https://dzone.com/articles/composite-design-pattern-in-java-1

The composite pattern is meant to "compose objects into a tree structure to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly"

• Composite design patterns describe groups of objects that can be treated in the same way as a single instance of the same object type.
• The composite pattern allows us to "compose" objects into tree structures to represent part-whole hierarchies.
• In addition, the composite patterns also allow our clients to treat individual objects and compositions in the same way.
• Composite patterns allow us to have a tree structure for each node that performs a task.
• In object-oriented programming, a composite is an object designed as a composition of one-or-more similar objects, all exhibiting similar functionality. This is known as a “has-a”relationship between objects.

Below is the list of classes/objects used in the composite pattern, which has four :

• Component – Component is the interface (or abstract class) for the composition of the objects and methods for accessing/processing its child or node components. It also implements a default interface to define common functionalities/behaviors for all component classes.
• Leaf – The leaf class defines a concrete component class, which does not have any further composition. The leaf class implements the component interface. It performs the command/task at its end only.
• Composite – The composite class defines a concrete component class, which stores its child components. The composite class implements the component interface. It forwards the command/task to the composite objects it contains. It may also perform additional operations before and after forwarding the command/task.
• Client – The client class uses the component interface to interact/manipulate the objects in the composition (Leaf and Composite).

To better understand this, let's take a look at an example of employees working in an organization.

Steps

• We create an interface to define functionalities we like to perform as composite and leaf objects. Below is the code of the Work interface, which has methods for assignWork() and performWork(). The Work interface will act as a component of the composite pattern in the example.

package design.composite;public interface Work {      void assignWork(Employee manager, String work);      void performWork();}

 

• We will create an abstract class of Employee to carry the common code for all various concrete subclasses of the employees.

package design.composite;public abstract class Employee implements Work {      protected long employeeId;      protected String employeeName;      protected String designation;      protected Department department;      public Employee(long employeeId, String employeeName, String designation, Department department) {                super();                this.employeeId = employeeId;                this.employeeName = employeeName;                this.designation = designation;                this.department = department;      }      public long getEmployeeId() {      return employeeId;      }      public void setEmployeeId(long employeeId) {      this.employeeId = employeeId;      }      public String getEmployeeName() {      return employeeName;      }      public void setEmployeeName(String employeeName) {      this.employeeName = employeeName;      }      public String getDesignation() {      return designation;      }      public void setDesignation(String designation) {      this.designation = designation;      }      public Department getDepartment() {      return department;      }      public void setDepartment(Department department) {      this.department = department;      }      @Override      public String toString() {                StringBuilder builder = new StringBuilder();                builder.append("Employee [").append(employeeId)                .append(", ").append(employeeName)                .append(", ").append(Designation)                .append(", ").append(department)                .append("]");                return builder.toString();      }}

 

• Now, we will create two concrete subclasses of the Employee — Manager and Engineer. For this, I am keeping the example simple to keep a focus on the pattern.
• We will create the Engineer class to use as the Leaf object and, hence, does not have any other employee object as a reference via composition.

package design.composite;public class Engineer extends Employee {        private String work;        public Engineer(long employeeId, String employeeName, String designation, Department department) {        super(employeeId, employeeName, designation, department);        }        @Override        public void assignWork(Employee manager, String work) {                        this.work = work;                        System.out.println(this  + " has assigned work of '" + work + "' by manager " + manager);        }        @Override        public void performWork() {        System.out.println(this  + " is performing work of '" + work + "'");        }}

 

• We will create the Manager class to use as the composite object, and we will have another Employee object as a collection via the composition.

package design.composite;import java.util.ArrayList;import java.util.List;public class Manager extends Employee {        protected List
    
      managingEmployees = new ArrayList
     
      ();        public Manager(long employeeId, String employeeName, String designation, Department department) {        super(employeeId, employeeName, designation, department);        }        public boolean manages(Employee employee) {          return managingEmployees.add(employee);        }        public boolean stopManaging(Employee employee) {        return managingEmployees.remove(employee);        }        @Override        public void assignWork(Employee manager, String work) {                    System.out.println(this  + " has assigned work of '" + work + "' by manager " + manager);                    System.out.println();                    System.out.println(this + " distributing work '" + work + "' to managing-employees..");                    managingEmployees.stream().forEach(employee -> {                        System.out.println("Assigning to " + employee);                        employee.assignWork(this, work);                    });                    System.out.println();                    System.out.println(this + " distributed work of '" + work + "'");                    System.out.println();        }        @Override        public void performWork() {                    System.out.println(this + " is asking his managing employees to perfom assigned work");                    System.out.println();                    managingEmployees.stream().forEach(employee -> employee.performWork());                    System.out.println();                    System.out.println(this + " has completed assigned work with the help of his manahging employees");                    System.out.println();        }}
     
    

 

• In the end, we will write the Main class as the Clien to execute and test our composite pattern code.

package design.composite;public class Main {public static void main(String[] args) {          Engineer ajay = new Engineer(1001l, "Ajay", "Developer", Department.ENG);          Engineer vijay = new Engineer(1002l, "Vijay", "SR. Developer", Department.ENG);          Engineer jay = new Engineer(1003l, "Jay", "Lead", Department.ENG);          Engineer martin = new Engineer(1004l, "Martin", "QA", Department.ENG);          Manager kim = new Manager(1005l, "Kim", "Manager", Department.ENG);          Engineer anders = new Engineer(1006l, "Andersen", "Developer", Department.ENG);          Manager niels = new Manager(1007l, "Niels", "Sr. Manager", Department.ENG);          Engineer robert = new Engineer(1008l, "Robert", "Developer", Department.ENG);          Manager rachelle = new Manager(1009l, "Rachelle", "Product Manager", Department.ENG);          Engineer shailesh = new Engineer(1010l, "Shailesh", "Engineer", Department.ENG);          kim.manages(ajay);          kim.manages(martin);          kim.manages(vijay);          niels.manages(jay);          niels.manages(anders);          niels.manages(shailesh);          rachelle.manages(kim);          rachelle.manages(robert);          rachelle.manages(niels);          rachelle.assignWork(rachelle, "develop web-service code for Product Catalogue");          rachelle.performWork();}}

 

• Below is the output of the program:

Employee [1009, Rachelle, Product Manager, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1009, Rachelle, Product Manager, ENG]Employee [1009, Rachelle, Product Manager, ENG] distributing work 'develop web-service code for Product Catalogue' to managing-employees..Assigning to Employee [1005, Kim, Manager, ENG]Employee [1005, Kim, Manager, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1009, Rachelle, Product Manager, ENG]Employee [1005, Kim, Manager, ENG] distributing work 'develop web-service code for Product Catalogue' to managing-employees..Assigning to Employee [1001, Ajay, Developer, ENG]Employee [1001, Ajay, Developer, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1005, Kim, Manager, ENG]Assigning to Employee [1004, Martin, QA, ENG]Employee [1004, Martin, QA, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1005, Kim, Manager, ENG]Assigning to Employee [1002, Vijay, SR. Developer, ENG]Employee [1002, Vijay, SR. Developer, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1005, Kim, Manager, ENG]Employee [1005, Kim, Manager, ENG] distributed work of 'develop web-service code for Product Catalogue'Assigning to Employee [1008, Robert, Developer, ENG]Employee [1008, Robert, Developer, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1009, Rachelle, Product Manager, ENG]Assigning to Employee [1007, Niels, Sr. Manager, ENG]Employee [1007, Niels, Sr. Manager, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1009, Rachelle, Product Manager, ENG]Employee [1007, Niels, Sr. Manager, ENG] distributing work 'develop web-service code for Product Catalogue' to managing-employees..Assigning to Employee [1003, Jay, Lead, ENG]Employee [1003, Jay, Lead, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1007, Niels, Sr. Manager, ENG]Assigning to Employee [1006, Andersen, Developer, ENG]Employee [1006, Andersen, Developer, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1007, Niels, Sr. Manager, ENG]Assigning to Employee [1010, Shailesh, Engineer, ENG]Employee [1010, Shailesh, Engineer, ENG] has assigned work of 'develop web-service code for Product Catalogue' by manager Employee [1007, Niels, Sr. Manager, ENG]Employee [1007, Niels, Sr. Manager, ENG] distributed work of 'develop web-service code for Product Catalogue'Employee [1009, Rachelle, Product Manager, ENG] distributed work of 'develop web-service code for Product Catalogue'Employee [1009, Rachelle, Product Manager, ENG] is asking his managing employees to perfom assigned workEmployee [1005, Kim, Manager, ENG] is asking his managing employees to perfom assigned workEmployee [1001, Ajay, Developer, ENG] is performing work of 'develop web-service code for Product Catalogue'Employee [1004, Martin, QA, ENG] is performing work of 'develop web-service code for Product Catalogue'Employee [1002, Vijay, SR. Developer, ENG] is performing work of 'develop web-service code for Product Catalogue'Employee [1005, Kim, Manager, ENG] has completed assigned work with the help of his manahging employeesEmployee [1008, Robert, Developer, ENG] is performing work of 'develop web-service code for Product Catalogue'Employee [1007, Niels, Sr. Manager, ENG] is asking his managing employees to perfom assigned workEmployee [1003, Jay, Lead, ENG] is performing work of 'develop web-service code for Product Catalogue'Employee [1006, Andersen, Developer, ENG] is performing work of 'develop web-service code for Product Catalogue'Employee [1010, Shailesh, Engineer, ENG] is performing work of 'develop web-service code for Product Catalogue'Employee [1007, Niels, Sr. Manager, ENG] has completed assigned work with the help of his manahging employeesEmployee [1009, Rachelle, Product Manager, ENG] has completed assigned work with the help of his manahging employees