In software engineering, a software design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into source or machine code. It is a description or template for how to solve a problem that can be used in many different situations.
- Design patterns are not a silver bullet to all your problems.
- Do not try to force them; bad things are supposed to happen, if done so.
- Keep in mind that design patterns are solutions to problems, not solutions finding problems; so don't overthink.
- If used in a correct place in a correct manner, they can prove to be a savior; or else they can result in a horrible mess of a code.
In plain words
Creational patterns are focused towards how to instantiate an object or group of related objects.
Real world example
Consider, you are building a house and you need doors. You can either put on your carpenter clothes, bring some wood, glue, nails and all the tools required to build the door and start building it in your house or you can simply call the factory and get the built door delivered to you so that you don't need to learn anything about the door making or to deal with the mess that comes with making it.
In plain words
Simple factory simply generates an instance for client without exposing any instantiation logic to the client
When to Use?
When creating an object is not just a few assignments and involves some logic, it makes sense to put it in a dedicated factory instead of repeating the same code everywhere.
Real world example
Consider the case of a hiring manager. It is impossible for one person to interview for each of the positions. Based on the job opening, she has to decide and delegate the interview steps to different people.
In plain words
It provides a way to delegate the instantiation logic to child classes.
Useful when there is some generic processing in a class but the required sub-class is dynamically decided at runtime. Or putting it in other words, when the client doesn't know what exact sub-class it might need.
Real world example
Extending our door example from Simple Factory. Based on your needs you might get a wooden door from a wooden door shop, iron door from an iron shop or a PVC door from the relevant shop. Plus you might need a guy with different kind of specialities to fit the door, for example a carpenter for wooden door, welder for iron door etc. As you can see there is a dependency between the doors now, wooden door needs carpenter, iron door needs a welder etc.
In plain words
A factory of factories; a factory that groups the individual but related/dependent factories together without specifying their concrete classes.
When to use?
When there are interrelated dependencies with not-that-simple creation logic involved
Real world example
Imagine you are at Hardee's and you order a specific deal, lets say, "Big Hardee" and they hand it over to you without any questions; this is the example of simple factory. But there are cases when the creation logic might involve more steps. For example you want a customized Subway deal, you have several options in how your burger is made e.g what bread do you want? what types of sauces would you like? What cheese would you want? etc. In such cases builder pattern comes to the rescue.
In plain words
Allows you to create different flavors of an object while avoiding constructor pollution. Useful when there could be several flavors of an object. Or when there are a lot of steps involved in creation of an object.
Having said that let me add a bit about what telescoping constructor anti-pattern is. At one point or the other we have all seen a constructor like below:
When to use?
When there could be several flavors of an object and to avoid the constructor telescoping. The key difference from the factory pattern is that; factory pattern is to be used when the creation is a one step process while builder pattern is to be used when the creation is a multi step process.
Real world example
Remember dolly? The sheep that was cloned! Lets not get into the details but the key point here is that it is all about cloning
In plain words
Create object based on an existing object through cloning.
When to use?
When an object is required that is similar to existing object or when the creation would be expensive as compared to cloning.
Real world example
There can only be one president of a country at a time. The same president has to be brought to action, whenever duty calls. President here is singleton.
In plain words
Ensures that only one object of a particular class is ever created.
In plain words
Structural patterns are mostly concerned with object composition or in other words how the entities can use each other. Or yet another explanation would be, they help in answering "How to build a software component?"
Real world example
Consider that you have some pictures in your memory card and you need to transfer them to your computer. In order to transfer them you need some kind of adapter that is compatible with your computer ports so that you can attach memory card to your computer. In this case card reader is an adapter. Another example would be the famous power adapter; a three legged plug can't be connected to a two pronged outlet, it needs to use a power adapter that makes it compatible with the two pronged outlet. Yet another example would be a translator translating words spoken by one person to another
In plain words
Adapter pattern lets you wrap an otherwise incompatible object in an adapter to make it compatible with another class.
Real world example
Consider you have a website with different pages and you are supposed to allow the user to change the theme. What would you do? Create multiple copies of each of the pages for each of the themes or would you just create separate theme and load them based on the user's preferences? Bridge pattern allows you to do the second i.e.
In Plain Words
Bridge pattern is about preferring composition over inheritance. Implementation details are pushed from a hierarchy to another object with a separate hierarchy.
Real world example
Every organization is composed of employees. Each of the employees has the same features i.e. has a salary, has some responsibilities, may or may not report to someone, may or may not have some subordinates etc.
In plain words
Composite pattern lets clients treat the individual objects in a uniform manner.
Real world example
Imagine you run a car service shop offering multiple services. Now how do you calculate the bill to be charged? You pick one service and dynamically keep adding to it the prices for the provided services till you get the final cost. Here each type of service is a decorator.
In plain words
Decorator pattern lets you dynamically change the behavior of an object at run time by wrapping them in an object of a decorator class.
Real world example
How do you turn on the computer? "Hit the power button" you say! That is what you believe because you are using a simple interface that computer provides on the outside, internally it has to do a lot of stuff to make it happen. This simple interface to the complex subsystem is a facade.
In plain words
Facade pattern provides a simplified interface to a complex subsystem.
Real world example
Did you ever have fresh tea from some stall? They often make more than one cup that you demanded and save the rest for any other customer so to save the resources e.g. gas etc. Flyweight pattern is all about that i.e. sharing.
In plain words
It is used to minimize memory usage or computational expenses by sharing as much as possible with similar objects.
Real world example
Have you ever used an access card to go through a door? There are multiple options to open that door i.e. it can be opened either using access card or by pressing a button that bypasses the security. The door's main functionality is to open but there is a proxy added on top of it to add some functionality. Let me better explain it using the code example below.
In plain words
Using the proxy pattern, a class represents the functionality of another class.
In plain words
It is concerned with assignment of responsibilities between the objects. What makes them different from structural patterns is they don't just specify the structure but also outline the patterns for message passing/communication between them. Or in other words, they assist in answering "How to run a behavior in software component?"
- Chain of Responsibility
- Command
- Iterator
- Mediator
- Memento
- Observer
- Visitor
- Strategy
- State
- Template Method
Real world example
For example, you have three payment methods (
A,BandC) setup in your account; each having a different amount in it.Ahas 100 USD,Bhas 300 USD andChaving 1000 USD and the preference for payments is chosen asAthenBthenC. You try to purchase something that is worth 210 USD. Using Chain of Responsibility, first of all accountAwill be checked if it can make the purchase, if yes purchase will be made and the chain will be broken. If not, request will move forward to accountBchecking for amount if yes chain will be broken otherwise the request will keep forwarding till it finds the suitable handler. HereA,BandCare links of the chain and the whole phenomenon is Chain of Responsibility.
In plain words
It helps building a chain of objects. Request enters from one end and keeps going from object to object till it finds the suitable handler.
Real world example
A generic example would be you ordering food at a restaurant. You (i.e.
Client) ask the waiter (i.e.Invoker) to bring some food (i.e.Command) and waiter simply forwards the request to Chef (i.e.Receiver) who has the knowledge of what and how to cook. Another example would be you (i.e.Client) switching on (i.e.Command) the television (i.e.Receiver) using a remote control (Invoker).
In plain words
Allows you to encapsulate actions in objects. The key idea behind this pattern is to provide the means to decouple client from receiver.
Command pattern can also be used to implement a transaction based system. Where you keep maintaining the history of commands as soon as you execute them. If the final command is successfully executed, all good otherwise just iterate through the history and keep executing the undo on all the executed commands.
Real world example
An old radio set will be a good example of iterator, where user could start at some channel and then use next or previous buttons to go through the respective channels. Or take an example of MP3 player or a TV set where you could press the next and previous buttons to go through the consecutive channels or in other words they all provide an interface to iterate through the respective channels, songs or radio stations.
In plain words
It presents a way to access the elements of an object without exposing the underlying presentation.
Real world example
A general example would be when you talk to someone on your mobile phone, there is a network provider sitting between you and them and your conversation goes through it instead of being directly sent. In this case network provider is mediator.
In plain words
Mediator pattern adds a third party object (called mediator) to control the interaction between two objects (called colleagues). It helps reduce the coupling between the classes communicating with each other. Because now they don't need to have the knowledge of each other's implementation.
Real world example
Take the example of calculator (i.e. originator), where whenever you perform some calculation the last calculation is saved in memory (i.e. memento) so that you can get back to it and maybe get it restored using some action buttons (i.e. caretaker).
In plain words
Memento pattern is about capturing and storing the current state of an object in a manner that it can be restored later on in a smooth manner.
Real world example
A good example would be the job seekers where they subscribe to some job posting site and they are notified whenever there is a matching job opportunity.
In plain words
Defines a dependency between objects so that whenever an object changes its state, all its dependents are notified.
Wikipedia says
The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
Real world example
Consider someone visiting Dubai. They just need a way (i.e. visa) to enter Dubai. After arrival, they can come and visit any place in Dubai on their own without having to ask for permission or to do some leg work in order to visit any place here; just let them know of a place and they can visit it. Visitor pattern lets you do just that, it helps you add places to visit so that they can visit as much as they can without having to do any legwork.
In plain words
Visitor pattern lets you add further operations to objects without having to modify them.
Real world example
Consider the example of sorting, we implemented bubble sort but the data started to grow and bubble sort started getting very slow. In order to tackle this we implemented Quick sort. But now although the quick sort algorithm was doing better for large datasets, it was very slow for smaller datasets. In order to handle this we implemented a strategy where for small datasets, bubble sort will be used and for larger, quick sort.
In plain words
Strategy pattern allows you to switch the algorithm or strategy based upon the situation.
Real world example
Imagine you are using some drawing application, you choose the paint brush to draw. Now the brush changes its behavior based on the selected color i.e. if you have chosen red color it will draw in red, if blue then it will be in blue etc.
In plain words
It lets you change the behavior of a class when the state changes.
Real world example
Suppose we are getting some house built. The steps for building might look like
- Prepare the base of house
- Build the walls
- Add roof
- Add other floors
The order of these steps could never be changed i.e. you can't build the roof before building the walls etc but each of the steps could be modified for example walls can be made of wood or polyester or stone.
In plain words
Template method defines the skeleton of how a certain algorithm could be performed, but defers the implementation of those steps to the children classes.