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Object Oriented Programming Principles
- 1. Object Oriented Programming Principles PortlandData Science Group Created by Andrew Ferlitsch Community Outreach Officer July, 2017
- 2. Why do OOP? •Pre-OOP Days – Write Code Fast as One Can. • Code was not designed – code smells bad. • Poor reuse of code – lots of copy and paste. • Code became unreadable – spaghetti code. • Difficult to trace – hard to fix bugs. • Fix one bug - breaks something downstream. • Object Oriented Programming (Design) • Design First, then Code. • Promotes reuse of code. • Code becomes modular with clearly defined interfaces. • Easy to maintain and modify. • Add new objects (classes) with small differences to existing objects.
- 3. OOP Example Banking Application- we might have checking accounts, savings accounts, money market accounts, and lines of credit. • Some of these accounts would have similar data fields (e.g., account number, balance). • Some of these accounts would have the same actions (e.g., withdraw money, get balance). • Some of these accounts would have some data and actions specific to the account type not shared with the other accounts. Pre-OOP Days : • Write a component (set of functions) for each account. • Common data fields and actions would be duplicated. • When maintaining/debugging, maybe confusing which data (e.g., account number goes with which account (e.g., checking, savings, money market, line of credit).
- 4. OOP Principles –Class & Objects • Class – A means to construct objects from predefined specifications (e.g, forms/templates), which may contain: • Initialized Data • Placeholders for Data • Methods for Accessing and Manipulating Data • Object – An instance of a class, i.e., built from a specification A collection of specifications for building Something. Class A Class B Class C Specifications on what to build and how to build it Class A ConstructionRequirements Object Class Constructor Parameters Class Definition Class Instance (Object)
- 5. OOP Principles –Inheritance • Class Inheritance – Derived (Sub) Class • Assembly of itself (derived or sub) with one or more other classes (base or super). • Hierarchical Classes – One class inherits a class, which inherits another, etc. Class A Class B Class C Class D A collection of specifications for base (super) classes. A collection of specifications for derived (sub) classes. Data/Methods of D Inherit Data/Methods of class B and C. Car Truck Chassis Wheels Motor Legend: Inheritance Parameters Base (super) classes are inherited Upwards into derived (sub) classes. Parameters are passed downwards From derived (sub) to the constructors of base (super) classes.
- 6. Derived Class OOP Principles– Data Encapsulation • Data Encapsulation – i.e., data hiding • Data Accessibility Object A Data is hidden Object B (data) A can see B A cannot see B’s data B can see its data Object B (public) (protected) (private) Object A A can access B’s data A cannot access B’s data A cannot access B’s data Derived Class can access B’s data Inheritance
- 7. OOP Principles –Polymorphism • Method Overloading – The same method (function) can have multiple implementations for different parameters. • Operator Overloading – The same operator (e.g., +) can have multiple implementations for different data types. Method Name: A Parameters Parameters Implementation Implementation One (same) name for the Method Switch to implementation based on parameters Parameters specific implementations Operator (e.g., +) Data Type Data Type Implementation Implementation Can be different number and/or data type of parameters. Same operator Switch to implementation based on data type of parameters. Data Type specific implementations
- 8. OOP Principles –Abstraction • Abstract Classes – Reduce Complexity by Hiding Details • Has Method Signatures (declarations), but not implementation. • Abstract Methods must be implemented by derived (sub) class. Method Name: A Method Name: B Parameters Parameters Abstract Class Derived Class Method Name: C Parameters Method Name: A Parameters Method Name: B Parameters Implementation Implementation Implementation Inheritance Derived Class specific implementations of inherited abstract methods