Classes and Objects and Traits And Other Miscellany

Classes and Objects and Traits And Other Miscellany 29 -Sep-20

Classes, objects, case classes, traits n A class is a template for objects n n n An object is an object created directly, not from a class n n n All constructor parameters are implicitly val (fields) Free implementations of equals, hash. Code, to. String, copy, and a factory method Case classes support pattern matching A trait is something that can be “mixed in” to a class n n 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 n n 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 Traits may include both abstract and concrete methods In Scala you can nest almost anything in anything else n This is actually a problem, until you learn when to do and when not to do this

Classes n n Syntax: class Class. Name(parameters) { body } or class Class. Name(parameters) extends Class. Name { body } The class definition is the (primary) constructor n n Parameters and body are optional Parameters, if any, are marked with: n var n n val n n A val parameter will create a field and a getter, but no setter Neither val nor var n n A var parameter will cause a field, getter, and setter to be included: var p: Int gives the methods p: () => Int and p_=: Int => () 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

Constructors n The class definition is the primary constructor n n 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 n n Auxiliary (additional) constructors have the syntax n n This may include any Scala: Loops, I/O, etc. 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 n new Person("Dave")

Examples I n n n n scala> class Person(val first. Name: String, var last. Name: String, age: Int) defined class Person scala> val mary = new Person("Mary", "Smith", 23) mary: Person = Person@d 73 c 3 c scala> mary. first. Name res 22: String = Mary scala> mary. last. Name res 23: String = Smith scala> mary. first. Name = "Sally" <console>: 7: error: reassignment to val scala> mary. last. Name = "Jones" res 24: String = Jones scala> mary. age <console>: 8: error: value age is not a member of Person scala> mary. last. Name res 25: String = Jones 5

Examples II n Again, but this time with a method: n n n scala> class Person(val first. Name: String, | var last. Name: String, age: Int) { | override def to. String = first. Name + | " " + last. Name + ", 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 6

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

Companion objects n The companion object of a class n n n The purpose of a companion object is n n has the same name as the class is defined in the same file as the class 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

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

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

Case classes can be pattern matched n 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

Traits n n n Traits are like classes that can be “mixed in” to other classes Syntax: trait Trait. Name { body } Traits may have concrete (defined) methods A class extends exactly one other class, but may with any number of traits Syntax: n n n class Class. Name(parameters) extends Other. Class with Trait 1, …, Trait. N { body of class } class Class. Name(parameters) extends Trait 1 with Trait 2, …, Trait. N { body of class } I consider this use of extends to be confusing

Pattern matching with match n You have seen pattern matching with match and literals n n today match { case "Saturday" => println("Party!") case "Sunday" => println("Pray. . ") case day => println(day + " is a workday. : ( ") } You can match with types n 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! : ( ") }

Pattern matching in assignments n You can pattern match on tuples: n n val (a, b, c) = (3, 5, 7) = 3 = 5 = 7 But… n n scala> a: Int b: Int c: Int scala> val a, a: (Int, b: (Int, c: (Int, b, c Int) = = (3, 5, 7) You can pattern match on lists: n n 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)

Operations and methods As operation As method call Unary prefix scala> -5 res 4: Int = -5 scala> 5 unary_res 5: Int = -5 Unary scala> " abc " trim res 6: java. lang. String = abc scala> " abc ". trim() res 7: java. lang. String = abc Binary scala> "abc" + "xyz" res 8: java. lang. String = abcxyz scala> "abc". +("xyz") res 9: java. lang. String = abcxyz scala> "abcdef" substring 2 res 10: java. lang. String = cdef scala> "abcdef". substring(2) res 11: java. lang. String = cdef scala> "abcdef" substring (1, 3) res 12: java. lang. String = bc scala> "abcdef". substring(1, 3) res 13: java. lang. String = bc >2 operands 15

Parameters in braces n A block consists of any number of statements inside braces, { } n n n When a method takes just one parameter (in addition to the object), you can put that parameter inside braces instead of parentheses n n n The last value in the block is the value of the block Parentheses, ( ), can’t enclose multiple statements scala> "abcdefg" substring { 2 } res 0: java. lang. String = cdefg This example is pointless and looks silly Sometimes, you may want to compute that parameter by a series of statements n n scala> println { | var x = 2 | while (x < 1000) x *= 2 | x | } 1024 This isn’t a great example either, but it does make the point 16

Methods with no parameters n You can define a “parameterless” method: n n You can define an “empty paren” method: n n n scala> def hello = println("Hello!") hello: Unit scala> hello Hello! scala> hello() <console>: 7: error: hello of type Unit does not take parameters 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) 17

Uniform access n In Java, the length of an array is a field, so you have to say my. Array. length; but the length of a String is a field, so you have to say my. String. length() n n 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: n n n This violates the principle of uniform access: The user shouldn’t have to know whether it’s a field or a method 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 18

Types n Types can be “aliased” (named) n n 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

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