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Scala Functional Programming Patterns
Scala Functional Programming Patterns

Scala Functional Programming Patterns: Grok and perform effective functional programming in Scala

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Scala Functional Programming Patterns

Chapter 2. Singletons, Factories, and Builders

It may sound funny—and I may be stating the obvious—but everyone needs to be born at some point. Go ahead and have a good laugh at that. It is that obvious. Objects, too, need to be born at some point to do useful work. Objects have a lifetime as well. An object is constructed—and hopefully it does something useful before it eventually dies.

In Java we can see the object such as:

           Point p = new Point(23, 94);

We know what is going on—an object of class point is created; its constructor-invoked p is a reference to this newly created object.

At times, we want explicit control of the object-creation process. There are times when we want to allow creation of only one instance of a class. Creational design patterns deal with object-creation mechanisms. Refer to https://sourcemaking.com/design_patterns/creational_patterns for more information on creational patterns.

Creational patterns help create objects...

Singletons – being one and only one

A singleton is a class of which only a single instance can exist. How do we prevent anyone from creating yet another instance? The solution is to make the constructor inaccessible. Here it is:

public class Singleton {
      // Eager initialization
  private static final Singleton instance = new Singleton(); // 1
 
  private Singleton() { // 2
  /* client code cannot create instance */
 }
 
      // Static factory method 
 public static Singleton getInstance() { // 3
  return instance;
 }

 // Driver code
 public static void main(String[] args) {
  System.out.println(Singleton.getInstance());
  System.out.println(Singleton.getInstance());
 }
}

Dissecting the code:

  • At 1, the static initializer creates the instance—also the final keyword ensures that the instance cannot be redefined.
  • At 2, the constructor access is private, so only the class methods can access it.
  • At 3, the public factory method gives access to the client code.

If you run the Java...

Null Objects – singletons in another garb

One popular form in which singletons are used is the Null Object pattern. Let's see what a Null Object is.

Java gives us the null reference to indicate a missing value. We are not supposed to call a method on it as there is no object. If, erroneously, we do, we are greeted with a Null Pointer Exception (NPE). When we design methods to return nulls, the client code that calls the method needs to check assiduously. This is what a typical null check-based Java code looks like:

       Point p = makeAPoint(); // a method that returns null in some cases
       if (p != null) { // the dreaded null check – onus is on us...
            // We are on sure grounds
       }

The problem is that the onus term is on us to check for a reference being null. Every call to makeAPoint() needs to be checked, as shown earlier. This soon becomes tedious.

Note

The inventor of the null keyword called it his billion-dollar mistake!!! Please see http://www.infoq...

Scala singletons

Scala has singleton objects called companion objects. A companion object is an object with the same name as a class. A companion object also can access private methods and fields of its companion class. Both a class and its companion object must be defined in the same source file. The companion object is where the apply() factory method may be defined. Let's have a look at the following example of a companion class:

class Singleton {  // Companion class
  def m() {
    println("class")
  }         
}

And then its companion object as:

object Singleton { // Companion Object
  def m() {
    println("companion")
  }         
}

It is that simple, when a case class is defined, Scala automatically generates a companion object for it.

The apply() factory method

If a companion object defines an apply() method, the Scala compiler calls it when it sees the class name followed by (). So, for example, when Scala sees something like:

                 Singleton(arg1...

Singletons – being one and only one


A singleton is a class of which only a single instance can exist. How do we prevent anyone from creating yet another instance? The solution is to make the constructor inaccessible. Here it is:

public class Singleton {
      // Eager initialization
  private static final Singleton instance = new Singleton(); // 1
 
  private Singleton() { // 2
  /* client code cannot create instance */
 }
 
      // Static factory method 
 public static Singleton getInstance() { // 3
  return instance;
 }

 // Driver code
 public static void main(String[] args) {
  System.out.println(Singleton.getInstance());
  System.out.println(Singleton.getInstance());
 }
}

Dissecting the code:

  • At 1, the static initializer creates the instance—also the final keyword ensures that the instance cannot be redefined.

  • At 2, the constructor access is private, so only the class methods can access it.

  • At 3, the public factory method gives access to the client code.

If you run the Java program, you...

Null Objects – singletons in another garb


One popular form in which singletons are used is the Null Object pattern. Let's see what a Null Object is.

Java gives us the null reference to indicate a missing value. We are not supposed to call a method on it as there is no object. If, erroneously, we do, we are greeted with a Null Pointer Exception (NPE). When we design methods to return nulls, the client code that calls the method needs to check assiduously. This is what a typical null check-based Java code looks like:

       Point p = makeAPoint(); // a method that returns null in some cases
       if (p != null) { // the dreaded null check – onus is on us...
            // We are on sure grounds
       }

The problem is that the onus term is on us to check for a reference being null. Every call to makeAPoint() needs to be checked, as shown earlier. This soon becomes tedious.

Note

The inventor of the null keyword called it his billion-dollar mistake!!! Please see http://www.infoq.com/presentations...

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Key benefits

  • Understand functional programming patterns by comparing them with the traditional object-oriented design patterns
  • Write robust, safer, and better code using the declarative programming paradigm
  • An illustrative guide for programmers to create functional programming patterns with Scala

Description

Scala is used to construct elegant class hierarchies for maximum code reuse and extensibility and to implement their behavior using higher-order functions. Its functional programming (FP) features are a boon to help you design “easy to reason about” systems to control the growing software complexities. Knowing how and where to apply the many Scala techniques is challenging. Looking at Scala best practices in the context of what you already know helps you grasp these concepts quickly, and helps you see where and why to use them. This book begins with the rationale behind patterns to help you understand where and why each pattern is applied. You will discover what tail recursion brings to your table and will get an understanding of how to create solutions without mutations. We then explain the concept of memorization and infinite sequences for on-demand computation. Further, the book takes you through Scala’s stackable traits and dependency injection, a popular technique to produce loosely-coupled software systems. You will also explore how to currying favors to your code and how to simplify it by de-construction via pattern matching. We also show you how to do pipeline transformations using higher order functions such as the pipes and filters pattern. Then we guide you through the increasing importance of concurrent programming and the pitfalls of traditional code concurrency. Lastly, the book takes a paradigm shift to show you the different techniques that functional programming brings to your plate. This book is an invaluable source to help you understand and perform functional programming and solve common programming problems using Scala’s programming patterns.

Who is this book for?

If you have done Java programming before and have a basic knowledge of Scala and its syntax, then this book is an ideal choice to help you to understand the context, the traditional design pattern applicable, and the Scala way. Having previous knowledge of design patterns will help, though it is not strictly necessary.

What you will learn

  • Get to know about functional programming and the value Scala s FP idioms bring to the table
  • Solve day-to-day programming problems using functional programming idioms
  • Cut down the boiler-plate and express patterns simply and elegantly using Scala s concise syntax
  • Tame system complexity by reducing the moving parts
  • Write easier to reason about concurrent code using the actor paradigm and the Akka library
  • Apply recursive thinking and understand how to create solutions without mutation
  • Reuse existing code to compose new behavior
  • Combine the object-oriented and functional programming approaches for effective programming using Scala
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Publication date : Dec 29, 2015
Length: 298 pages
Edition : 1st
Language : English
ISBN-13 : 9781783985845
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Table of Contents

12 Chapters
1. Grokking the Functional Way Chevron down icon Chevron up icon
2. Singletons, Factories, and Builders Chevron down icon Chevron up icon
3. Recursion and Chasing your Own Tail Chevron down icon Chevron up icon
4. Lazy Sequences – Being Lazy, Being Good Chevron down icon Chevron up icon
5. Taming Multiple Inheritance with Traits Chevron down icon Chevron up icon
6. Currying Favors with Your Code Chevron down icon Chevron up icon
7. Of Visitors and Chains of Responsibilities Chevron down icon Chevron up icon
8. Traversals – Mapping/Filtering/Folding/Reducing Chevron down icon Chevron up icon
9. Higher Order Functions Chevron down icon Chevron up icon
10. Actors and Message Passing Chevron down icon Chevron up icon
11. It's a Paradigm Shift Chevron down icon Chevron up icon
Index Chevron down icon Chevron up icon

Customer reviews

Rating distribution
Full star icon Full star icon Full star icon Half star icon Empty star icon 3.5
(2 Ratings)
5 star 0%
4 star 50%
3 star 50%
2 star 0%
1 star 0%
cust: Feb 05, 2024
Full star icon Full star icon Full star icon Full star icon Empty star icon 4
-gets you up to speed in knowning basics in fp world
Amazon Verified review Amazon
Mark V. Jan 14, 2016
Full star icon Full star icon Full star icon Empty star icon Empty star icon 3
While this book introduces functional programming principles, it does so without any real plan. Deconstruction and pattern matching are mixed in a chapter on recursion. The book does pick up some pace after the first half, where it does a decent job of introducing flatMap, map, monads and monoids.This book contains a lot of Java (namely unit tests) along with Scala. I feel like the tests should have been written in Scala too, this is a Scala book after all.What annoyed me most about this book, were the links to more information. Instead of explaining the concepts briefly, there are some 10 links provided, such as "Some expensive objects may need to be lazy-initialized. Refer to http://martinfowler.com/bliki/LazyInitialization.html for a very nice introduction to lazy initialization." This makes the book feel unfinished.
Amazon Verified review Amazon
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