Scala for Java Developers

The ultimate reason why you should adopt Scala is readability: code that is similar to plain English will make it easier for anyone (including yourself) to understand, maintain, and refactor it. Scala is unique in that it unifies the object-oriented side that Java has in order to make code modular with the power of functional languages to express transformations concisely. To illustrate how to achieve conciseness by the introduction of anonymous functions (also called lambdas) into the language, take a look at the following line of code:

List(1,2,3,4,5) filter (element => element < 4)

As a Java programmer, the line might look awkward at first since it does not follow the usual pattern of invoking method signatures on classes. A possible Java translation of the previous code could be as follows:

import java.util.*;

public class ListFilteringSample {

  public static void main(String[] args) {

    List<Integer> elements = Arrays.asList(1, 2, 3, 4, 5);

    List<Integer> filteredElements = new ArrayList<Integer>();

    for (Integer element : elements)
      if (element < 4) filteredElements.add(element);

    System.out.println("filteredElements:" + filteredElements);

  }
}

We first create a List with five integers, then create an empty List that will hold the result of the filtering and then loop over the elements of the List to retain only the ones that match the if predicate (element < 4) and finally print out the result. Even if this is straightforward to write, it requires a few lines of code, whereas the Scala line could just be read like the following:

"From the given List, filter each element such that this element is lower than 4".

The fact that the code becomes really concise but expressive makes it easier for the programmer to comprehend at once a difficult or lengthy algorithm.

Having a compiler that performs a lot of type checking and works as a personal assistant, is in our opinion, a significant advantage over languages that check types dynamically at runtime, and the fact that Java is a statically-typed language is probably one of the main reasons that made it so popular in the first place. The Scala compiler belongs to this category as well and goes even further by finding out many of the types automatically, often relieving the programmer from specifying these types explicitly in the code. Moreover, the compiler in your IDE gives instant feedback, and therefore, increases your productivity.

Scala integrates seamlessly with Java, which is a very attractive feature, to avoid reinventing the wheel. You can start running Scala today in a production environment. Large corporations such as Twitter, LinkedIn, or Foursquare (to name a few) have done that on large-scale deployments for many years now, followed recently by other big players such as Intel or Amazon. Scala compiles to Java bytecode, which means that performance will be comparable. Most of the code that you are running while executing Scala programs is probably Java code, the major difference being what programmers see and the advanced type checking while compiling code.

To achieve better performance and handle more load, modern Java frameworks and libraries for web development are now tackling difficult problems that are tied to multi-core architectures and the integration with unpredictable external systems. Scala's incentive to use immutable data structures and functional programming constructs as well as its support for parallel collections has a better chance to succeed in writing concurrent code that will behave correctly. Moreover, Scala's superior type system and macro support enable DSLs for trivially safe asynchronous constructs, for example, composable futures and asynchronous language extensions.

In summary, Scala is the only language that has it all. It is statically typed, runs on the JVM and is totally Java compatible, is both object-oriented and functional, and is not verbose, thereby leading to better productivity, less maintenance, and therefore more fun.

If you are now getting impatient to start experimenting with the promising features of Scala that were briefly described previously, this is a good time to open a browser, access the Typesafe page URL at http://www.typesafe.com/platform/getstarted, and download the Typesafe Activator.

The intent of the rest of the chapter is to incrementally introduce some of the basic concepts of Scala by typing commands in an interactive shell and get direct feedback from the compiler. This method of learning by experimentation should feel like a breath of fresh air and has already proven to be a very effective way of learning the syntax and useful constructs of the language. While Scala continues to evolve at École Polytechnique Fédérale de Lausanne (EPFL), many large and small corporations are now taking advantage of the features of the Typesafe platform.

As stated on their website, the Typesafe Activator is "a local web and command-line tool that helps developers get started with the Typesafe platform". We will cover the Activator in more detail in a later chapter dedicated to programming tools, but for now, we will only take the shortest path in getting up and running and get familiar with some of the syntax of the language.

You should now be able to extract the downloaded zip archive to your system in a directory of your choice.

Locate the activator script within the extracted archive and either right-click on it and select Open if you are running Windows or enter the following command in a terminal window if you are on Linux/Mac:

> ./activator ui 

In both cases, this will start the Activator UI in a browser window.

In the New application section of the HTML page of the Activator, click on the [Basics] Hello-Scala! template.

Notice the Location field of the HTML form in the following screenshot. It indicates where your project will be created:

At present, you don't need to pay too much attention to all of the things that happen in the background nor to the generated structure of the project. Out of curiosity, you may click on the Code view & Open in IDE tab and then on the Run tab to execute this Hello World Scala project, which should print, well, "Hello, world !".

Start a terminal window and navigate to the root directory of the hello-scala project that we just created, by entering the following command on the command line (assuming our project is under C:\Users\Thomas\hello-scala):

> cd C:\Users\Thomas\hello-scala
C:\Users\Thomas\hello-scala> activator console

This command will start the Scala interpreter, also known as Scala REPL (Read-Eval-Print-Loop), a simple command-line tool to program interactively.

In Java, you would declare a new variable by putting in order its type, followed by the name, and then the optional value:

String yourPast = "Good Java Programmer";

In Scala, the order of declaring the variable name and type is inverted, with the name appearing before its type. Let's enter the following line into the REPL:

scala> val yourPast : String = "Good Java Programmer"  [Hit Enter]
yourPast : String = "Good Java Programmer"

Inverting the order of declaring the variables, type, and name as compared to Java might at first seem a strange idea if you want to make it as easy as possible for a Java developer to grab the Scala syntax. However, it makes sense for several reasons:

You probably noticed the val variable in front of the declaration; it means that we explicitly declare the variable as immutable. We can try to modify it as shown in the following code snippet:

scala> yourPast = "Great Scala Programmer"
<console>:8: error: reassignment to val
  yourPast = "Great Scala Programmer"
           ^

The preceding code will not only give you a clear explanation of what was wrong but also the exact place where the parser did not agree (notice the ^ character precisely showing where the error lies in the line).

If we want to create a mutable variable instead, we should declare it with var as shown in the following code snippet:

scala> var yourFuture = "Good Java Programmer"
yourFuture: String = "Good Java Programmer"
scala> yourFuture = "Great Scala Programmer"
yourFuture: String = "Great Scala Programmer"

In summary, you cannot change yourPast but you can change yourFuture!

The semicolon at the end of the lines is optional in Scala; a small but pleasant feature of the language.

Let's move on to an important difference. In Java, you have primitive types such as int, char, or boolean (eight in total), as well as operators to manipulate data such as + or >. In Scala, there are only classes and objects, making Scala more "object-oriented" than Java in some way. For instance, enter the following value into the REPL:

scala> 3
res1: Int = 3

By default, the compiler created an immutable Int (integer) variable with the res1 name (that is, result 1) in case you need to reuse it later on.

Now, enter the following line in REPL:

scala> 3 + 2
res2: Int = 5

The preceding code resembles the usage of an operator (as in Java) but is in fact the invocation of a method named + called on object 3 with the input parameter 2, equivalent to the slightly less clear statement:

scala> (3).+(2)
res3: Int = 5

Syntactic sugar (that is, syntax designed to make things easier to read or express) was added here by removing the necessity to specify the parenthesis. This also means that we can now implement similar methods on our own defined types to express code elegantly. For example, we can express the addition of two Money objects of different currencies (note that the Money type does not exist in the default Scala library) by simply stating Money(10,"EUR") + Money(15,"USD"). Let's try to do that in the REPL.

First, we can define a new class Money that has a constructor parameter named amount of type Int as follows:

scala> class Money(amount:Int)
defined class Money

Now, we can create a Money instance as shown in the following code snippet:

scala> val notMuch = new Money(2)
notMuch : Money = Money@76eb235

You get back an object with its displayed reference. The REPL provides you with TAB completion, so type notMuch. and hit the Tab key:

scala> notMuch. [Tab]
asInstanceOf isInstanceOf toString

By using the preceding autocompletion, you will get suggestions of the available methods for that class, as you will get with most Java IDEs.

As shown previously, you can construct new instances of Money by invoking the constructor, but you do not have access to the amount variable since it is not a field. To make it a field of the Money class, you have to add a 'val' or 'var' declaration in front of it, as shown in the following code snippet:

scala> class Money(val amount:Int)
defined class Money

This time, instead of again typing the line that created an instance, we will use the up arrow (the shortcut to display previous expressions: the history of the console) and navigate to it:

scala> val notMuch = new Money(2)
notMuch : Money = Money@73cd15da

Invoking autocompletion on this new instance will display the following:

scala> notMuch. [Tab ]
amount asInstanceOf isInstanceOf toString

So, we can simply read the value of the getter for this amount field by referring to it:

scala> notMuch.amount
res4: Int = 2

Similarly, if we had declared the amount to be a var variable instead of val, we would also have access to the setter method:

scala> class Money(var amount:Int)
defined class Money
scala> val notMuch = new Money(2)
notMuch: Money = Money@6517ff0
scala> notMuch. [ Tab ]
amount   amount_=   asInstanceOf   isInstanceOf   toString

The setter method is invoked when we use the following code snippet:

scala> notMuch.amount=3
notMuch.amount: Int = 3

As Java developers, we are accustomed to the JavaBean style domain classes that not only include fields with getters and setters but also constructors as well as hashCode, equals, and toString methods, as shown in the following code snippet:

public class Money {

    private Integer amount;
    private String currency;

    public Money(Integer amount, String currency) {

        this.amount = amount;
        this.currency = currency;

    }

    public Integer getAmount() {
        return amount;
    }

    public void setAmount(Integer amount) {
        this.amount = amount;
    }

    public String getCurrency() {
        return currency;
    }
    public void setCurrency(String currency) {
        this.currency = currency;
    }

    @Override
    public int hashCode() {
        int hash = 5;
        hash = 29 * hash + (this.amount != null ? this.amount.hashCode() : 0);
        hash = 29 * hash + (this.currency != null ? this.currency.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object obj) {

        if (obj == null) {
            return false;
        }

        if (getClass() != obj.getClass()) {
            return false;
        }

        final Money other = (Money) obj;
        return true;
    }

    @Override
    public String toString() {
        return "Money{" + "amount=" + amount + ", currency=" + currency + '}';

    }
}

Achieving this in Scala is very straightforward and only requires the addition of the case word in front of the class declaration:

scala> case class Money(amount:Int=1, currency:String="USD")
defined class Money

We have just defined a class Money with two immutable fields named amount and currency with default values.

Without going too much into the details of the case classes, we can say that in addition to the preceding features of a traditional JavaBean style domain class, they have a powerful mechanism for pattern matching. The case word is analogous to the switch statement in Java, though it is more flexible, as we will see later on. The case classes contain additional features among which one is a factory method to create instances (no need to use the new keyword to create one).

By default, the fields declared in Scala classes are public, unlike Java, where they have a package-private scope, defined between private and protected. We could have written case class Money(private val amount: Int, private val currency: String) to make them private instead, or used var instead of val to make the fields mutable.

The shortest way to create an instance of Money is very straightforward:

scala> val defaultAmount = Money()
defaultAmount: Money = Money(1,USD)
scala> val fifteenDollars = Money(15,"USD")
fifteenDollars: Money = Money(15,USD)
scala> val fifteenDollars = Money(15)
fifteenDollars: Money = Money(15,USD) 

In the previous instance declaration, since only one parameter is given instead of two, the compiler matched it against the first declared field, that is, amount. Since the value 15 is of the same type as amount (that is, Integer), the compiler was able to populate the instance with this amount, using the default value "USD" as the currency.

Unlike the amount variable, invoking the Money constructor with the sole currency parameter will fail, as seen in the following statement:

scala> val someEuros = Money("EUR")
<console>:9: error: type mismatch;
 found   : String("EUR")
 required: Int
       val someEuros = Money("EUR")
                             ^

The preceding code does not work because the compiler could not guess which parameter we were referring to, and therefore tried to match them in order of declaration. To be able to use the default value for amount with the given "EUR" string, we need to include the parameter name explicitly, as shown in the following code snippet:

scala> val someEuros = Money(currency="EUR")
someEuros: Money = Money(1,EUR)

We could therefore also mark all parameters explicitly, which can be recommended when there are many parameters as shown in the following code snippet:

scala> val twentyEuros = Money(amount=20,currency="EUR")
twentyEuros: Money = Money(20,EUR)

An additional useful method when constructing instances is the copy method, which creates a new instance out of the original and eventually replaces given parameters:

scala> val tenEuros = twentyEuros.copy(10)
tenEuros: Money = Money(10,EUR)

We can use the copy method with explicitly named parameters, as follows:

scala> val twentyDollars = twentyEuros.copy(currency="USD")
twentyDollars: Money = Money(20,USD)

The copy method can be very useful when writing test fixtures, in particular, when the mockup instances to be initialized have constructors with many fields that are similar.

Let's move on by creating an addition operation of our Money class. For simplicity, we will pretend for a moment that we only deal with amounts of the same currency, the default USD.

In Java, we would probably add such a method with the following signature and simple content:

public class Money {

    Integer amount;
    String currency;

    public Money(Integer amount, String currency) {
        this.amount = amount;
        this.currency = currency;
    }

    public Money add(Money other) {
        return new Money(this.amount +
        other.amount, this.currency);
    }
    ...
}

In Scala, we use the def keyword to define a class method or a function. In the REPL, we can have multiline expressions. The following case class declaration, containing the implementation of a summing method + is an example of such features:

scala> case class Money(val amount:Int=1, val currency:String="USD"){
     |   def +(other: Money) : Money = Money(amount + other.amount)
     | }
defined class Money

Notice that we can use + as a method name. We have also included the return type Money in the signature declaration, which is only optional since the type inference of Scala will deduct it, but including it explicitly is a good documentation practice for public methods (and methods are public by default if no other scope is specified). Moreover, in Scala, since the return word at the end of the method is optional, the last statement is always the one that is returned to the caller of the method. Furthermore, it is generally considered a good practice to omit the return keyword since it is not mandatory.

We can now write the addition of two Money instances with the following simple expression:

scala> Money(12) + Money(34)
res5: Money = Money(46,USD)

Things start becoming exciting once we start manipulating collections of objects, and the functional programming part of Scala helps very much for that matter. Since generics are part of the language (Java 5 onwards), Java can, for example, iterate over a list of integers by writing the following code snippet:

List<Integer> numbers = new ArrayList<Integer>();
numbers.add(1);
numbers.add(2);
numbers.add(5);
for(Integer n: numbers) {
    System.out.println("Number "+n);
}

The preceding code produces the following output:

Number 1
Number 2
Number 5

In Scala, the declaration of a list can be written as follows:

scala> val numbers = List(1,2,5)
numbers: List[Int] = List(1,2,5)

Scala collections systematically distinguish between immutable and mutable collections, but encourage immutability by constructing immutable collections by default. They simulate additions, updates, or removals by returning new collections from such operations instead of modifying them.

One way to print out the numbers is that we can follow Java's imperative style of programming and iterate over the collection by creating a for loop:

scala> for (n <- numbers) println("Number "+n)
Number 1
Number 2
Number 5

Another way to write the code in Scala (as well as many other languages on the JVM, such as Groovy, JRuby, or Jython) involves a more functional style, using lambda expressions (sometimes referred to as closures). In brief, lambdas are just functions that you can pass around as parameters. These functions take input parameters (in our case, the n integer) and return the last statement/line of their body. They are in the following form:

functionName { input =>
                body
             }

A typical example of lambda to iterate over the elements of the numbers list we have defined earlier, is given as follows:

scala> numbers.foreach { n:Int =>     
   | println("Number "+n)
   | }
Number 1
Number 2
Number 5

In that case, the body consists of only one statement (println...), and therefore returns Unit, that is, an empty result roughly equivalent to void in Java, except that void does not return anything.

As the time of writing this book, lambda expressions in Java are around the corner and will be introduced very soon as part of the JDK8 release, adopting a Scala-like style. Some of the functional constructs will therefore soon be available to Java developers.

It should become possible to write our tiny example in the following way:

numbers.forEach(n -> { System.out.println("Numbers "+n);});

As we stated previously, Scala collections are, by default, immutable. This is a very important aspect for making them behave as expected when dealing with multiprocessor architectures. One unique feature of the Scala collections compared to Java is that they include support for running operations in parallel.

The REPL is a great tool to try out the powerful operations that we can apply to the collection elements. Let's go back to our interpreter prompt:

scala> val numbers = List(1,2,3,4,5,6)
numbers: List[Int] = List(1,2,3,4,5,6)
scala> val reversedList = numbers.reverse
reversedList: List[Int] = List(6,5,4,3,2,1)
scala> val onlyAFew = numbers drop 2 take 3
onlyAFew: List[Int] = List(3, 4, 5)

The drop method indicates that we get rid of the first two elements of the list, and the take method indicates that we keep only three elements from the result obtained after the drop method.

This last command is interesting for two reasons:

Another way of writing elements in a given list is by using the :: method, generally referred to in Scala documentation as the "cons operator". This alternative syntax looks like the following expression:

scala> val numbers = 1 :: 2 :: 3 :: 4 :: 5 :: 6 :: Nil
numbers: List[Int] = List(1, 2, 3, 4, 5, 6)

If you are wondering why there is a Nil value at the end of this expression, this is because there is a simple rule in Scala that says that a method whose last character is : (that is, a colon) is applied on its right side rather than the left side (such a method is called as right-associative). So, the evaluation of 6 :: Nil is not equivalent to 6.::(Nil) in that case, but rather Nil.::(6). We can exhibit that into the REPL as follows:

scala> val simpleList = Nil.::(6)
simpleList: List[Int] = List(6)

The evaluation of 5 :: 6 :: Nil is therefore done by applying the :: method on the simpleList that we saw earlier, which is List(6):

scala> val twoElementsList = List(6).::(5)
twoElementsList: List[Int] = List(5, 6)

In this case, 5 was appended before 6. Repeating this operation several times will give you the final List(1,2,3,4,5,6).

This convenient way of expressing lists is not just for simple values such as integers but can be applied to any type. Moreover, we can concatenate two List instances by using the ::: method in a similar way:

scala> val concatenatedList = simpleList ::: twoElementsList
concatenatedList: List[Int] = List(6, 5, 6)

We can even mix elements of various types in the same List, for example, integers and Booleans, as shown in the following code snippet:

scala> val things = List(0,1,true)
things: List[AnyVal] = List(0, 1, true) 

However, as you probably noticed, the result type AnyVal chosen by the compiler in that case is the first common type between integers and Booleans encountered in their hierarchy. For instance, retrieving only the Boolean element (at index two in the list) will return an element of type AnyVal rather than a Boolean value:

scala> things(2)
res6: AnyVal = true

Now, if we put an element of type String within the list as well, we will get a different common type:

scala> val things = List(0,1,true,"false")
things: List[Any] = List(0, 1, true, false)

The reason for that can be directly visualized by looking at the hierarchy of Scala types. Classes representing primitive types such as Int, Byte, Boolean, or Char belong to value types of scala.AnyVal, whereas String, Vector, List, or Set belong to reference types of scala.AnyRef, both being subclasses of the common type Any, as shown in the following diagram:

The full hierarchy of Scala types is given in the official Scala documentation at http://docs.scala-lang.org/tutorials/tour/unified-types.html.

Let's manipulate objects that are more complex than integers. We can, for instance, create some collections of Money instances that we made earlier and experiment with them:

scala> val amounts = List(Money(10,"USD"),Money(2,"EUR"),Money(20,"GBP"),Money(75,"EUR"),Money(100,"USD"),Money(50,"USD"))
amounts: List[Money] = List(Money(10,USD), Money(2,EUR), Money(20,GBP), Money(75,EUR), Money(100,USD), Money(50,USD))
scala> val first = amounts.head
first: Money = Money(10,USD)
scala> val amountsWithoutFirst = amounts.tail
amountsWithoutFirst: List[Money] = List(Money(2,EUR), Money(20,GBP), Money(75,EUR), Money(100,USD), Money(50,USD))

scala> val euros = amounts.filter(money => money.currency=="EUR")
euros: List[Money] = List(Money(2,EUR), Money(75,EUR))

scala> val euros = amounts.filter(x => x.currency=="EUR")
euros: List[Money] = List(Money(2,EUR),Money(75,EUR))

scala> val euros = amounts.filter(_.currency=="EUR")
euros: List[Money] = List(Money(2,EUR),Money(75,EUR))

scala> val allAmounts = amounts.partition(amt =>
     |   amt.currency=="EUR")
allAmounts: (List[Money], List[Money]) = (List(Money(2,EUR), Money(75,EUR)),List(Money(10,USD), Money(20,GBP), Money(100,USD), Money(50,USD)))

scala> val euros = allAmounts._1
euros: List[Money] = List(Money(2,EUR),Money(75,EUR))
scala> val everythingButEuros= allAmounts._2
everythingButEuros: List[Money] = List(Money(10,USD),Money(20,GBP),Money(100,USD),Money(50,USD))

scala> val (euros,everythingButEuros) = amounts.partition(amt =>
     |   amt.currency=="EUR")
euros: List[Money] = List(Money(2,EUR), Money(75,EUR))
everythingButEuros: List[Money] = List(Money(10,USD), Money(20,GBP), Money(100,USD), Money(50,USD))

scala> euros
res2: List[Money] = List(Money(2,EUR), Money(75,EUR))

import java.util.HashMap;
import java.util.Map;

public class MapSample {
    public static void main(String[] args) {
        Map amounts = new HashMap<String,Integer>();
        amounts.put("USD", 10);
        amounts.put("EUR", 2);

        Integer euros = (Integer)amounts.get("EUR");
        Integer pounds = (Integer)amounts.get("GBP");

        System.out.println("Euros: "+euros);
        System.out.println("Pounds: "+pounds);
    }
}

Euros: 2
Pounds: null

scala> val wallet = Map( "USD" -> 10, "EUR" -> 2 )
wallet: scala.collection.immutable.Map[String,Int] = Map(USD -> 10, EUR -> 2)

scala> val tenDollars = "USD"-> 10
tenDollars : (String, Int) = (USD,10)
scala> val tenDollars = ("USD",10)
tenDollars : (String, Int) = (USD,10)

scala> val updatedWallet = wallet + ("GBP" -> 20)
wallet: scala.collection.immutable.Map[String,Int] = Map(USD -> 10, EUR -> 2, GBP -> 20)
scala> val someEuros = wallet("EUR")
someEuros: Int = 2

scala> val somePounds = wallet("GBP")
java.util.NoSuchElementException: key not found: GBP  (followed by a full stacktrace)

scala> val mayBeSomeEuros = wallet.get("EUR")
mayBeSomeEuros: Option[Int] = Some(2)
scala> val mayBeSomePounds = wallet.get("GBP")
mayBeSomePounds: Option[Int] = None

scala> val status = mayBeSomeEuros match {
     |   case None => "Nothing of that currency"
     |   case Some(value) => "I have "+value+" Euros"
     | }
status: String = I have 2 Euros

scala> List(1,2,3,4).map(x => x+1)
res6: List[Int] = List(2,3,4,5)

scala> def increment = (x:Int) => x + 1
increment: Int => Int

scala> List(1,2,3,4).map(increment)
res7: List[Int] = List(2,3,4,5)

scala> List(1,2,3,4) map increment
res8: List[Int] = List(2, 3, 4, 5)

scala> val printedAmounts =
     |   amounts map(m=> ""+  m.amount + " " + m.currency)
printedAmounts: List[String] = List(10 USD, 2 EUR, 20 GBP, 75 EUR, 100 USD, 50 USD)

scala> val many = 10000.2345
many: Double = 10000.2345
scala> val amount = s"$many euros"
amount: String = 10000.2345 euros 

scala> val amount = f"$many%12.2f euros"
amount: String = "    10000.23 euros"

scala> val amount = s"${many*2} euros"
amount: String = 20000.469 euros

scala> val printedAmounts =
     |   amounts map(m=> s"${m.amount} ${m.currency}")
printedAmounts: List[String] = List(10 USD, 2 EUR, 20 GBP, 75 EUR, 100 USD, 50 USD)

scala> val sortedAmounts = amounts groupBy(_.currency)
sortedAmounts: scala.collection.immutable.Map[String,List[Money]] = Map(EUR -> List(Money(2,EUR), Money(75,EUR)), GBP -> List(Money(20,GBP)), USD -> List(Money(10,USD), Money(100,USD), Money(50,USD)))

scala> val sumOfNumbers = numbers.foldLeft(0) { (total,element) =>
     |   total + element
     | }
sumOfNumbers: Int = 21

scala> val sumOfNumbers = numbers.sum
sumOfNumbers: Int = 21