the Employee interface.

public class CompositePatternDemo {  
    public static void main(String args[]){  
         Employee emp1=new Cashier(101,"Sohan Kumar", 20000.0);  
         Employee emp2=new Cashier(102,"Mohan Kumar", 25000.0);  
         Employee emp3=new Accountant(103,"Seema Mahiwal", 30000.0);   
         Employee manager1=new BankManager(100,"Ashwani Rajput",100000.0);  
            
          manager1.add(emp1);  
          manager1.add(emp2);  
          manager1.add(emp3);  
          manager1.print();  
        }  
}

Applicability

Use the Composite pattern when

you want to represent part-whole hierarchies of objects

you want clients to be able to ignore the difference between compositions of objects and individual objects. Clients will treat all objects in the composite structure uniformly