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Scala for Java Developers
Scala for Java Developers

Scala for Java Developers: Build reactive, scalable applications and integrate Java code with the power of Scala.

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Scala for Java Developers

Chapter 2. Code Integration

Being able to make Java and Scala cooperate on the same code base is a prerequisite to guarantee a smooth transition between the two languages.

In this chapter, we are going to quickly create a small Java web application in which we will show you how to add Scala code to it. Then, we will cover some of the most common integration points between Java and Scala and how programming styles differ so that programmers who want to refactor and extend their Java application can do it according to some guidelines.

To avoid spending too much time on creating, understanding, and documenting a sample Java project, we are going to use a small database that is already available as part of the Oracle's NetBeans IDE distribution and create a JPA persistence layer as well as a REST API from it using the code generation features of the IDE.

Tip

Download the sample Java project

If you are impatient to directly jump into the Scala code integration features of this chapter, you may skip...

Creating a REST API from an existing database


The sample database bundled with the NetBeans IDE can be downloaded from the www.netbeans.org website. Just click on the Download button on this website and pick the JavaEE version of the IDE.

Once you have run the installation wizard, seen the The installation was successful! message, and started the IDE (Version 8.0 in our case), we are ready to create a fully functional web app in five minutes. The first time you use it, just click on the upper-left corner of the NetBeans IDE to close the startup screen and you should see the three tabs: Projects, Files, and Services on the left-hand side of the IDE.

The sample database

Our reference database can be seen from the IDE by clicking on the Services panel. Under the Databases menu that is part of the Services tab, double-click on the jdbc:derby://localhost:1527/sample [app on APP] Database Connection link to connect to the sample database on port 1527 (the default port for Derby databases) with the...

Adding a test in Scala


For now, we have only Java code in the small sample Maven project. We are ready to introduce a few lines of Scala to the same codebase in order to show how both languages seamlessly interoperate. Let's create a src/test/scala directory, next to the existing java/ directory, where we will put our following new CustomerScalaTest.scala class, which is a similar test to the one we already have under src/test/java:

package com.demo.sample

import org.junit._
import Assert._

class CustomerScalaTest {

  @Before
  def setUp: Unit = {
  }

  @After
  def tearDown: Unit = {
  }

  @Test
  def testGetCustomerId = {
    System.out.println("getCustomerId")
    val instance = new Customer()
    val expResult: Integer = null
    val result: Integer = instance.getCustomerId()
    assertEquals(expResult, result)
  }
}

If we run the tests again, that is, type >mvn clean test again, the class will just be ignored as it is not a .java source file.

Setting up Scala within a Java Maven project


In order to be able to start writing a Scala unit test and compile Scala code into our Java project, we need to add a few dependencies and the scala-maven-plugin to the pom.xml file. The dependencies are as follows:

  • Dependency for the core scala-library:

    <dependency>
      <groupId>org.scala-lang</groupId>
      <artifactId>scala-library</artifactId>
      <version>2.10.0</version>
    </dependency>
  • Dependency for scalatest (a framework for testing in Scala that supports JUnit and other styles; we will cover it in detail in Chapter 4, Testing Tools):

    <dependency>
      <groupId>org.scalatest</groupId>
      <artifactId>scalatest_2.10</artifactId>
      <version>2.0/version>
      <scope>test</scope>
    </dependency>
  • Dependency for JUnit to use Java Assert statements in our test case:

    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId...

Scala and Java collaboration


Going back to the REPL, we are going to experiment further with mixing Scala and Java to explore some common integration needs, and in particular, testing and manipulating the Java REST API that we have built at the beginning of the chapter.

As a reminder on how to restart the REPL from the hello-scala project introduced in Chapter 1, Programming Interactively within Your Project, if you closed it in the meantime, just start a new terminal window, navigate to the root of the hello-scala project, and enter the following command in the command prompt:

> ./activator console

Converting between collection types

Let's start by comparing Java and Scala collection classes and see how we can go from one to the other. For instance, a Scala List (from the scala.collection.immutable package) is different from java.util.List, and sometimes, it can be useful to convert from one to the other. A convenient way in Java to create java.util.List is to use the java.util.Arrays...

Differences in style between Java and Scala code


If you are going to refactor or rewrite Java code into Scala code, there are a number of style differences that are useful to be aware of. Obviously, programming style is largely a matter of taste; however, a few guidelines generally acknowledged by the Scala community can help someone new to Scala to write easier-to-read and more maintainable code. This section is dedicated to showing some of the most common differences.

Writing an algorithm in Java follows an imperative style, that is, a sequence of statements that change a program state. Scala, focusing primarily on functional programming, adopts a more declarative approach, where everything is an expression rather than a statement. Let's illustrate this in an example.

In Java, you would commonly find the following code snippet:

...
String customerLevel = null;
if(amountBought > 3000) {
    customerLevel = "Gold"; 
} else {
    customerLevel = "Silver";
}
...

The Scala equivalent consists...

Summary


In this chapter, we have covered how to start integrating Scala code into a Java codebase as well as how to refactor some of the most common Java constructs into Scala by following some style guidelines. A much more exhaustive list of style recommendations is available at http://docs.scala-lang.org/style/ if you are interested in learning more.

So far we have been mostly addressing the Scala language and syntax. In the next chapter, we are going to introduce the tools that complement it and that are necessary to make our Scala programming both productive and enjoyable.

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Table of Contents

10 Chapters
Programming Interactively within Your Project Chevron down icon Chevron up icon
Code Integration Chevron down icon Chevron up icon
Understanding the Scala Ecosystem Chevron down icon Chevron up icon
Testing Tools Chevron down icon Chevron up icon
Getting Started with the Play Framework Chevron down icon Chevron up icon
Database Access and the Future of ORM Chevron down icon Chevron up icon
Working with Integration and Web Services Chevron down icon Chevron up icon
Essential Properties of Modern Applications – Asynchrony and Concurrency Chevron down icon Chevron up icon
Building Reactive Web Applications Chevron down icon Chevron up icon
Scala Goodies Chevron down icon Chevron up icon

Customer reviews

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Rating distribution
Full star icon Full star icon Full star icon Full star icon Half star icon 4.3
(16 Ratings)
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2 star 12.5%
1 star 0%
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MK Oct 07, 2016
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Meets expectations of Java developer
Amazon Verified review Amazon
Jens Richnow May 29, 2014
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Book ReviewI enjoyed reading the book 'Scala for Java Developers'! It covers so many topics a developer is faced with every day and across the entire development stack (testing, DB interaction, UI, Web Services, etc. - the table of contents says it all). Despite of approaching such a wide range of topics each gives one gives a gentle introduction with hands-on examples that are easy to understand and follow. Also, there are always many pointers on where to go next and references for further reading.Especially now with Java 8 introducing streams and lambdas it will be a compelling cross-over as Java developers get to know functional programming which makes using both languages and/or migration from Java Scala easier.Another goog point is the advice to pick small(ish) projects/features to get into Scala, to get to run Scala alongside an existing Java project. Once a certain confidence is achieved new projects, features or modules can be written entirely in Scala to complement existing Java projects.As a disclaimer: I have been a Java developer for 15+ years and actively looking at Scala for more than a year now. I implemented a few small projects using the Typesafe stack, did read a few books about Scala and followed the courses at Coursera. I wished that this book had been available at the beginning of my journey as it really eases the introduction into Scala as an alternative/complement to Java and highlights the advantages of Scala. Nevertheless, the book provided so many new tricks and ways of implementing certain solution that it was invaluable. I applied quite a number of the nuggads straight to existing applications.In summary, the book is well written, rather pragmatic, no-nonsense in approach and easy to follow. I will certainly keep it as reference handy at all times. I can recommend the book not only to any Java developer looking into getting a new tool under the toolbelt but also developers who are new(ish) to Scala as they will pick up many tricks and tips.
Amazon Verified review Amazon
Prashanth Jul 22, 2014
Full star icon Full star icon Full star icon Full star icon Full star icon 5
Scala for Java Developers takes your journey through what is scala? How Scala integrates with Java.The book covers all the components of scala ecosystem. Scala, Play Framework, Akka and Reactive applications topics can have their own book. This book will be helpful for beginners and as well some one who are adopting and want to take quick overlook of what each technology does.The only thing I didnt like is the author used Netbeans in one chapter and eclipse in another chapter? The book should ideally have used one IDE.Note: I got the book as part of the reviewers program by Packtpub.
Amazon Verified review Amazon
Eric Le Goff May 05, 2014
Full star icon Full star icon Full star icon Full star icon Full star icon 5
I wish this book had existed when I fitst started learning Scala . Coming from the Java (object-oriented / imperative) school, all I could do is just some trial and error attempts (this is where the Scala REPL helped a lot).While other books are targetting readers from different backgrounds, this one is essentially written for those coming from the Java (object-oriented / imperative)The most interesting aspects in my opinion of this books are :- Introduction to scala collections- Overview of the interoperability with Java- Scala tools and ecosystem (SBT, ScalaTest...)- XML and JSON manipulation- Concurrency programming and how to deal with asynchronous actionsMost of all, the book is full of real life examples which take advantage of technologies such as Play / Akka / NoSql... Those are easy to test in your local environment.It should provide the reader a pragmactic guide to scala adoption.
Amazon Verified review Amazon
Priyadharshini N. Dec 07, 2015
Full star icon Full star icon Full star icon Full star icon Full star icon 5
I read this book after reading the Scala Cookbook and I found it extremely useful. It's more of "what" rather than "how".
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