Design Patterns


  1. Identify the varying parts and separate them from the invariant.

  2. Program to an interface (supertype), not an implementation.

    Animal animal = new Dog();
    List<Integer> list = new ArrayList<Integer>();
  3. Favor composition over inheritance. HAS-A can be better than IS-A.

  4. SOLID:

    • Single responsibility: a class should have only one reason to change.
    • Open-closed: classes should be open for extension, but closed for modification.
    • Liskov Substitution: objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program
    • Interface Segregation: many client-specific interfaces are better than one general-purpose interface.
    • Dependency inversion: depend upon abstractions. Do not depend upon concrete classes. High-level components should not depend on low-level components; rather, they should both depend on abstractions.
      • No variables should hold a reference to a concrete class.
      • No class should derive from a concrete class.
  5. Loose coupling: minimize the interdependency between objects.

  6. Design by contract: preconditions, postconditions, and invariants.

  7. Principle of Least Knowledge aka (Law of Demeter): talk only to your immediate friends.

  8. The Hollywood Principle (aka. Inversion of Control): (High-level components) Don’t call us, we’ll call you (low-level components).

    • A form of IoC: Dependency injection - a technique in which an object receives other objects that it depends on.
  9. Tell-Don’t-Ask: A->B instead of B->A->B for data.

  10. Build end-to-end (incrementally) instead of top-down nor bottom-up.

  11. In a Test-driven development (TDD), there’s no need to do a big design up front.

  12. There’s no need to prefix m_ for private stuff.

  13. We shouldn’t always use patterns in today’s agile and XP trends. But if our specific solution is similar to a pattern, refactor it to make it more generic if needed.

The Strategy Pattern

  1. The strategy pattern enables selecting an algorithm at runtime. Instead of implementing a single algorithm directly, code receives run-time instructions as to which in a family of algorithms to use.

The Observer Pattern

  1. Publishers (Subject) + Subscribers (Observers) = Observer Pattern.

  2. It defines a one-to-many dependency. When the subject changes, all dependents are notified.

  3. In Java, we can use the built-in Observable - Observer superclasses.

    // extends Observable
    // extends Observer
    // in the constructor
  4. In Java, Observable is a class, which means we have to subclass it.

  5. UML:

    • /images/observer.png
    • Example: /images/observer_example.png
  6. Used principles:

    • Loose coupling.

The Decorator Pattern

  1. Decorators have the same supertype as the object they decorate.

  2. You can use one or more decorators to wrap (HAS-A) an object.

  3. The decorator adds its own behavior either before and/or after delegating to the object it decorates to do the rest of the job.

  4. The decorator pattern attaches additional responsibilities to an object dynamically.

  5. UML:

    • /images/decorator.png
    • Example: /images/decorator_example.png
  6. is mainly using the decorator pattern.

  7. Used principles:

    • Open-closed.

The Factory Pattern

  1. The factory method pattern defines an interface for creating an object, but lets subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.

  2. UML:

    • /images/factory.png
    • Example: /images/factory_example.png
  3. The abstract factory pattern provides an interface for creating families of related or dependent objects without specifying their concrete classes.

  4. Used principles:

    • Dependency inversion.

The Singleton Pattern

  1. The singleton pattern restricts the instantiation of a class to one single instance, and provides a global point of access to it:

    public class Singleton {
        private static Singleton uniqueInstance;
        private Singleton() {} // PRIVATE constructor
        public static synchronized Singleton getInstance() { // remove synchronized if there's no multithreading
            if (uniqueInstance == null) {
                uniqueInstance = new Singleton();
            return uniqueInstance;

The Command Pattern

  1. The command pattern encapsulates a request as an object, thereby letting you parameterize other objects with different requests and support undoable operations.

  2. UML:

    • /images/command.png

The Adapter Pattern

  1. The adapter pattern converts the interface of a class into another interface the clients expect. It lets classes work together that couldn’t otherwise because of incompatible interfaces.

  2. UML:

    • /images/adapter1.png
    • /images/adapter2.png

The Facade Pattern

  1. The facade pattern provides a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

  2. Used principles:

    • Least knowledge.

The Template Method Pattern

  1. The template method pattern defines the skeleton of an algorithm in a method, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure:

    abstract class AbstractClass {
        final void templateMethod() {
        abstract void primitiveOperation1();
        abstract void primitiveOperation2();
        void concreteOperation() {
            // implementation here
  2. Used principles:

    • Inversion of control.
    • Single responsibility.

The State Pattern

  1. The state pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

  2. It’s close to the concept of finite-state machines.

The Proxy Pattern

  1. A remote proxy acts as a local representative to a remote object.

  2. Java RMI’s (remote method invocation) client helper is a “stub” and the service helper is a “skeleton”. The stub is the proxy.

  3. The proxy pattern provides a surrogate or placeholder for another object to access it.


  1. The MVC is using:
    • the strategy pattern: the view delegates to the controllers to handle user actions. (Swappable controllers.)
    • the composite pattern: the view is a composite of GUI components.
    • the observer pattern: the model is the observable, and the view & controller are observers.

Design Pattern Categories

  • Creational: Factory, Singleton
  • Behavioral (how classes communicate): State, Iterator, Command
  • Structural: Adapter, Composite, Decorator, Facade, Proxy