Scala — OOP Basic

Object-Oriented Basics

Constructors

class Person(name: String, age: Int) // This is the constructor

val person = new Person("iris", 99)// a new person is constructed

Difference between class parameter and class member

• class parameters are not fields
• class members can be accessed by a “.”
• to convert parameter > field: var & val definition inside of class are fields

“This” keyword

• “this” access the method field incase there are same class property name and method property name

Overloading

• overloading — defining method with the same name but different signature.
• compiler will get confused when fields are same, but return type is different.
• used def this() to overload constructor

Immutability

• extremely important in functional programming
• says that an instance is fixed, and cannot be modified.
• whenever we need to modify the content of an instance, we need to return a new instance with the modified content

Syntactic Sugar: Method Notations

when an apply() method is defined, when a class is called as a method, the compiler looks for the definition of apply() inside of the class, and execute.

Scala Objects

• class level functionality (static)
• means an instance of the object can access the class level definition
• scala does not have class level functionality
• in scala, to use class level definition, we use object as a singleton instance. for example, we define the type of the person object, and that is the only instance.
• the advantage: scala objects are singleton instance by definition
• a common pattern — companions:

In order to separate class and instance level functionality

• companion design pattern — write object and class in the same file or same scope to split static(class) and instance level functionality

Factory Method

• inside an object. to build an object like a constructor

Inheritance

• Scala offers single class inheritance
• private will mark a method within its own class
• protected will mark a method as accessible within its own and subclasses
• seal will mark a method as accessible within the same file ONLY

Overriding

• directly specify in the constructor
• or overriding inside the class definition
• all instances of derived classes will use the most recent definition

Polymorphism

• a method call will go to the most overridden definition

Prevent override

• use final keyword on member to prevent override
• use final on entire class
• seal class

Abstract data types

Meaning of abstract

• for the situation we leave the definition of method and field blank or unimplemented
• abstract class cannot be inistantiate
• to extend an abstract class, provide a definition
• both abstract classes and traits can have either abstract or non-abstract members

Trait

• the ultimate abstract data type
• describe behavior of a class
• can be inherited along with classes
• do not have constructor parameters