Design pattern categories

The fact that software development is an extremely broad topic leads to a number of things that can be done with programming. Everything is different and this leads to various requirements about the qualities of programs. All these facts have caused many different design patterns to be invented. This is further contributed to by the existence of various programming languages with different features and levels of expressiveness.

This book focuses on the design patterns from the point of view of Scala. As we mentioned previously, Scala is a hybrid language. This leads us to a few famous design patterns that are not needed anymore—one example is the null object design pattern, which can simply be replaced by Scala's Option. Other design patterns become possible using different approaches—the decorator design pattern can be implemented using stackable traits. Finally, some new design patterns become available, which are applicable specifically to the Scala programming language—the cake design pattern, pimp my library, and so on. We will focus on all of these and make it clear where the richness of Scala helps us to make our code even cleaner and simpler.

Even if there are many different design patterns, they can all be collated into a few main groups:

• Creational
• Structural
• Behavioral
• Functional
• Scala-specific design patterns

Some of the design patterns that are specific to Scala can be assigned to the previous groups. They can either be additional or replacements of the already existing ones. They are typical to Scala and take advantage of some advanced language features or simply features not available in other languages.

The first three groups contain the famous Gang of Four design patterns. Every design pattern book covers them and so will we. The rest, even if they can be assigned to one of the first three groups, will be specific to Scala and functional programming languages. In the next few subsections, we will explain the main characteristics of these groups and briefly present the actual design patterns that fall under them.

Creational design patterns

The creational design patterns deal with object creation mechanisms. Their purpose is to create objects in a way that is suitable to the current situation, which could lead to unnecessary complexity and the need of extra knowledge if they were not there. The main ideas behind the creational design patterns are as follows:

• Knowledge encapsulation about the concrete classes
• Hiding details about the actual creation and how objects are combined

We will be focusing on the following creational design patterns in this book:

• The abstract factory design pattern
• The factory method design pattern
• The lazy initialization design pattern
• The singleton design pattern
• The object pool design pattern
• The builder design pattern
• The prototype design pattern

The following few sections give a brief definition of what these patterns are. They will be looked at in depth individually later in this book.

The abstract factory design pattern

This is used to encapsulate a group of individual factories that have a common theme. When used, the developer creates a specific implementation of the abstract factory and uses its methods in the same way as in the factory design pattern to create objects. It can be thought of as another layer of abstraction that helps instantiating classes.

The factory method design pattern

This design pattern deals with creation of objects without explicitly specifying the actual class that the instance will have—it could be something that is decided at runtime based on many factors. Some of these factors can include operating systems, different data types, or input parameters. It gives developers the peace of mind of just calling a method rather than invoking a concrete constructor.

The lazy initialization design pattern

This design pattern is an approach to delay the creation of an object or the evaluation of a value until the first time it is needed. It is much more simplified in Scala than it is in an object-oriented language such as Java.

The singleton design pattern

This design pattern restricts the creation of a specific class to just one object. If more than one class in the application tries to use such an instance, then this same instance is returned for everyone. This is another design pattern that with the use of basic Scala features can be easily achieved.

The object pool design pattern

This design pattern uses a pool of objects that are already instantiated and ready for use. Whenever someone requires an object from the pool, it is returned, and after the user is finished with it, it puts it back into the pool manually or automatically. A common use for pools are database connections, which generally are expensive to create; hence, they are created once and then served to the application on request.

The builder design pattern

The builder design pattern is extremely useful for objects with many possible constructor parameters, which would otherwise require developers to create many overrides for the different scenarios an object could be created in. This is different to the factory design pattern, which aims to enable polymorphism. Many of the modern libraries today employ this design pattern. As we will see later, Scala can achieve this pattern really easily.

The prototype design pattern

This design pattern allows object creation using a clone() method from an already created instance. It can be used in cases when a specific resource is expensive to create or when the abstract factory pattern is not desired.

Structural design patterns

Structural design patterns exist in order to help establish the relationships between different entities in order to form larger structures. They define how each component should be structured so that it has very flexible interconnecting modules that can work together in a larger system. The main features of structural design patterns include the following:

• The use of composition to combine the implementations of multiple objects
• Help build a large system made of various components by maintaining a high level of flexibility

In this book, we will focus on the following structural design patterns:

• The adapter design pattern
• The decorator design pattern
• The bridge design pattern
• The composite design pattern
• The facade design pattern
• The flyweight design pattern
• The proxy design pattern

The next subsections will put some light on what these patterns are about before we delve in them later in this book.

The adapter design pattern

The adapter design pattern allows the interface of an existing class to be used from another interface. Imagine that there is a client who expects your class to expose a doWork() method. You might have the implementation ready in another class but the method is called differently and is incompatible. It might require extra parameters too. This could also be a library that the developer doesn't have access to for modifications. This is where the adapter can help by wrapping the functionality and exposing the required methods. The adapter is useful for integrating the existing components. In Scala, the adapter design pattern can be easily achieved using implicit classes.

The decorator design pattern

Decorators are a flexible alternative to subclassing. They allow developers to extend the functionality of an object without affecting other instances of the same class. This is achieved by wrapping an object of the extended class into one that extends the same class and overrides the methods whose functionality is supposed to be changed. Decorators in Scala can be built much more easily using another design pattern called stackable traits.

The bridge design pattern

The purpose of the bridge design pattern is to decouple an abstraction from its implementation so that the two can vary independently. It is useful when the class and its functionality vary a lot. The bridge reminds us of the adapter pattern, but the difference is that the adapter pattern is used when something is already there and you cannot change it, while the bridge design pattern is used when things are being built. It helps us to avoid ending up with multiple concrete classes that will be exposed to the client. You will get a clearer understanding when we delve deeper in the topic, but for now, let's imagine that we want to have a FileReader class that supports multiple different platforms. The bridge will help us end up with FileReader, which will use a different implementation, depending on the platform. In Scala, we can use self-types in order to implement a bridge design pattern.

The composite design pattern

The composite is a partitioning design pattern that represents a group of objects that are to be treated as only one object. It allows developers to treat individual objects and compositions uniformly and to build complex hierarchies without complicating the source code. An example of composite could be a tree structure where a node can contain other nodes, and so on.

The facade design pattern

The purpose of the facade design pattern is to hide the complexity of a system and its implementation details by providing a simpler interface to use to the client. This also helps to make the code more readable and to reduce the dependencies of the outside code. It works as a wrapper around the system that is being simplified, and of course, it can be used in conjunction with some of the other design patterns we mentioned previously.

The flyweight design pattern

The flyweight design pattern provides an object that is used to minimize memory usage by sharing it throughout the application. This object should contain as much data as possible. A common example given is a word processor, where each character's graphical representation is shared with the other same characters. The local information then is only the position of the character, which is stored internally.

The proxy design pattern

The proxy design pattern allows developers to provide an interface to other objects by wrapping them. They can also provide additional functionality, for example, security or thread-safety. Proxies can be used together with the flyweight pattern, where the references to shared objects are wrapped inside proxy objects.

Behavioral design patterns

Behavioral design patterns increase communication flexibility between objects based on the specific ways they interact with each other. Here, creational patterns mostly describe a moment in time during creation, structural patterns describe a more or less static structure, and behavioral patterns describe a process or flow. They simplify this flow and make it more understandable.

The main features of behavioral design patterns are as follows:

• What is being described is a process or flow
• The flows are simplified and made understandable
• They accomplish tasks that would be difficult or impossible to achieve with objects

In this book, we will focus our attention on the following behavioral design patterns:

• The value object design pattern
• The null object design pattern
• The strategy design pattern
• The command design pattern
• The chain of responsibility design pattern
• The interpreter design pattern
• The iterator design pattern
• The mediator design pattern
• The memento design pattern
• The observer design pattern
• The state design pattern
• The template method design pattern
• The visitor design pattern

The following subsections will give brief definitions of the aforementioned behavioral design patterns.

The value object design pattern

Value objects are immutable and their equality is based not on their identity, but on their fields being equal. They can be used as data transfer objects, and they can represent dates, colors, money amounts, numbers, and so on. Their immutability makes them really useful in multithreaded programming. The Scala programming language promotes immutability, and value objects are something that naturally something occurs there.

The null object design pattern

Null objects represent the absence of a value and they define a neutral behavior. This approach removes the need to check for null references and makes the code much more concise. Scala adds the concept of optional values, which can replace this pattern completely.

The strategy design pattern

The strategy design pattern allows algorithms to be selected at runtime. It defines a family of interchangeable encapsulated algorithms and exposes a common interface to the client. Which algorithm is chosen could depend on various factors that are determined while the application runs. In Scala, we can simply pass a function as a parameter to a method, and depending on the function, a different action will be performed.

The command design pattern

This design pattern represents an object that is used to store information about an action that needs to be triggered at a later time. The information includes the following:

• The method name
• The owner of the method
• Parameter values

The client then decides which commands to be executed and when by the invoker. This design pattern can easily be implemented in Scala using the by-name parameters feature of the language.

The chain of responsibility design pattern

The chain of responsibility is a design pattern where the sender of a request is decoupled from its receiver. This way, it makes it possible for multiple objects to handle the request and to keep logic nicely separate. The receivers form a chain where they pass the request, and if possible, they process it, and if not, they pass it to the next receiver. There are variations where a handler might dispatch the request to multiple other handlers at the same time. This somehow reminds of function composition, which in Scala can be achieved using the stackable traits design pattern.

The interpreter design pattern

The interpreter design pattern is based on the possibility to characterize a well-known domain with a language with strict grammar. It defines classes for each grammar rule in order to interpret sentences in the given language. These classes are likely to represent hierarchies as grammar is usually hierarchical as well. Interpreters can be used in different parsers, for example, SQL or other languages.

The iterator design pattern

The iterator design pattern is when an iterator is used to traverse a container and access its elements. It helps to decouple containers from the algorithms performed on them. What an iterator should provide is sequential access to the elements of an aggregate object without exposing the internal representation of the iterated collection.

The mediator design pattern

This pattern encapsulates the communication between different classes in an application. Instead of interacting directly with each other, objects communicate through the mediator, which reduces the dependencies between them, lowers the coupling, and makes the overall application easier to read and maintain.

The memento design pattern

This pattern provides the ability to rollback an object to its previous state. It is implemented with three objects: originator, caretaker, and memento. The originator is the object with the internal state; the caretaker will modify the originator, and a memento is an object that contains the state that the originator returns. The originator knows how to handle a memento in order to restore its previous state.

The observer design pattern

This design pattern allows the creation of publish/subscribe systems. There is a special object called subject that automatically notifies all the observers when there are any changes in the state. This design pattern is popular in various GUI toolkits and generally where event handling is needed. It is also related to reactive programming, which is enabled by libraries such as Akka, and we will see an example of this toward the end of this book.

The state design pattern

This design pattern is similar to the strategy design pattern, and it uses a state object to encapsulate different behavior for the same object. It improves the code's readability and maintainability by avoiding the use of large conditional statements.

The template method design pattern

This design pattern defines the skeleton of an algorithm in a method and then passes some of the actual steps to the subclasses. It allows developers to alter some of the steps of an algorithm without having to modify its structure. An example of this could be a method in an abstract class that calls other abstract methods, which will be defined in the children.

The visitor design pattern

The visitor design pattern represents an operation to be performed on the elements of an object structure. It allows developers to define a new operation without changing the original classes. Scala can minimize the verbosity of this pattern compared to the pure object-oriented way of implementing it by passing functions to methods.

Functional design patterns

We will be looking into all of the preceding design patterns from the point of view of Scala. This means that they will look different than in other languages, but they've still been designed not specifically for functional programming. Functional programming is much more expressive than object-oriented programming. It has its own design patterns that help making the life of a programmer easier. We will focus on:

• Monoids
• Monads
• Functors

After we've looked at some Scala functional programming concepts, and we've been through these, we will mention some interesting design patterns from the Scala world.

A brief explanation of the preceding listed patterns will follow in the next few subsections.

Monoids

Monoid is a concept that comes from mathematics. We will take a look at it in more detail with all the theory needed to understand it later in this book. For now, it will be enough to remember that a monoid is an algebraic structure with a single associative binary operation and an identity element. Here are the keywords you should remember:

• The associative binary operation. This means (a+b)+c = a+(b+c).
• The identity element. This means a+i = i+a = a. Here, the identity is i.

What is important about monoids is that they give us the possibility to work with many different types of values in a common way. They allow us to convert pairwise operations to work with sequences; the associativity gives us the possibility for parallelization, and the identity element allows us to know what to do with empty lists. Monoids are great to easily describe and implement aggregations.

Monads

In functional programming, monads are structures that represent computations as sequences of steps. Monads are useful for building pipelines, adding operations with side effects cleanly to a language where everything is immutable, and implementing compositions. This definition might sound vague and unclear, but explaining monads in a few sentences seems to be something hard to achieve. Later in this book, we will focus on them and try and clear things up without the use of a complex mathematical theory. We will try to show why monads are useful and what they can help with, as long as developers understand them well.

Functors

Functors come from a category theory, and as for monads, it takes time to explain them properly. We will look at functors later in this book. For now, you could remember that functors are things that can allow us to lift a function of the type A => B to a function of the type F[A] => F[B].

Scala-specific design patterns

The design patterns in this group could be assigned to some of the previous groups. However, they are specific to Scala and exploit some of the language features that we will focus on in this book, and we've decided to place them in their own group.

We will focus our attention on the following ones:

• The lens design pattern
• The cake design pattern
• Pimp my library
• Stackable traits
• The type class design pattern
• Lazy evaluation
• Partial functions
• Implicit injection
• Duck typing
• Memoization

The next subsections will give you some brief information about these patterns before we properly study them later in this book.

The lens design pattern

The Scala programming language promotes immutability. Having objects immutable makes it harder to make mistakes. However, sometimes mutability is required and the lens design pattern helps us achieve this nicely.

The cake design pattern

The cake design pattern is the Scala way to implement dependency injection. It is something that is used quite a lot in real-life applications, and there are numerous libraries that help developers achieve it. Scala has a way of doing this using language features, and this is what the cake design pattern is all about.

Pimp my library

Many times, engineers need to work with libraries, which are made to be as generic as possible. Sometimes, we need to do something more specific to our use case, though. The pimp my library design pattern provides a way to write extension methods for libraries, which we cannot modify. We can also use it for our own libraries as well. This design pattern also helps to achieve a better code readability.

Stackable traits

The stackable traits is the Scala way to implement the decorator design pattern. It can also be used to compose functions, and it's based on a few advanced Scala features.

The type class design pattern

This design pattern allows us to write generic code by defining a behavior that must be supported by all members of a specific type class. For example, all numbers must support the addition and subtraction operations.

Lazy evaluation

Many times, engineers have to deal with operations that are slow and/or expensive. Sometimes, the result of these operations might not even be needed. Lazy evaluation is a technique that postpones the operation execution until it is actually needed. It could be used for application optimization.

Partial functions

Mathematics and functional programming are really close together. As a consequence, there are functions that exist that are only defined for a subset of all the possible input values they can get. A popular example is the square root function, which only works for non-negative numbers. In Scala, such functions can be used to efficiently perform multiple operations at the same time or to compose functions.

Implicit injection

Implicit injection is based on the implicit functionality of the Scala programming language. It automatically injects objects whenever they are needed, as long as they exist in a specific scope. It can be used for many things, including dependency injection.

Duck typing

This is a feature that is available in Scala and is similar to what some dynamic languages provide. It allows developers to write code, which requires the callers to have some specific methods (but not implement an interface). When someone uses a method with a duck type, it is actually checked during compile time whether the parameters are valid.

Memoization

This design pattern helps with optimization by remembering function results, based on the inputs. This means that as long as the function is stable and will return the same result when the same parameters are passed, one can remember its results and simply return them for every consecutive identical call.