Sample of Java Pattern -- Composite

Department.java

package composite;

import java.util.List;

public interface Department {
    public Department getDepartment();
    public String getDepartmentname();
    public void add(Department department);
    public void remove(Department department);
    public boolean hasSubDepartments();
    public List<Department> getSubDepartments();
}

CompositeDepartment.java

package composite;

import java.util.ArrayList;
import java.util.List;

public class CompositeDepartment implements Department {
    private String name;
    private List<Department> departments = new ArrayList<Department>();
    
    public CompositeDepartment(String name) {
        super();
        System.out.println("Department " + name + " is created.");
        this.name = name;
    }

    public Department getDepartment() {
        return this;
    }

    public String getDepartmentname() {
        return this.name;
    }
    
    public void add(Department department){
        System.out.println("Department " + department.getDepartmentname() + " is added to Department " + this.name + ".");
        departments.add(department);
    }

    public void remove(Department department){
        System.out.println("Department " + department.getDepartmentname() + " is removed from Department " + this.name + ".");
        departments.remove(department);
    }

    public boolean hasSubDepartments(){
        if(departments == null || departments.isEmpty())
            return false;
        return true;
    }
    
    public List<Department> getSubDepartments(){
        if(hasSubDepartments())
            return departments;
        return null;
    }
}

SingleDepartment.java

package composite;

import java.util.List;

public class SingleDepartment implements Department {
    private String name;
    
    public SingleDepartment(String name) {
        super();
        System.out.println("Department " + name + " is created.");
        this.name = name;
    }

    public Department getDepartment() {
        return this;
    }

    public String getDepartmentname() {
        return this.name;
    }

    public void add(Department department){
        System.out.println("Could not add a child department in Department " + this.name + ".");
    }

    public void remove(Department department){
        System.out.println("Department " + department.getDepartmentname() + " doesn't belong to Department " + this.name + ".");
    }
    
    public boolean hasSubDepartments(){
        return false;
    }
    
    public List<Department> getSubDepartments(){
        return null;
    }
}

CompositeTest.java

package composite;

public class CompositeTest {
    public static void main(String[] args) {
        Department hq = new CompositeDepartment("HQ");
        Department financing = new SingleDepartment("Financing");
        Department marketing = new CompositeDepartment("Marketing");
        Department preSales = new SingleDepartment("PreSales");
        Department sales = new SingleDepartment("Sales");
        Department postSales = new SingleDepartment("PostSales");
        Department techSupport = new SingleDepartment("TechSupport");
        hq.add(financing);
        hq.add(marketing);
        hq.add(techSupport);
        marketing.add(preSales);
        marketing.add(sales);
        marketing.add(postSales);
        System.out.println("hq has sub departments: " + hq.hasSubDepartments());
        System.out.println("marketing has sub departments: " + marketing.hasSubDepartments());
        System.out.println("techSupport has sub departments: " + techSupport.hasSubDepartments());
        marketing.remove(preSales);
        marketing.remove(sales);
        marketing.remove(postSales);
        sales.remove(preSales);
        hq.remove(financing);
        hq.remove(marketing);
        hq.remove(techSupport);
    }
}

you may download the source code.