17 February 2018
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This is an introduction to Scala for programmers who have completed the equivalent of the Java-based Computer Science I-II-III (CSci 111-112-211) sequence at the University of Mississippi.
I adapt and expand the document “A Scala Tutorial for Java Programmers” by Michel Schinz and Phillipp Haller to better meet the needs of my Scala-based courses (to explain some items more thoroughly, improve the narrative, and make the document accessible).
A “Hello, world!” program is the obligatory first example to give when introducing a new language. We can write a program HelloWorld as follows in Scala:
What Scala features do we use here in relation to Java?
Keyword object declares a singleton object named HelloWorld. An object is essentially a class with a single instance. The body of the object is enclosed in braces following the name.
The keyword def introduces a method definition.
In a declaration, a colon (:) separates the name from its type.
Method main takes the command line arguments as its parameter, which is an array of strings.
The main method is a procedure and, hence, has no return type declared. The body of the method is enclosed in braces following the method header.
The main method is not declared as static as in Java. Static members do not exist in Scala. We can use singleton objects instead.
The body of main has a single call to predefined method println.
We can use the scalac command (similar to the javac command) to invoke the Scala compiler. If the above Scala program is stored in file HelloWorld.scala, we can compile it from the command line as follows:
The above compiles the Scala source file and generates a few class files in the current directory.
File HelloWorld.class contains a class that can be executed.
We can use the scala command (similar to the java command) to execute the main method. Execution of the program prints the “Hello, World” string to the console.
Scala code can interact with Java code. Package java.lang is imported by default and other packages can be imported explicitly.
Consider a program to obtain and format the current date according to the conventions used in a specific country, say France.
import java.util.{Date, Locale}
import java.text.DateFormat
import java.text.DateFormat._
object FrenchDate {
def main(args: Array[String]) {
val now = new Date
val df = getDateInstance(LONG, Locale.FRANCE)
println(df format now)
}
}Java’s class libraries such as Date and DateFormat can be called directly from Scala code.
The Scala import statement is more powerful than Java’s. It can:
Import multiple classes by enclosing names in braces.
The first line above imports the Data and Locale classes from Java package java.util.
Import everything in a package or class by using an underscore character (_) instead of a Java’s asterisk (*).
The third line imports all members of the DateFormat class, making static method getDateInstance and static field LONG visible.
The main method in the FrenchDate object:
Creates an instance of Java’s Date class, thus getting the current date
Uses the imported static method getDateInstance to format the date appropriately for France using Locale.France
Prints the current date formatted according to the localized DateFormat instance
The main method declares two local “variables” in its body: now and df. Note that:
Both are declared with val. After initialization, Scala does not allow further assignments to a val.
The alternative is var. Scala does allow assignments to a var; it is like an ordinary variable in Java.
Although we cannot change the binding of a val to an object, Scala does allow the internal state of the object to be changed.
Although Scala is statically typed like Java, neither of these variables are given explicit types. The types of the variables are [inferred ]https://en.wikipedia.org/wiki/Type_inference) by the compiler from the type of the initializing expression.
Scala methods taking one explicit argument can be written in infix syntax such as
which is the same as the method call
This feature has important consequences, as we discuss below.
It is also possible to inherit from Java classes and implement Java interfaces directly in Scala.
Scala is a pure object-oriented language. Everything is an object, including numbers and functions.
Scala does not distinguish primitive types (e.g., boolean and int) from reference types (e.g., objects). It enables us to manipulate functions as first-class values.
Since numbers are objects, they also have methods. For example, the expression
is equivalent to the expression
which shows the method calls explicitly.
In Scala, the “operator” symbols (e.g., +, *, /) are valid identifiers. The parentheses around numbers are necessary because Scala’s lexical analyzer uses a longest-match rule for tokens, breaking expression
into the tokens 1., +, and 2. This results in 1. being interpreted as a Double.
Because Scala functions are objects, we can:
pass a function as an argument to or return a function as a result from another function.
That is, Scala has higher-order functions.
store a function in a variable or data structure.
That is, Scala’s functions are first-class values.
These are key features of functional programming.
Consider the Timer program below. It includes a timer function named oncePerSecond that performs some action every second. The specific action performed is encoded as a call-back function passed into oncePerSecond as an argument.
object Timer {
def oncePerSecond(callback: () => Unit) {
while (true) { callback(); Thread sleep 1000 }
}
def timeFlies() {
println("time flies like an arrow...")
}
def main(args: Array[String]) {
oncePerSecond(timeFlies)
}
}The type of the call-back function is () => Unit. This means it is a function that takes no arguments and returns nothing. The type Unit is similar to void in C/C++.
Function oncePerSecond calls the Thread.sleep method (from java.lang) and uses an infinite while loop to repeat the call-back action every second.
The main function calls this timer function, passing a call-back function that prints the string “time flies like an arrow…” on the console. The program endlessly prints this string every second.
The program uses the predefined Scala method println instead the like-named Java method from System.out.
The Timer program in the subsection above can be refined by replacing the function timeFlies by an anonymous function as shown below:
object TimerAnonymous {
def oncePerSecond(callback: () => Unit) {
while (true) { callback(); Thread sleep 1000 }
}
def main(args: Array[String]) {
oncePerSecond(() =>
println("time flies like an arrow..."))
}
}In the anonymous function, a right arrow => separates the function’s argument list from its body expression.
In this example, the argument list is empty, shown by the empty pair of parentheses on the left.
Like Java, Scala is a class-based language. However, Scala classes can have parameters. Consider the following class Complex for representing complex numbers:
The Complex class takes two arguments, the real and imaginary parts of the complex number. In the Complex class, these become val fields of the object (i.e., class instance) that are only visible and accessible from inside the object. (This visibility is private[this], a type of visibility that does not occur in Java.)
The default constructor is built into the class syntax. We pass values for these arguments when we create an instance of class Complex, as follows:
This causes all statements in the class definition to be executed.
The Complex class defines function methods re and im, which provide access to the two parts of the complex number. That is, these are accessor, or getter, methods.
We declare a function with an = between the function’s header (i.e., name and parameter list) and its body. The body is an expression that is evaluated when the function is called. If the body itself consists of a sequence of expressions, we enclose it in braces. The value of last expression executed is the value of the body.
In Complex, the compiler infers the return types for methods re() and im() by examining the right-hand sides and deducing that both return a value of type Double. The compiler gives an error message when it cannot infer the type of a method or variable.
Some Scala programmers suggest that the types be omitted whenever possible and only inserted when necessary.
However, your instructor considers it better software engineering practice to explicitly specify the types of all public features of a class or package such as the re() and im() methods.
But, for internal features of a class or method, it is convenient and safe to use type inference. For example, we did not give explicit types for the now and df values in the FrenchDate object shown in a previous section.
To call the methods re and im, we must put an empty pair of parentheses after their names:
object ComplexNumbers {
def main(args: Array[String]) {
val c = new Complex(1.2, 3.4)
println("imaginary part: " + c.im())
}
}We can eliminate the empty parameter list by defining re and im as methods without arguments instead of with zero arguments. We simply omit the parentheses in the method definition, as shown below:
All classes in Scala inherit from a superclass. If no super class is given explicitly, then the superclass is scala.AnyRef by default.
Like Java, a Scala class inherits all methods from its superclass by default.
Like Java, a Scala class may override individual methods by giving new definitions.
Unlike Java, a Scala method that overrides a superclass method must be explicitly declared with the override modifier. This is to reduce the possibility of class overriding a superclass method by accident.
For example, theComplex class can redefine the toString method inherited from AnyRef.
class Complex(real: Double, imaginary: Double) {
def re = real
def im = imaginary
override def toString() =
"" + re + (if (im < 0) "" else "+") + im + "i"
}The new definition of toString() overrides the definition for Complex objects. However, in doing so, it uses the default definitions of toString() for Double by its references to re and im in the string concatenation.
In programming, we often use trees and other hierarchical data structures.
We can illustrate how to implement a tree in Scala using a small calculator program for simple arithmetic expressions composed of sums, integer constants, and variables. Examples of such expressions are 1+2 and (x+x)+(7+y).
We can represent expressions naturally with a tree, where nodes are operations (e.g., addition) and leaves are values (e.g., constants or variables).
In Java, we can represent such a tree using an abstract superclass for the trees and a concrete subclass for each kind of internal and leaf node. The abstract class defines the common interface to the tree and the concrete subclasses define the specific features of a node.
In functional programming languages such as Haskell, we can define an algebraic data type for the same purpose. These types often enable us to express programs concisely by using pattern matching constructs.
Scala combines the concepts of classes and algebraic data types into its case classes. Consider the example:
abstract class Tree
case class Sum(l: Tree, r: Tree) extends Tree
case class Var(n: String) extends Tree
case class Const(v: Int) extends Tree Here, we define Sum, Var and Const as case classes. These differ from from standard classes in several ways:
We can create an instance without using the keyword new keyword, for example, we can write Const(5) instead of new Const(5).
The compiler automatically generates getter functions for the constructor parameters. That is, it is possible to get the value of the v constructor parameter of some instance c of class Const just by writing c.v.
The compiler supplies default definitions for the methods equals and hashCode. These work on the structure of the instances and not on their identities.
The compiler provides a default definition for method toString, which prints the value in a “source” form. For example, the tree for expression x+1 prints as Sum(Var(x),Const(1)).
Instances of these classes can be decomposed through pattern matching as we see below.
These features enable us to use Scala case classes much like we would use algebraic data types in a functional language.
To explore the use of case classes, consider a function to evaluate an expression in some environment. The purpose of an environment is to associate values with variables.
For example, the expression x+1 evaluated in an environment that associates the value 5 with the variable x, written { x -> 5 }, gives 6 as result.
An environment is just a function that associates a value with a (variable) name. The environment { x -> 5 } given above can be written as a Scala function as follows:
This notation defines a function that, when given the string "x" as argument, returns the integer 5, and, otherwise, fails and throws an exception.
An environment is a function of type String => Int. To simplify our evaluation program, we define the name Environment to be an alias for this type using the following declaration:
We can now define the evaluation function in Scala as follows:
def eval(t: Tree, env: Environment): Int = t match {
case Sum(l, r) => eval(l, env) + eval(r, env)
case Var(n) => env(n)
case Const(v) => v
}This evaluation function performs pattern matching on the tree t (i.e., using the match operator).
The pattern match first checks whether the tree t is a Sum object (i.e., an instance of the Sum case class).
If the tree is a Sum, the pattern match binds the left subtree to a new variable l and the right subtree to a new variable r, then evaluates the expression following the arrow =>.
This expression uses the values of the variables bound by the pattern match, i.e., l and r in the Sum case.
If the first check does not succeed, that is, the tree is not a Sum object, the pattern match then checks whether t is a Var object.
If the tree is a Var, the pattern match binds the name contained in the Var node to a new variable n and then evaluates the right-hand-side expression.
If the second check also fails, that is, if t is neither a Sum nor a Var, the pattern match then checks whether the expression is a Const object.
If the tree is a Const, the pattern match binds the value contained in the Const node to a new variable v and then evaluates the right-hand-side expression.
Finally, if all checks fail, the pattern match expression raises an exception to signal failure. In this version of eval, failure occurs only when there are additional subclasses of Tree that we have declared but not yet defined in the eval pattern match.
Pattern matching attempts to match a value to a series of patterns. As soon as a pattern matches (moving top to bottom, left to right in the source code), the program extracts and names various parts of the value and then evaluates the associated expression using the values of these named parts.
An object-oriented programmer might ask why we did not define eval as a method of class Tree and its subclasses.
We could do that, because Scala allows method definitions in case classes just as in normal classes.
Deciding whether to use pattern matching or methods is partly a matter of taste. But the choice also affects the extensibility the program.
Using methods, we can easily add a new kind of node by defining a new subclass of Tree. But adding a new operation to manipulate the tree is tedious because it requires us to modify every subclass of Tree.
Using pattern matching, the situation is reversed: adding a new kind of node requires us to modify all functions that do pattern matching on the tree, to take the new node into account. But adding a new operation is easy, we just define it as an independent function.
To explore pattern matching further, consider another operation on arithmetic expressions: symbolic derivation. Looking back at our calculus class, we see the following rules for differentiation:
The derivative of a sum is the sum of the derivatives.
The derivative of some variable v is 1 if v is the variable relative to which the derivation takes place, and is 0 otherwise.
The derivative of a constant is 0.
We can directly translate these rules into a Scala function that uses the above case classes and pattern matching, as follows:
def derive(t: Tree, v: String): Tree = t match {
case Sum(l, r) => Sum(derive(l, v), derive(r, v))
case Var(n) if (v == n) => Const(1)
case _ => Const(0)
}Function derive introduces two new concepts related to pattern matching.
The case expression for variables has a guard, an expression following the if keyword. This guard prevents pattern matching from succeeding unless its expression is true.
Here the guard ensures that the function returns the constant 1 only if the name of the variable being derived is the same as the derivation variable v.
A pattern can include a wildcard, written _, that matches any value, without giving it a name.
Consider an example with a simple main function that performs several operations on the expression (x+x)+(7+y).
It first computes its value in the environment { x -> 5, y -> 7 } and then computes its derivative relative to x and then to y.
def main(args: Array[String]) {
val exp: Tree = Sum(Sum(Var("x"),Var("x")),Sum(Const(7),Var("y")))
val env: Environment = { case "x" => 5 case "y" => 7 }
println("Expression: " + exp)
println("Evaluation with x=5, y=7: " + eval(exp, env))
println("Derivative relative to x:\n " + derive(exp, "x"))
println("Derivative relative to y:\n " + derive(exp, "y"))
}Executing this program, we get the expected output:
Expression: Sum(Sum(Var(x),Var(x)),Sum(Const(7),Var(y)))
Evaluation with x=5, y=7: 24
Derivative relative to x:
Sum(Sum(Const(1),Const(1)),Sum(Const(0),Const(0)))
Derivative relative to y:
Sum(Sum(Const(0),Const(0)),Sum(Const(0),Const(1)))The result of the derivative is complex. It should be simplified before printing. Defining a basic simplification function using pattern matching is an interesting (but surprisingly tricky) problem, left as an exercise for the reader.
The Scala source code for the case class version of the Expression Tree program is in file ExprCase.scala.
A traditional object-oriented version of the Expression Tree program is in file ExprObj.scala.
In addition to inheriting code from a superclass, a Scala class can also reuse code from one or several traits.
From a Java perpective, traits can be viewed as interfaces that can also contain code. In Scala, when a class inherits from a trait, it implements that trait’s interface, and inherits all the code contained in the trait.
Note: In older Java implementations, an interface consisted of just method signatures and constants. In Java 8, interfaces can now include default definitions of methods, somewhat similar to Scala traits.
To see the usefulness of traits, consider a classic example: ordered objects. We want to compare objects of a given class among themselves. For example, we need to compare objects according to some total order to sort them.
In Java, objects that can be compared implement the Comparable interface.
In Scala, we can do better by defining the equivalent of Comparable as a trait, which we call Ord.
When comparing objects, six different comparison operations are useful: smaller, smaller or equal, equal, not equal, greater or equal, and greater.
But it is tedious to define all of these for every class whose instances we wish to compare.
We observe that we can define four of the six in terms of the other two. For example, given the equal and smaller comparison operators, we can define the other four comparison operators.
In Scala, we can capture this observation in the following trait declaration:
trait Ord {
def < (that: Any): Boolean
def <=(that: Any): Boolean = (this < that) || (this == that)
def > (that: Any): Boolean = !(this <= that)
def >=(that: Any): Boolean = !(this < that)
}This definition both creates a new type called Ord, which plays the same role as Java’s Comparable interface, and generates default implementations of three comparison operators in terms of a fourth, abstract operator. All classes inherit the equality and inequality operators and, thus, those operators do not need to be defined in Ord.
The type Any used above, is the supertype of all other types in Scala. It is essentially a more general version of Java’s Object type that is also a supertype of basic types like Int, Float, etc.
To make objects of a class comparable, it is sufficient to define equality and inferiority operators and then “mix in” the Ord trait.
For example, consider a Date class representing dates in the Gregorian calendar. Such dates are composed of a day, a month, and a year, each of which we can represent with an integer. We can start the definition of the Date class as follows:
class Date(y: Int, m: Int, d: Int) extends Ord {
def year = y
def month = m
def day = d
override def toString(): String = year + "-" + month + "-" + day
// ... two methods defined below
}The extends Ord declaration specifies that the Date class inherits from the Ord trait (in addition to the default superclass).
To make the comparisons work correctly, we redefine the equals method, inherited from Java’s Object class, to compare dates by comparing their individual fields. The default implementation of equals does not work because it compares objects physically. In Scala, the comparison becomes:
override def equals(that: Any): Boolean =
that.isInstanceOf[Date] && {
val o = that.asInstanceOf[Date]
o.day == day && o.month == month && o.year == year
}This method uses the predefined methods isInstanceOf and asInstanceOf.
isInstanceOf corresponds to the Java instanceof operator. It returns true if and only if the object to which it is applied is an instance of the given type.
asInstanceOf corresponds to the Java cast operator: if the object is an instance of the given type, it is viewed as such, otherwise it throws a ClassCastException.
To complete our implementation, we need to define the < operator. It uses the predefined Scala method error to throw an exception with the given error message.
def <(that: Any): Boolean = {
if (!that.isInstanceOf[Date])
error("cannot compare " + that + " and a Date")
val o = that.asInstanceOf[Date]
(year < o.year) ||
(year == o.year && (month < o.month ||
(month == o.month && day < o.day)))
}This completes the definition of the Date class.
Instances of the Date class can be seen either as dates or as comparable objects. They define all six comparison operators:
equals and < because they appear directly in the definition of the Date class
the other four because they are inherited from the Ord trait
Traits are useful in many other situations, as you will see as you program more in Scala.
Scala supports generics. Java did not support generics until Java 5.
Genericity is the ability to write code parameterized by types.
For example, suppose we are writing a library for linked lists. What type do we give the elements of the list?
If we choose a specific concrete type such as Int, we severely limit the usefulness of the library. It is impractical to include different implementation for every possible concrete type.
If follow choose a default supertype like Any, then users of our library would need to their lace code with many type checks and type casts. (In Java, there was also the problem that basic types do not inherit from Object.)
Scala supports generic classes and methods to solve this problem. As an example, consider the simplest possible container class: a reference, which can either be empty or point to an object of some type.
class Reference[T] {
private var contents: T = _
def set(value: T) { contents = value }
def get: T = contents
}This example parameterizes the class Reference with a type T, which is the type of its element. The body of the class declares the contents variable, the argument of the set method, and the return type of the get method to have type ‘T’.
This is our first example to use mutable variables declare with var. In this example, we initialize the contents variable to have the value _, which represents the default value for the type. This default value is 0 for numeric types, false for the Boolean type, () for the Unit type, and null for all object types.
To use this Reference class, we need to specify what type to use for the type parameter T, that is, the type of the element contained in the cell. For example, to create and use a cell holding an integer, one could write the following:
object IntegerReference {
def main(args: Array[String]) {
val cell = new Reference[Int]
cell.set(13)
println("Reference contains the half of " + (cell.get * 2))
}
}This example does not need to cast the value returned by the get method before using it as an integer. It is also not possible to store anything but an integer in that particular cell, because it was declared as holding an integer.
The goal of this document is to provide a quick overview of Scala for programmers already familiar with Java. To learn more, use a textbook, reference book, other tutorials, and manuals.
TODO: More exercises
Extend the case class version of the expression-tree calculator program as described below.
Modify the Tree algebraic data type as follows:
Change node type Const to represent a floating point number instead of an integer.
Add node types Sub, Prod, and Div to represent subtraction, multiplication, and division of the two values, respectively. (Assume Sub means that the right operand is subtracted from the left operand.)
add node type Neg to represent negating a value.
add node types Sin and Cos to represent the sine and cosine trigonometric functions, respectively.
Extend functions eval and derive to support the above modifications of Tree. Test the extended functions thoroughly. (What should be done for division by zero?)
Further extend the expression Tree program developed in the previous exercise as described below.
Develop a new function simplify that takes an extended expression Tree, simplifies it by evaluating all subexpressions involving only constants (i.e., not evaluating variables), and returns the new expression Tree.
simplify(Add(Const(2),Const(3))) yields Const(5)simplify(Add(Add(Const(1),Const(1)),Mul(Const(1),Const(3)))) yields Const(5).simplify(Neg(Const(5))) yields Const(-5)Extend the simplify function to exploit mathematical properties such as identity elements (x + 0 = x = 0 + x), and zeros
Further extend the simplify function to exploit associativity ((x + y) + z = x + (y + z), commutativity (x + 1 = 1 + x), etc.
Modify and extend the traditional object-oriented version of the expression-tree calculator program in the same manner as described in the previous two exercises.
In February 2016, I created these notes by adapting and expanding the web document “A Scala Tutorial for Java Programmers” by Michel Schinz and Phillipp Haller to better meet the needs of my Scala-based course (to explain some items more thoroughly, improve the narrative, and make the document accessible). The original document can be found on the Scala Language website at http://docs.scala-lang.org/tutorials/scala-for-java-programmers.html.
In Spring 2017, I updated the format to be better compatible with Pandoc. I revised the document further in Spring 2018 (e.g., adding the first exercises).
I maintain these notes as text in Pandoc’s dialect of Markdown using embedded LaTeX markup for the mathematical formulas and then translate the notes to HTML, PDF, and other forms as needed.
TODO: Add
TODO: Add