(6) Concepts In OOP (ITI1121)

(6) Concepts In OOP (ITI1121)

1. (1) OOP and Encapsulation

2. (2) Polymorphism and Inheritance

   • Inheritance
     • To inherit from another class...
       • public class Subclass extends Parent { ... }
     • Java only has single-inheritance
       • Object has no parent
     • A subclass can...
       • Add new methods
       • Override and hide parent methods
         • Overrides instance method
           • Whether it is invoked from a subclass or a parent class, the method that will get called is always the one from the SUBCLASS
         • Hides static method
           • That is to say, hides it from the parent object.
           • If invoked from subclass, the subclass static method is called
           • If invoked from parent class, the superclass static method is called
     • Using the keyword final, we can prevent subclasses from overriding methods
     • An attribute declared private cannot be accessed by child class
       • Circumvent this with the keyword protected
     • super()
       • Explicit call to immediate super class's constructor
       • Can only be first statement of constructor
       • Can only appear in constructors
       • If the first statement of a constructor is not super(), java automatically inserts a call to super.
         • The super class must have a constructor with an arity of 0, or else a compiler error will occur.
         • The default arity 0 constructor is no longer present if a constructor is defined
     • Inheritance allows you to re-use code, fixing a bug in the superclass will fix all subclasses
     • Dynamic binding (virtual, late)
       • Method names are searched from bottom-to-top at run time
   • (2) Types of Polymorphism
     • A variable or method is polymorphic if it refers to objects of more than one class
     • (1) Ad hoc (overloading)
       • Several methods or constructors with the same name can be added to a class as long as their signatures are different (return type and arguments)
       • Same name, different signatures.
       • Operator overloading
         • Some operators like + are overloaded, but a user cannot overload an operator.
     • (2) Inclusion (subtyping, data)
       • An instance variable of the superclass can hold any reference to one of its subtypes
         • However, the superclass variable now loses all references to methods defined in subtypes.
           • Even if the method gets "hidden" by overriding a static method, Java will still call the static method from the superclass.
       • A class that has an abstract method must be abstract (public abstract class...)
         • One cannot create an object of an abstract class.
         • An abstract method MUST be declared by any sublcass that wants to inherit

3. (3) Polymorphism and interfaces

   • Implementation
     • public interface comparable { ... }
     • The implementation must provide every single method and variable in an interface
       • public class Implementation implements Interface1. Interface2,... {... }
       • It's possible to implement multiple interfaces
   • It's possible to have a reference variable to an interface -- it can hold any class that implements the interface.
     • However, we cannot call any methods through it.
   • Properties, uses...
     • Designates a list of public methods and variables of a class
     • Implements the relationship "can be seen as" as opposed to "is a". It can be seen as a contract.
     • Formalism used to express abstract data types

4. (4) Abstract Data Types

   • Queue
   • List
   • Stack

5. (5) Iterators

6. (6) Binary Search Tree

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