Behavioral Patterns


Behavioral patterns improve communication and flexibility between objects.

All Patterns


Observer pattern establishes a one to many relationship, so if an observable object is modified all the observers listening to changes on that object get notified.

This pattern is used to reflect changes in an object, when there are changes in another object without having these two objects coupled. This pattern allows you to add additional observers in the future with minimal changes.

If we have many observers and observables, the communication between them can become complex.

An Observable object needs to be created and then, the observers subscribe to the observable. Each time the observable state changes, attached observers are notified .


Observer Pattern - UML
Observer Pattern - UML
  1. Create an observer interface:

     public interface Observer {
         void update();
  2. Create a subject interface:

     public interface Subject {
         void register(Observer obj);
         void unregister(Observer obj);
         void notifyObservers();
  3. Create a class implementing the Subject interface. This class will be responsible for registering, unregistering and notifying observers:

     public class MessageNotifier implements Subject, Runnable {
         private List<Observer> observers;
         private String message;
         public MessageNotifier() {
             observers = new ArrayList<>();
         public void register(Observer observer) {
         public void unregister(Observer observer) {
         public void notifyObservers() {
             for (Observer observer : observers) {
         public String showMessage() {
             return this.message;
         public void setMessage(String message) {
             this.message = message;
         public void run() {
             Scanner sc =  new Scanner(;
             while (true) {
                 String response =;
  4. Create concrete classes for observers implementing the Observer interface. The class implementing the subject interface is included by composition, so it has access to the updates:

     public class Mobile implements Observer {
         private MessageNotifier messageNotifier;
         public Mobile(MessageNotifier messageNotifier) {
             this.messageNotifier = messageNotifier;
         public void update() {
             System.out.println(messageNotifier.showMessage() + " from Mobile");
  5. The client is responsible for creating the subject class and registering the observers:

     System.out.println("Enter message: ");
     MessageNotifier messageNotifier =  new MessageNotifier();
     Mobile observerOne = new Mobile(messageNotifier);
     Desktop observerTwo = new Desktop(messageNotifier);
     Tablet observerThree = new Tablet(messageNotifier);
     new Thread(messageNotifier).start();


Command pattern is a data driven design pattern that allows you to create a set of behaviors for a particular subject. It decouples the object that invokes the action from the object that performs the action.

It is used to queue commands and keep a history of them to do and undo actions.


  1. Create a command interface:

     public interface Command {
         void execute(UUID policyId);
  2. Create a request class:

     public class Policy {
         public void create(UUID policyId) {
             System.out.println("Create policy " + policyId);
         public void update(UUID policyId) {
             System.out.println("Update policy " + policyId);
         public void cancel(UUID policyId) {
             System.out.println("Cancel policy " + policyId);
  3. Create concrete classes implementing the Command interface:

     public class CreatePolicy implements Command {
         private Policy policy;
         public CreatePolicy(Policy policy) {
             this.policy = policy;
         public void execute(UUID policyId) {

  4. Create a command invoker class:

     public class PolicyCommandsInvoker {
         private List<Command> commands = new ArrayList<>();
         public void takeCommand(Command command) {
         public void runCommands(UUID policyId) {
             for (Command command : commands) {
  5. The client can use the invoker class to execute commands:

     Policy policy = new Policy();
     CreatePolicy createPolicy = new CreatePolicy(policy);
     UpdatePolicy updatePolicy = new UpdatePolicy(policy);
     CancelPolicy cancelPolicy = new CancelPolicy(policy);
     PolicyCommandsInvoker policyCommandsInvoker = new PolicyCommandsInvoker();


Strategy pattern defines a family of behaviors which are interchangeable. This pattern allows you to change at runtime the way an object behaves. Therefore, instead of having multiple instances, there will be only one that is able to interchange different behaviors, and the only requirement for these strategies or behaviors is having a common interface required by the context object.


Strategy Pattern - UML
Strategy Pattern - UML
  1. Create strategy interface:

     public interface Operation {
         int doOperation(int number1, int number2);
  2. Create concrete class implementing the strategy interface:

     public class Addition implements Operation {
         public int doOperation(int number1, int number2) {
             return number1 + number2;

  3. Create a context class that accepts :

     public class CalculatorContext {
         private final Operation operation;
         public CalculatorContext(Operation operation) {
             this.operation = operation;
         public int executeStrategy(int num1, int num2) {
             return operation.doOperation(num1, num2);
  4. The client will use the context class to execute strategies:

     System.out.println("Do Addition Operation...");
     CalculatorContext calculator = new CalculatorContext(new Addition());
     System.out.println(NUM1 + " + " + NUM2 + ": " + calculator.executeStrategy(NUM1, NUM2));
     System.out.println("Do Substration Operation...");
     calculator = new CalculatorContext(new Subtraction());
     System.out.println(NUM1 + " - " + NUM2 + ": " + calculator.executeStrategy(NUM1, NUM2));
     System.out.println("Do Division Operation...");
     calculator = new CalculatorContext(new Division());
     System.out.println(NUM1 + " / " + NUM2 + ": " + calculator.executeStrategy(NUM1, NUM2));
     System.out.println("Do Multiplication Operation...");
     calculator = new CalculatorContext(new Multiplication());
     System.out.println(NUM1 + " * " + NUM2 + ": " + calculator.executeStrategy(NUM1, NUM2));