It takes a week to travel the 8,000 miles overland from Java to Kotlin. If you're an experienced Java developer who has tried the Kotlin language, you were probably productive in about the same time.
You'll have found that they do things differently in Kotlin, though. Nullability is important, collections are different, and classes are final by default. Kotlin is more functional, but what does that mean, and how should it change the way that you program? And what about all that Java code that you still have to support?
Your tour guides Duncan and Nat first made the trip in 2015, and they've since helped many teams and individuals follow in their footsteps. Travel with them as they break the route down into legs like Optional to Nullable, Beans to Values, and Open to Sealed Classes. Each explains a key concept and then shows how to refactor production Java to idiomatic Kotlin, gradually and safely, while maintaining interoperability.
The resulting code is simpler, more expressive, and easier to change. By the end of the journey, you'll be confident in refactoring Java to Kotlin, writing Kotlin from scratch, and managing a mixed language codebase as it evolves over time.
Author(s): Duncan McGregor, Nat Pryce
Edition: 1
Publisher: O'Reilly Media
Year: 2021
Language: English
Commentary: Vector PDF
Pages: 424
City: Sebastopol, CA
Tags: Java; Functional Programming; Refactoring; Object-Oriented Programming; Kotlin
Cover
Copyright
Table of Contents
Preface
How This Book Is Organized
How Did We Choose the Topics?
Complexity
Perfect Code
Code Formatting
Conventions Used in This Book
Using Code Examples
O’Reilly Online Learning
How to Contact Us
Acknowledgments
Duncan’s Bit
Nat’s Bit
Chapter 1. Introduction
The Grain of a Programming Language
An Opinionated History of Java Programming Style
Primeval Style
Bean Style
Enterprise Style
Modern Style
The Future
The Grain of Kotlin
Refactoring to Kotlin
Refactoring Principles
We Assume Good Test Coverage
We Commit for Git Bisect
What Are We Working On?
Let’s Get Started!
Chapter 2. Java to Kotlin Projects
Strategy
Adding Kotlin Support to a Java Build
Moving On
Chapter 3. Java to Kotlin Classes
A Simple Value Type
The Limitations of Data Classes
Moving On
Chapter 4. Optional to Nullable
Representing Absence
Refactoring from Optional to Nullable
Refactoring to Idiomatic Kotlin
Moving On
Chapter 5. Beans to Values
Beans
Values
Why Should We Prefer Values?
Refactoring Beans to Values
Moving On
Chapter 6. Java to Kotlin Collections
Java Collections
Kotlin Collections
Refactoring from Java to Kotlin Collections
Fix Up the Java
Convert to Kotlin
Moving On
Chapter 7. Actions to Calculations
Functions
Calculations
Actions
Why Should We Care?
Why Prefer Calculations?
Refactoring Actions to Calculations
Existing Code
A Better Design
End Game
Moving On
Chapter 8. Static Methods to Top-Level Functions
Java Statics
Kotlin Top-Level Functions, Objects, and Companions
Refactoring from Static Methods to Top-Level Functions
Move to Top Level
Kotlinify
Moving On
Chapter 9. Multi- to Single-Expression Functions
Take 1: Inlining
Take 2: Introduce a Function
Take 3: Let
Take 4: Stepping Back
Moving On
Chapter 10. Functions to Extension Functions
Functions and Methods
Extension Functions
Extensions and Function Types
Extension Properties
Conversions
Nullable Parameters
Nullable Receivers
Generics
Extension Functions as Methods
Refactoring to Extension Functions
Moving On
Chapter 11. Methods to Properties
Fields, Accessors, and Properties
How to Choose
Mutable Properties
Refactoring to Properties
Moving On
Chapter 12. Functions to Operators
A Foundational Class: Money
Adding a User-Defined Operator
Calling Our Operator from Existing Kotlin Code
Operators for Existing Java Classes
Conventions for Denoting Values
Moving On
Chapter 13. Streams to Iterables to Sequences
Java Streams
Kotlin Iterables
Kotlin Sequences
Multiple Iterations
Choosing Between Streams, Iterables, and Sequences
Algebraic Transformation
Refactoring from Streams to Iterables and Sequences
Iterables First
Then Sequences
Extracting Part of a Pipeline
Final Tidy
Moving On
Chapter 14. Accumulating Objects to Transformations
Calculating with Accumulator Parameters
Refactoring to Functions over Immutable Data
Let’s Do That Again
Enriching the Abstraction We Discovered
Moving On
Chapter 15. Encapsulated Collections to Type Aliases
Composing Domain Collections
Collections with Other Properties
Refactoring Encapsulated Collections
Convert Operations to Extensions
Substitute a Type Alias
Refactoring Collections with Other Properties
Moving On
Chapter 16. Interfaces to Functions
Object-Oriented Encapsulation
Functional Encapsulation
Function Types in Java
Mix and Match
Comparing the Approaches
Coupling
Object-Oriented or Functional?
The Legacy of Java
Traceability
Refactoring from Interfaces to Functions
Introduce Functions
Moving On
Chapter 17. Mocks to Maps
Replacing Mocks with Maps
Have We Really Weaned Off Mocks, Though?
Moving On
Chapter 18. Open to Sealed Classes
Polymorphism or Sealed Classes?
Converting an Interface to a Sealed Class
Moving On
Chapter 19. Throwing to Returning
Error Handling Before Exceptions
Error Handling with Exceptions
Java and Checked Exceptions
Kotlin and Exceptions
Beyond Exceptions: Functional Error Handling
Error Handling in Kotlin
Refactoring Exceptions to Errors
Our Conversion Strategy
Starting at the Bottom
Contract
Stepping Out
More Fixup
Layers
Moving On
Chapter 20. Performing I/O to Passing Data
Listening to Tests
I/O to Data
Efficient Writing
Efficient Reading
Moving On
Chapter 21. Exceptions to Values
Identifying What Can Go Wrong
Representing Errors
What About I/O?
Moving On
Chapter 22. Classes to Functions
An Acceptance Test
Unit Testing
Headers
Different Field Separators
Sequences
Reading from a File
Comparison with Commons CSV
Moving On
Chapter 23. Continuing the Journey
Grain
Functional Thinking
Simple Design
Functional Programming and Textual Reasoning
Refactoring
Refactoring and Functional Thinking
Bibliography
Learning Kotlin
Testing and Test-Driven Development
Working with Legacy Code
Software Design and Development
Java and the JVM
Index
About the Authors
Colophon
