Classes and Objects and Traits
And Other Miscellany
12-Mar-16
Classes, objects, case classes, traits
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A Scala class is like a Java class
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A Scala object is like a Java “singleton” class
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All constructor parameters are implicitly val (fields)
Free implementations of equals, hashCode, toString, copy, and a factory
method
Case classes support pattern matching
A Scala trait is like a Java interface
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If you want exactly one of a given kind of object, just define it directly; no need
to create a class first
A Scala case class is just a class with extra goodies
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Use the class to create “objects” or “instances” of the class
Scala traits may include “concrete” methods, not just abstract ones
Scala has no special “inner classes” as in Java
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In Scala you can nest almost anything in anything else
Classes
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Syntax: class ClassName(parameters) { body }
The class definition is the (primary) constructor
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Parameters and body are optional
Parameters, if any, are marked with:
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var
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val
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A val parameter will create a field and a getter, but no setter
Neither val nor var
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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
Generated methods may be overridden in the class
Constructors
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The class definition is the primary constructor
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Auxiliary (additional) constructors have the syntax
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When creating a new object, the code within the class is
executed
def this(parameters) {
call to a constructor declared earlier (this is required)
rest of code
}
Calling a constructor is just as in Java
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new Person("Dave")
Examples I
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scala> class Person(val firstName:String, var lastName: String, age: Int)
defined class Person
scala> val mary = new Person("Mary", "Smith", 23)
mary: Person = Person@d73c3c
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
5
Examples II
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Again, but this time with a method:
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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
6
Object
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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:
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def main(args: Array[String]) { body }
This is exactly analogous to Java’s main method
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public static void main(String[] args) { body }
Companion objects
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Scala’s equivalent of static is the companion object
The companion object of a class
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has the same name as the class
is defined in the same file as the class
The object and class 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
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To define a method as abstract, simply omit its body
To define a field as abstract, omit its initial value
A class containing abstract methods or fields must be
declared abstract
As in Java, an abstract class is one that cannot be
instantiated
In a concrete subclass, you do not need the override
keyword
Case classes
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Syntax: case class ClassName(parameters) { body }
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All the parameters are implicitly val
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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
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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
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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
Case classes in pattern matching
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sealed abstract class Move
case object Left extends Move
case object Right extends Move
case object Up
extends Move
case object Down extends Move
direction match {
case Left => x -= 1
case Right => x += 1
case Up
=> y -= 1
case Down => y += 1
}
Traits
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Traits are like Java’s interfaces
Syntax: trait TraitName { body }
Unlike Java, traits may have concrete (defined) methods
A class extends exactly one other class, but may with
any number of traits
Syntax:
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class ClassName(parameters) extends OtherClass with
Trait1, …, TraitN { body }
class ClassName(parameters) extends Trait1 with Trait2,
…, TraitN { body }
Pattern matching with match
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You have seen pattern matching with match and literals
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today match {
case "Saturday" => println("Party! Party! Party!")
case "Sunday" => println("Pray....")
case day => println(day + " is a workday. :( ")
}
You can match with types
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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
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You can pattern match on tuples:
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But…
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scala> val (a, b, c) = (3, 5, 7)
a: Int = 3
b: Int = 5
c: Int = 7
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:
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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
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"
scala> "abc".+("xyz")
res8: java.lang.String = abcxyz res9: java.lang.String = abcxyz
scala> "abcdef" substring 2
res10: java.lang.String = cdef
>2 operands
scala> "abcdef".substring(2)
res11: java.lang.String = cdef
scala> "abcdef" substring (1, 3) scala> "abcdef".substring(1, 3)
res12: java.lang.String = bc
res13: java.lang.String = bc
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Parameters in braces
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A block consists of any number of statements inside braces, { }
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When a method takes just one parameter, you can put that parameter inside
braces instead of parentheses
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The last value in the block is the value of the block
Parentheses, ( ), can’t enclose multiple statements
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
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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
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Methods with no parameters
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You can define a “parameterless” method:
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You can define an “empty paren” method:
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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)
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Uniform access
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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()
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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:
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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
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Types
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Types can be “aliased” (named)
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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
The End