Composite Design Pattern in Java

Want to learn more about the composite design pattern in Java? Take a look at this tutorial to learn how to implement this design pattern in your project.

Brijesh Saxena

CORE ·

Updated Oct. 20, 20 · Tutorial

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Here I am with another useful design pattern for you — the Composite Design Pattern. I will try to point out the key features to remember while implementing the composite pattern for you.

Composite Design Pattern

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

The 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.
The 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 Worker interface, which has methods for assignWork() and performWork(). The Work interface will act as a component of the composite pattern in the example.

     Java 
   
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package org.trishinfotech.composite;
public interface Worker {
    void assignWork(Employee manager, Work work);
    void performWork();
}

The Work Class is defined as below:

     Java 
   
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package org.trishinfotech.composite;
import java.util.ArrayList;
import java.util.List;
public class Work {
    private Calculator workType;
    private List<String> work = new ArrayList<String>();
    public Work(Calculator workType, List<String> work) {
        super();
        this.workType = workType;
        this.work = work;
    }
    public Calculator getWorkType() {
        return workType;
    }
    public void setWorkType(Calculator workType) {
        this.workType = workType;
    }
    public List<String> getWork() {
        return work;
    }
    public void setWork(List<String> work) {
        this.work = work;
    }
    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("Work [workType=").append(workType).append(", work=").append(work).append("]");
        return builder.toString();
    }
}

To keep the example simple, Here I am dealing only with simple calculations like Factorial value and Palindrome and Armstrong check. Since Pandirome can be a string as well, I have kept work as String in theWork class. Below class defines the algorithms/formulas to perform Factorial, Palindrome and Armstrong. Here's the code for Calculator enum:

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package org.trishinfotech.composite;
import java.math.BigInteger;
public enum Calculator {
    FACTORIAL {
        @Override
        public String calculate(String value) {
            String answer = "NA";
            try {
                long longValue = Long.parseLong(value);
                BigInteger factorialValue = BigInteger.valueOf(1);
                for (long i = 1; i <= longValue; i++) {
                    factorialValue = factorialValue.multiply(BigInteger.valueOf(i));
                }
                answer = factorialValue.toString();
            } catch (NumberFormatException exp) {
                System.out.println("Can't calculate factorial of " + value);
            }
            return answer;
        }
    },
    PALINDROME {
        @Override
        public String calculate(String value) {
            String answer = "false";
            if (value != null && !value.trim().isEmpty()) {
                String reverse = (new StringBuilder(value).reverse().toString());
                answer = Boolean.toString(reverse.equals(value));
            }
            return answer;
        }
    },
    ARMSTRONG {
        @Override
        public String calculate(String value) {
            String answer = "false";
            try {
                long longValue = Long.parseLong(value);
                long number = longValue;
                long armstrongValue = 0;
                while (number != 0) {
                    long temp = number % 10;
                    armstrongValue = armstrongValue + temp * temp * temp;
                    number /= 10;
                }
                answer = Boolean.toString(String.valueOf(armstrongValue).equals(value));
            } catch (NumberFormatException exp) {
                System.out.println("Can't calculate armstrong of " + value);
            }
            return answer;
        }
    };
    public abstract String calculate(String value);
}
 
      
 
     

I have created these algorithms as enums. To read more on Java-Enums, please refer to my article (Java Enums: How to Make Enums More Useful).

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

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package org.trishinfotech.composite;
public abstract class Employee implements Worker {
    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;
    }
    public abstract int teamSize();
    public String fullDetails() {
        StringBuilder builder = new StringBuilder();
        builder.append("Employee [").append(employeeId).append(", ").append(employeeName).append(", ")
                .append(designation).append(", ").append(department).append(", Team=").append(teamSize()).append("]");
        return builder.toString();
    }
    public String shortDetails() {
        StringBuilder builder = new StringBuilder();
        builder.append("'").append(employeeName).append("'");
        return builder.toString();
    }
    @Override
    public String toString() {
        return shortDetails();
    }
}
 
      
 
     

The Employeeclass declares abstract method teamSize() which I will use to split the work load assigned from a manager to anEngineer. So teamSize() will will be 1 forEngineer where as it will give the number of employees theManager is managing.

TheEngineer class is as below:

     Java 
   
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package org.trishinfotech.composite;
import java.util.ArrayList;
import java.util.List;
public class Engineer extends Employee {
    private List<Work> works = new ArrayList<Work>();
    public Engineer(long employeeId, String employeeName, String designation, Department department) {
        super(employeeId, employeeName, designation, department);
    }
    @Override
    public int teamSize() {
        return 1;
    }
    @Override
    public void assignWork(Employee manager, Work work) {
        this.works.add(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 '" + works + "'");
        works.stream().forEach(work -> {
            work.getWork().stream().forEach(value -> {
                Calculator calculator = work.getWorkType();
                System.out.println(this + " has result of work of '" + work + "' as : " + calculator.calculate(value));
            });
        });
        works.clear();
    }
}

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.

The Manager class is as below:

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package org.trishinfotech.composite;
import java.util.ArrayList;
import java.util.List;
public class Manager extends Employee {
    List<Employee> managingEmployees = new ArrayList<Employee>();
    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 int teamSize() {
        return managingEmployees.stream().mapToInt(employee -> employee.teamSize()).sum();
    }
    @Override
    public void assignWork(Employee manager, Work 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..");
        int fromIndex = 0;
        int toIndex = 0;
        int totalWork = work.getWork().size();
        List<String> assignWork = null;
        while (toIndex < totalWork) {
            for (Employee employee : managingEmployees) {
                System.out.println("Assigning to " + employee);
                int size = employee.teamSize();
                toIndex = fromIndex + size;
                assignWork = work.getWork().subList(fromIndex, toIndex);
                if (assignWork.isEmpty()) {
                    return;
                }
                employee.assignWork(this, new Work(work.getWorkType(), assignWork));
                fromIndex = toIndex;
            }
            break;
        }
    }
    @Override
    public void performWork() {
        System.out.println(this + " is asking his managing employees to perform 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 managing employees");
        System.out.println();
    }
}
 
      
 
     

We define Work in the property file instead of hard-coding into theMain class. The work.propertiesis defined as below:

     Java 
   
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Calculate.Palindrome=1234321, 12341234, ABCDEDCBA, 4567887654, XYZZYX, 45676543, 3456543
Calculate.Armstrong=153, 8208, 2104, 4210818, 345321, 32164049651, 876412347
Calculate.Factorial=20, 43, 15, 120, 543, 35, 456, 432, 350, 44, 26, 17, 8

The Main class usesWorkLoader class to load work from the work.properties. TheWorkLoader class is defined as below:

     Java 
   
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package org.trishinfotech.composite;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.Properties;
import java.util.Set;
public class WorkLoader {
    protected Properties properties = new Properties();
    public WorkLoader(String fileName) {
        try (InputStream input = new FileInputStream(fileName)) {
            // load a properties file
            properties.load(input);
        } catch (IOException exp) {
            exp.printStackTrace();
        }
    }
    public Properties getProperties() {
        return properties;
    }
    public List<Work> getWorkList() {
        List<Work> workList = new ArrayList<Work>();
        Set<Object> keys = properties.keySet();
        for (Object key : keys) {
            String workType = key.toString().substring("Calculate".length() + 1).toUpperCase();
            String values = properties.getProperty(key.toString());
            Work work = new Work(Calculator.valueOf(workType), Arrays.asList(values.split(", ")));
            workList.add(work);
        }
        return workList;
    }
}

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

     Java 
   
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package org.trishinfotech.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);
        WorkLoader workLoad = new WorkLoader("work.properties");
        workLoad.getWorkList().stream().forEach(work -> {
            rachelle.assignWork(rachelle, work);
        });
        rachelle.performWork();
    }
}

Below is the output of the program:

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'Rachelle' has assigned work of 'Work [workType=FACTORIAL, work=[20, 43, 15, 120, 543, 35, 456, 432, 350, 44, 26, 17, 8]]' by manager 'Rachelle'
'Rachelle' distributing work 'Work [workType=FACTORIAL, work=[20, 43, 15, 120, 543, 35, 456, 432, 350, 44, 26, 17, 8]]' to managing-employees..
Assigning to 'Kim'
'Kim' has assigned work of 'Work [workType=FACTORIAL, work=[20, 43, 15]]' by manager 'Rachelle'
'Kim' distributing work 'Work [workType=FACTORIAL, work=[20, 43, 15]]' to managing-employees..
Assigning to 'Ajay'
'Ajay' has assigned work of 'Work [workType=FACTORIAL, work=[20]]' by manager 'Kim'
Assigning to 'Martin'
'Martin' has assigned work of 'Work [workType=FACTORIAL, work=[43]]' by manager 'Kim'
Assigning to 'Vijay'
'Vijay' has assigned work of 'Work [workType=FACTORIAL, work=[15]]' by manager 'Kim'
Assigning to 'Robert'
'Robert' has assigned work of 'Work [workType=FACTORIAL, work=[120]]' by manager 'Rachelle'
Assigning to 'Niels'
'Niels' has assigned work of 'Work [workType=FACTORIAL, work=[543, 35, 456]]' by manager 'Rachelle'
'Niels' distributing work 'Work [workType=FACTORIAL, work=[543, 35, 456]]' to managing-employees..
Assigning to 'Jay'
'Jay' has assigned work of 'Work [workType=FACTORIAL, work=[543]]' by manager 'Niels'
Assigning to 'Andersen'
'Andersen' has assigned work of 'Work [workType=FACTORIAL, work=[35]]' by manager 'Niels'
Assigning to 'Shailesh'
'Shailesh' has assigned work of 'Work [workType=FACTORIAL, work=[456]]' by manager 'Niels'
'Rachelle' has assigned work of 'Work [workType=PALINDROME, work=[1234321, 12341234, ABCDEDCBA, 4567887654, XYZZYX, 45676543, 3456543]]' by manager 'Rachelle'
'Rachelle' distributing work 'Work [workType=PALINDROME, work=[1234321, 12341234, ABCDEDCBA, 4567887654, XYZZYX, 45676543, 3456543]]' to managing-employees..
Assigning to 'Kim'
'Kim' has assigned work of 'Work [workType=PALINDROME, work=[1234321, 12341234, ABCDEDCBA]]' by manager 'Rachelle'
'Kim' distributing work 'Work [workType=PALINDROME, work=[1234321, 12341234, ABCDEDCBA]]' to managing-employees..
Assigning to 'Ajay'
'Ajay' has assigned work of 'Work [workType=PALINDROME, work=[1234321]]' by manager 'Kim'
Assigning to 'Martin'
'Martin' has assigned work of 'Work [workType=PALINDROME, work=[12341234]]' by manager 'Kim'
Assigning to 'Vijay'
'Vijay' has assigned work of 'Work [workType=PALINDROME, work=[ABCDEDCBA]]' by manager 'Kim'
Assigning to 'Robert'
'Robert' has assigned work of 'Work [workType=PALINDROME, work=[4567887654]]' by manager 'Rachelle'
Assigning to 'Niels'
'Niels' has assigned work of 'Work [workType=PALINDROME, work=[XYZZYX, 45676543, 3456543]]' by manager 'Rachelle'
'Niels' distributing work 'Work [workType=PALINDROME, work=[XYZZYX, 45676543, 3456543]]' to managing-employees..
Assigning to 'Jay'
'Jay' has assigned work of 'Work [workType=PALINDROME, work=[XYZZYX]]' by manager 'Niels'
Assigning to 'Andersen'
'Andersen' has assigned work of 'Work [workType=PALINDROME, work=[45676543]]' by manager 'Niels'
Assigning to 'Shailesh'
'Shailesh' has assigned work of 'Work [workType=PALINDROME, work=[3456543]]' by manager 'Niels'
'Rachelle' has assigned work of 'Work [workType=ARMSTRONG, work=[153, 8208, 2104, 4210818, 345321, 32164049651, 876412347]]' by manager 'Rachelle'
'Rachelle' distributing work 'Work [workType=ARMSTRONG, work=[153, 8208, 2104, 4210818, 345321, 32164049651, 876412347]]' to managing-employees..
Assigning to 'Kim'
'Kim' has assigned work of 'Work [workType=ARMSTRONG, work=[153, 8208, 2104]]' by manager 'Rachelle'
'Kim' distributing work 'Work [workType=ARMSTRONG, work=[153, 8208, 2104]]' to managing-employees..
Assigning to 'Ajay'
'Ajay' has assigned work of 'Work [workType=ARMSTRONG, work=[153]]' by manager 'Kim'
Assigning to 'Martin'
'Martin' has assigned work of 'Work [workType=ARMSTRONG, work=[8208]]' by manager 'Kim'
Assigning to 'Vijay'
'Vijay' has assigned work of 'Work [workType=ARMSTRONG, work=[2104]]' by manager 'Kim'
Assigning to 'Robert'
'Robert' has assigned work of 'Work [workType=ARMSTRONG, work=[4210818]]' by manager 'Rachelle'
Assigning to 'Niels'
'Niels' has assigned work of 'Work [workType=ARMSTRONG, work=[345321, 32164049651, 876412347]]' by manager 'Rachelle'
'Niels' distributing work 'Work [workType=ARMSTRONG, work=[345321, 32164049651, 876412347]]' to managing-employees..
Assigning to 'Jay'
'Jay' has assigned work of 'Work [workType=ARMSTRONG, work=[345321]]' by manager 'Niels'
Assigning to 'Andersen'
'Andersen' has assigned work of 'Work [workType=ARMSTRONG, work=[32164049651]]' by manager 'Niels'
Assigning to 'Shailesh'
'Shailesh' has assigned work of 'Work [workType=ARMSTRONG, work=[876412347]]' by manager 'Niels'
'Rachelle' is asking his managing employees to perform assigned work
'Kim' is asking his managing employees to perform assigned work
'Ajay' is performing work of '[Work [workType=FACTORIAL, work=[20]], Work [workType=PALINDROME, work=[1234321]], Work [workType=ARMSTRONG, work=[153]]]'
'Ajay' has result of work of 'Work [workType=FACTORIAL, work=[20]]' as : 2432902008176640000
'Ajay' has result of work of 'Work [workType=PALINDROME, work=[1234321]]' as : true
'Ajay' has result of work of 'Work [workType=ARMSTRONG, work=[153]]' as : true
'Martin' is performing work of '[Work [workType=FACTORIAL, work=[43]], Work [workType=PALINDROME, work=[12341234]], Work [workType=ARMSTRONG, work=[8208]]]'
'Martin' has result of work of 'Work [workType=FACTORIAL, work=[43]]' as : 60415263063373835637355132068513997507264512000000000
'Martin' has result of work of 'Work [workType=PALINDROME, work=[12341234]]' as : false
'Martin' has result of work of 'Work [workType=ARMSTRONG, work=[8208]]' as : false
'Vijay' is performing work of '[Work [workType=FACTORIAL, work=[15]], Work [workType=PALINDROME, work=[ABCDEDCBA]], Work [workType=ARMSTRONG, work=[2104]]]'
'Vijay' has result of work of 'Work [workType=FACTORIAL, work=[15]]' as : 1307674368000
'Vijay' has result of work of 'Work [workType=PALINDROME, work=[ABCDEDCBA]]' as : true
'Vijay' has result of work of 'Work [workType=ARMSTRONG, work=[2104]]' as : false
'Kim' has completed assigned work with the help of his managing employees
'Robert' is performing work of '[Work [workType=FACTORIAL, work=[120]], Work [workType=PALINDROME, work=[4567887654]], Work [workType=ARMSTRONG, work=[4210818]]]'
'Robert' has result of work of 'Work [workType=FACTORIAL, work=[120]]' as : 6689502913449127057588118054090372586752746333138029810295671352301633557244962989366874165271984981308157637893214090552534408589408121859898481114389650005964960521256960000000000000000000000000000
'Robert' has result of work of 'Work [workType=PALINDROME, work=[4567887654]]' as : true
'Robert' has result of work of 'Work [workType=ARMSTRONG, work=[4210818]]' as : false
'Niels' is asking his managing employees to perform assigned work
'Jay' is performing work of '[Work [workType=FACTORIAL, work=[543]], Work [workType=PALINDROME, work=[XYZZYX]], Work [workType=ARMSTRONG, work=[345321]]]'
'Jay' has result of work of 'Work [workType=FACTORIAL, work=[543]]' as : 872891556790260456843586366934462570217404467584986862157951660218033476207283552939973350537069217537549478114229971312262991181125700413163606116971183695586311579361173915852193241844683585564114537080099157013082576149573523335369738442355599816710804068865355684599942047453968407440725166640261015140054933542126214585902983116193907129111747665196041855032660956095883694974186998924247749529335931094092429725640658310042199668214202637924631140057800119930627800378541111012271243379033822559451085315416955672912791395237932595257653475479021208979154389591618595449855779925751308303314870067464075830210893226767964127916083986194604372529850059441599156750579670067110415997949767860963439005485252514342425180564330056392659552281473502353159284918610493411467800558076001750687023376509841526414457026571388047691226042613278047076078395826335028468047405871941546558134196554638340479908186429178241794558954878506078646531853505428187507441610683354213870320835243780540993795316813622383436151335642255741817696689682127415809984693167490611336830354021351607247261703154384913620088006020923947745280000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
'Jay' has result of work of 'Work [workType=PALINDROME, work=[XYZZYX]]' as : true
'Jay' has result of work of 'Work [workType=ARMSTRONG, work=[345321]]' as : false
'Andersen' is performing work of '[Work [workType=FACTORIAL, work=[35]], Work [workType=PALINDROME, work=[45676543]], Work [workType=ARMSTRONG, work=[32164049651]]]'
'Andersen' has result of work of 'Work [workType=FACTORIAL, work=[35]]' as : 10333147966386144929666651337523200000000
'Andersen' has result of work of 'Work [workType=PALINDROME, work=[45676543]]' as : false
'Andersen' has result of work of 'Work [workType=ARMSTRONG, work=[32164049651]]' as : false
'Shailesh' is performing work of '[Work [workType=FACTORIAL, work=[456]], Work [workType=PALINDROME, work=[3456543]], Work [workType=ARMSTRONG, work=[876412347]]]'
'Shailesh' has result of work of 'Work [workType=FACTORIAL, work=[456]]' as : 150777392777717065903328562798297482932764849966301315324902295697797980802999492049275470580840593582700556154654997912467653672836190567363944536581444396786039028419417159553169852939652733499484374432647121409002713034716885273557660568294514238651304204026421026217797122437474581042706674997505548774529387552185264469304745879944335896334980134727576771262477699704913814778801164976379963316514713032786305083016847394455111607701177156363125206697642497352441989049637406799105387152093299654856194446887474831405921359722324720996553956200165400519069670468845686118517860926559421327845227712982865242890852011587912148558934925229259778865164753102371910801614732061965104129730561590839408147446252948841011789641706225763887234100676084552005497753764496546383864694159909979495432469993306110242973486330432796522331628915418533758582252153753291412897349335363154308911927972242304805109760000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
'Shailesh' has result of work of 'Work [workType=PALINDROME, work=[3456543]]' as : true
'Shailesh' has result of work of 'Work [workType=ARMSTRONG, work=[876412347]]' as : false
'Niels' has completed assigned work with the help of his managing employees
'Rachelle' has completed assigned work with the help of his managing employees
 
      
 
     

Here, I haven't use concurrency to keep the example simple. Please feel free to enhance the example as per your choice at your end.

Source Code can be found here: Composite-Design-Pattern-Sample-Code

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