1. Classes and Objects and Traits
And Other Miscellany
6-Jul-18

2. Classes, objects, case classes, traits
A class is a template for objects
Use the class to create “objects” or “instances” of the class
You don’t “talk to the class,” you talk to objects created from the class
An object is an object created directly, not from a class
You can “talk to the object”
Notice that, in this case, the name of the object begins with a capital letter
A case class is just a class with extra goodies
All constructor parameters are implicitly val (fields)
Free implementations of equals, hashCode, toString, copy, and a factory method
Case classes support pattern matching
A trait is something that can be “mixed in” to a class
Traits may include both abstract and concrete methods
In Scala you can nest almost anything in anything else
This is actually a problem, until you learn when to do and when not to do this

3. Classes
Syntax: class ClassName(parameters) { body } or class ClassName(parameters) extends ClassName { body }
The class definition is the (primary) constructor
Parameters and body are optional
Parameters, if any, are marked with:
var
A var parameter will cause a field, getter, and setter to be included: var p: Int gives the methods p: () => Int and p_=: Int => ()
val
A val parameter will create a field and a getter, but no setter
Neither val nor var
Can be used within the body of the class, but not create a field or any methods
Inherited methods may be overridden in the class

4. Constructors The class definition is the primary constructor
When creating a new object, the code within the class is executed
vars and vals are created for the new object
Function definitions are made available to the new object
“Loose” code, not contained within a function, is evaluated
This may include any Scala: Loops, I/O, etc.
Auxiliary (additional) constructors have the syntax
def this(parameters) { call to a constructor declared earlier (this is required, and must be first) rest of code }
Use the keyword new to call a constructor
new Person("Dave")

5. Examples I
scala> class Person(val firstName:String, var lastName: String, age: Int) defined class Person
scala> val mary = new Person("Mary", "Smith", 23) mary: Person =
scala> mary.firstName res22: String = Mary
scala> mary.lastName res23: String = Smith
scala> mary.firstName = "Sally" <console>:7: error: reassignment to val
scala> mary.lastName = "Jones" res24: String = Jones
scala> mary.age <console>:8: error: value age is not a member of Person
scala> mary.lastName res25: String = Jones

6. Examples II Again, but this time with a method:
scala> class Person(val firstName:String, | var lastName: String, age: Int) { | override def toString = firstName | " " + lastName + ", age " + age | } defined class Person
scala> val mary = new Person("Mary", "Smith", 23) mary: Person = Mary Smith, age 23
scala> println(mary) Mary Smith, age 23

7. Object
An object is defined similar to the way that a class is defined, but it cannot take parameters
Syntax: object ObjectName { body }
A program’s main method is defined in an object:
def main(args: Array[String]) { body }
Thus, a “complete” program requires at least one object definition

8. Companion objects The companion object of a class
has the same name as the class
is defined in the same file as the class
The purpose of a companion object is
To hold information that is the same for all objects of the class (so that the exact same data is not duplicated many times)
To hold methods useful to the class that do not depend on any specific object of the class
The class and its companion object can access each other’s private fields and methods
In the class, access to the fields and methods in the companion object must be qualified with the name of the object

9. Abstract classes To define a method as abstract, simply omit its body
def someMethod(n: Int)
To define a field as abstract, omit its initial value
A class containing abstract methods or fields must be declared abstract
abstract class LivingThing { … }
An abstract class is one that cannot be instantiated
In a concrete subclass, you do not need the override keyword

10. Case classes Syntax: case class ClassName(parameters) { body }
All the parameters are implicitly val
A parameter can be explicitly declared as var (not recommended)
toString, equals, hashCode, and copy are generated (unless you supply them)
apply and unapply are also generated
apply lets you omit the word new when you create objects
unapply lets you use the objects in pattern matching

11. Case classes can be pattern matched
scala> case class Person(age: Int, name: String) defined class Person scala> val dave = Person(40, "Dave") dave: Person = Person(40,Dave) scala> dave match { | case Person(a, n) if a > 30 => println(n + " is old!") | case _ => println("Whatever") | } Dave is old! scala> val quinn = Person(25, "Quinn") quinn: Person = Person(25,Quinn) scala> quinn match { | case Person(a, n) if a > 30 => println(n + " is old!") | case _ => println("Whatever") | } Whatever

12. Traits Traits are like classes that can be “mixed in” to other classes
Syntax: trait TraitName { body }
Traits may have concrete (defined) methods
A class extends exactly one other class, but may with any number of traits
Syntax:
class ClassName(parameters) extends OtherClass with Trait1, …, TraitN { body of class }
class ClassName(parameters) extends Trait1 with Trait2, …, TraitN { body of class }
I consider this use of extends to be confusing

13. Pattern matching with match
You have seen pattern matching with match and literals
today match { case "Saturday" => println("Party! Party! Party!") case "Sunday" => println("Pray....") case day => println(day + " is a workday. :( ") }
You can match with types
something match { case x: Int => println("I'm the integer " + x) case x: String => println("I'm the String \"" + x + "\"") println("My length is " + x.length) case _ => println("I don't know what I am! :( ") }

14. Pattern matching in assignments
You can pattern match on tuples:
scala> val (a, b, c) = (3, 5, 7) a: Int = 3 b: Int = 5 c: Int = 7
But…
scala> val a, b, c = (3, 5, 7) a: (Int, Int, Int) = (3,5,7) b: (Int, Int, Int) = (3,5,7) c: (Int, Int, Int) = (3,5,7)
You can pattern match on lists:
scala> val list = List("once", "upon", "a", "time") list: List[java.lang.String] = List(once, upon, a, time)
scala> val first :: second :: rest = list first: java.lang.String = once second: java.lang.String = upon rest: List[java.lang.String] = List(a, time)

15. Operations and methods
As operation
As method call
Unary prefix
scala> -5
res4: Int = -5
scala> 5 unary_-
res5: Int = -5
Unary
scala> " abc " trim
res6: java.lang.String = abc
scala> " abc ".trim()
res7: java.lang.String = abc
Binary
scala> "abc" + "xyz"
res8: java.lang.String = abcxyz scala> "abcdef" substring 2
res10: java.lang.String = cdef
scala> "abc".+("xyz")
res9: java.lang.String = abcxyz scala> "abcdef".substring(2)
res11: java.lang.String = cdef
>2 operands
scala> "abcdef" substring (1, 3)
res12: java.lang.String = bc
scala> "abcdef".substring(1, 3)
res13: java.lang.String = bc

16. Parameters in braces
A block consists of any number of statements inside braces, { }
The last value in the block is the value of the block
Parentheses, ( ), can’t enclose multiple statements
When a method takes just one parameter (in addition to the object), you can put that parameter inside braces instead of parentheses
scala> "abcdefg" substring { 2 } res0: java.lang.String = cdefg
This example is pointless and looks silly
Sometimes, you may want to compute that parameter by a series of statements
scala> println { | var x = | while (x < 1000) x *= | x | } 1024
This isn’t a great example either, but it does make the point

17. Methods with no parameters
You can define a “parameterless” method:
scala> def hello = println("Hello!") hello: Unit
scala> hello Hello!
scala> hello() <console>:7: error: hello of type Unit does not take parameters
You can define an “empty paren” method:
scala> def hi() = println("Hi!") hi: ()Unit
scala> hi Hi!
scala> hi() Hi!
If you define a method without parentheses, you can’t call it with parentheses
You can replace a parameterless method with an empty paren method, without affecting user code (but not vice versa)

18. Uniform access
In Java, the length of an array is a field, so you have to say myArray.length; but the length of a String is a field, so you have to say myString.length()
This violates the principle of uniform access: The user shouldn’t have to know whether it’s a field or a method
However, if I say foo = bar, or println(bar), I am using bar like a variable, so I expect bar to act like a variable:
bar should not do I/O
bar should not change mutable state
bar should not depend on values in mutable state
In other words, if bar is a function, it should be a pure function
Scala convention: When you call a method that does one of the above (impure) things, use parentheses

19. Types Types can be “aliased” (named)
type Word = String type Sentence = List[Word] type Paragraph = List[Sentence]
This is a simple thing that can be extremely helpful when dealing with complex data types

20. The End