Book Image

Java 9 with JShell

By : Gaston C. Hillar
Book Image

Java 9 with JShell

By: Gaston C. Hillar

Overview of this book

The release of Java 9 has brought many subtle and not-so-subtle changes to the way in which Java programmers approach their code. The most important ones are definitely the availability of a REPL, known as JShell, which will make experiments and prototyping much more straightforward than the old IDE-based project-led approach. Another, more subtle change can be seen in the module system, which will lead to more modularized, maintainable code. The techniques to take full advantage of object-oriented code, functional programming and the new modularity features in Java 9 form the main subjects of this book. Each chapter will add to the full picture of Java 9 programming starting out with classes and instances and ending with generics and modularity in Java.

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Java 9 with JShell
Gaston C. Hillar
Mar, 2017 | 408 pages
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Table of Contents (23 chapters)
Java 9 with JShell
Java 9 with JShell
Credits
Credits
About the Author
About the Author
Acknowledgement
Acknowledgement
About the Reviewer
About the Reviewer
www.PacktPub.com
www.PacktPub.com
Customer Feedback
Customer Feedback
Preface
Preface
Free Chapter
1
JShell – A Read-Evaluate-Print-Loop for Java 9
JShell – A Read-Evaluate-Print-Loop for Java 9
Getting ready for our journey toward OOP with Java 9
Installing the required software on Windows, macOS, or Linux
Running Java 9 code in JShell
Working with variables, methods, and sources
Editing the source code in our favorite external code editor
Loading source code
Test your knowledge
Summary
2
Real-World Objects to UML Diagrams and Java 9 via JShell
Real-World Objects to UML Diagrams and Java 9 via JShell
Identifying objects from applications requirements
Capturing real-world objects
Generating classes to create objects
Recognizing variables and constants
Identifying actions to create methods
Organizing classes with UML diagrams
Using feedback from domain experts
Test your knowledge
Summary
3
Classes and Instances
Classes and Instances
Understanding classes and instances in Java 9
Working with object initialization and its customization
Introducing garbage collection
Declaring classes
Customizing constructors and initialization
Understanding how garbage collection works
Creating instances of classes and understanding their scope
Exercises
Test your knowledge
Summary
4
Encapsulation of Data
Encapsulation of Data
Understanding members composing a class
Declaring immutable fields
Working with setters and getters
Combining setters, getters, and fields
Transforming values with setters and getters
Using static fields to provide class-level values
Using static methods to provide overridable class-level values
Test your knowledge
Summary
5
Mutable and Immutable Classes
Mutable and Immutable Classes
Creating mutable classes in Java 9
Working with mutable objects in JShell
Building immutable classes in Java 9
Working with immutable objects in JShell
Understanding the differences between mutating and non-mutating objects
Learning the advantages of non-mutating objects when writing concurrent code
Working with instances of the immutable String class
Creating the immutable version of an existing mutable class
Test your knowledge
Summary
6
Inheritance, Abstraction, Extension, and Specialization
Inheritance, Abstraction, Extension, and Specialization
Creating class hierarchies to abstract and specialize behavior
Understanding inheritance
Creating an abstract base class
Declaring classes that inherit from another class
Overriding and overloading methods
Test your knowledge
Summary
7
Members Inheritance and Polymorphism
Members Inheritance and Polymorphism
Creating concrete classes that inherit from abstract superclasses
Understanding polymorphism
Controlling overridability of members in subclasses
Controlling subclassing of classes
Creating methods that work with instances of different subclasses
Test your knowledge
Summary
8
Contract Programming with Interfaces
Contract Programming with Interfaces
Understanding how interfaces work in combination with classes
Declaring interfaces
Declaring classes that implement interfaces
Taking advantage of the multiple inheritance of interfaces
Combining class inheritance and interfaces
Test your knowledge
Summary
9
Advanced Contract Programming with Interfaces
Advanced Contract Programming with Interfaces
Working with methods receiving interfaces as arguments
Downcasting with interfaces and classes
Treating instances of an interface type as a different subclass
Taking advantage of default methods in interfaces in Java 9
Test your knowledge
Summary
10
Maximization of Code Reuse with Generics
Maximization of Code Reuse with Generics
Understanding parametric polymorphism, Java 9 generics, and generic code
Declaring an interface to be used as a type constraint
Declaring a class that conforms to multiple interfaces
Declaring subclasses that inherit the implementation of interfaces
Creating exception classes
Declaring a class that works with a constrained generic type
Using a generic class for multiple compatible types
Test your knowledge
Summary
11
Advanced Generics
Advanced Generics
Creating a new interface to be used as a constraint for a second type parameter
Declaring two classes that implement an interface to work with two type parameters
Declaring a class that works with two constrained generic types
Creating instances of a generic class with two generic type parameters
Test your knowledge
Summary
12
Object-Oriented, Functional Programming, and Lambda Expressions
Object-Oriented, Functional Programming, and Lambda Expressions
Understanding functions and methods as first-class citizens
Working with functional interfaces and lambda expressions
Creating a functional version of array filtering
Creating a data repository with generics and interfaces
Filtering collections with complex conditions
Using a map operation to transform values
Combining a map operation with reduce
Chaining many operations with map and reduce
Working with different collectors
Test your knowledge
Summary
13
Modularity in Java 9
Modularity in Java 9
Refactoring existing code to take advantage of object-oriented programming
Organizing object-oriented code with the new modularity in Java 9
Creating modular source code
Compiling multiple modules with the Java 9 compiler
Run modularized code with Java 9
Test your knowledge
Summary
Exercise Answers
Exercise Answers
Chapter 1, JShell – A Read-Evaluate-Print-Loop for Java 9
Chapter 2, Real-World Objects to UML Diagrams and Java 9 via JShell
Chapter 3, Classes and Instances
Chapter 4, Encapsulation of Data
Chapter 5, Mutable and Immutable Classes
Chapter 6, Inheritance, Abstraction, Extension, and Specialization
Chapter 7, Members Inheritance and Polymorphism
Chapter 8, Contract Programming with Interfaces
Chapter 9, Advanced Contract Programming with Interfaces
Chapter 10, Maximization of Code Reuse with Generics
Chapter 11, Advanced Generics
Chapter 12, Object-Oriented, Functional Programming, and Lambda Expressions
Chapter 13, Modularity in Java 9
Index
Index

Combining a map operation with reduce

The following lines show an imperative code version of a for loop that calculates the sum of all the lowestScore values for the mobile games. The code file for the sample is included in the java_9_oop_chapter_12_01 folder, in the example12_13.java file.

int lowestScoreSum = 0;
for (MobileGame mobileGame : repository.getAll()) {
    lowestScoreSum += mobileGame.lowestScore;
}
System.out.println(lowestScoreSum);

The code is very easy to understand. The lowestScoreSum variable has a starting value of 0, and each iteration of the for loop retrieves a MobileGame instance from the List<MobileGame> returned by the repository.getAll() method and increases the value of the lowestScoreSum variable with the value of the mobileGame.lowestScore field.

We can combine the map and reduce operations to create a functional version of the previous imperative code to calculate the sum of all the lowestScore values for the mobile games. The next lines chain a call to...

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