Scala: Class and Object Cheat Sheet

Classes

Basic Syntax:

class Class_name{
// methods and feilds
}

Some optional attributes:

• Keyword class: A class keyword is used to declare the type of class.
• Class name: The name should begin with an initial letter (capitalized by convention).
• Superclass(if any): The name of the class’s parent (superclass), if any, preceded by the keyword extends. A class can only extend (subclass) one parent.
• Traits(if any): A comma-separated list of traits implemented by the class, if any, preceded by the keyword extends. A class can implement more than one trait.
• Body: The class body is surrounded by { } (curly braces).

Object

An object consists of :

• State: It is represented by attributes of an object. It also reflects the properties of an object.
• Behavior: It is represented by methods of an object. It also reflects the response of an object with other objects.
• Identity: It gives a unique name to an object and enables one object to interact with other objects.

In Scala, an object of a class is created using the new keyword. The syntax of creating an object in Scala is:

var obj = new Objectname();

Object orientation

class C(x: R)	Constructor params - x is only available in class body.
class C(val x: R) var c = new C(4) c.x	Constructor params - automatic public member defined.
class C(var x: R) { assert(x > 0, "positive please") var y = x val readonly = 5 private var secret = 1 def this = this(42) }	Constructor is class body. Declare a public member. Declare a gettable but not settable member. Declare a private member. Alternative constructor.
new { ... }	Anonymous class.
abstract class D { ... }	Define an abstract class (non-createable).
class C extends D { ... }	Define an inherited class.
class D(var x: R) class C(x: R) extends D(x)	Inheritance and constructor params (wishlist: automatically pass-up params by default).
object O extends D { ... }	Define a singleton (module-like).
trait T { ... } class C extends T { ... } class C extends D with T { ... }	Traits. Interfaces-with-implementation. No constructor params.
trait T1; trait T2 class C extends T1 with T2 class C extends D with T1 with T2	Multiple traits.
class C extends D { override def f = ...}	Must declare method overrides.
new java.io.File("f")	Create object.