Programming Languages and Paradigms

Object-Oriented Programming

Object-Oriented Programming

    Objects, classes, fields, methods Encapsulation Inheritance Polymorphism and dynamic binding  Program units: Classes OOP Slide 2

Object

Definition: a thing that has identity, state, and behavior    identity: a distinguished instance of a class state: collection of values for its variables behavior: capability to execute methods * variables and methods are defined in a class OOP Slide 3

Examples of Objects

  LightBulb

state/attributes

 on (true or false)

behavior

  switch on switch off  check if on BankAccount  

state/attributes

 balance

behavior

   deposit withdraw check balance Car  

state/attributes

 # of liters of gas in tank   total # of km run so far efficiency (km/liter)

behavior

     drive load gas change efficiency check gas check odometer reading

Note



each object is an “instance” of that “class” of object



each instance has its own values for its attributes

 e.g., different accounts can have different balances OOP Slide 4

Class

Definition: a collection of data (fields/ variables) and methods that operate on that data    data/methods define the contents/capabilities of the instances (objects) of the class a class can be viewed as a factory for objects a class defines a recipe for its objects OOP Slide 5

Instantiation

   Object creation Memory is allocated for the object’s fields as defined in the class Initialization is specified through a

constructor

 a special method invoked when objects are created OOP Slide 6

A Class with a Constructor

BankAccount.java

public class BankAccount { private double balance;   

Constructor: special method that handles initialization Java Example: BankAccount A constructor is invoked during object construction: BankAccount b; b = new BankAccount(); b.deposit( 100.00 );

Constructor call Method call

public BankAccount() { balance = 0; }

public double getBalance() { return balance; } } public void deposit( double amount ) { balance = balance + amount; } … OOP Slide 7

Encapsulation

  A key OO concept: “Information Hiding” Key points    The user of an object should have access only to those methods (or data) that are essential Unnecessary implementation details should be hidden from the user In Java/C++, use classes and access modifiers (public, private, protected) OOP Slide 8

Inheritance

   Inheritance:  programming language feature that allows for the implicit definition of variables/methods for a class through an existing class Subclass relationship   B is a subclass of A B inherits all definitions (variables/methods) in A Superclass variables, subclass objects  Polymorphism and dynamic binding OOP Slide 9

Reuse

   Inheritance encourages software reuse Existing code need not be rewritten Successful reuse occurs only through careful planning and design  when defining classes, anticipate future modifications and extensions OOP Slide 10

Polymorphism

   “Many forms”  allow several definitions under a single method name Example:  “move” means something for a person object but means something else for a car object Dynamic binding:  capability of an implementation to distinguish between the different forms during run-time OOP Slide 11

OOP in Java and C++

      Program Structure and Execution Encapsulation and Inheritance Objects and Variables Methods Pointers Constructors OOP Slide 12

Program Structure

   Class definition similar in Java and C++ Java: two types of programs  application (with main() function)  applet (typically embedded in a web page) C++   a program is (still) a collection of functions that may use objects and classes main() function serves as driver OOP Slide 13

Program Execution

  Java: Virtual Machine (VM)   programs: both compiled and interpreted compiler produces .class from .java

 VM loads .class file(s) as needed C++: compiled, linked, and loaded    modules separately compiled linked to produce executable static vs dynamic libraries OOP Slide 14

Encapsulation

   Enforced through access keywords   public: for interface private: to make implementation inaccessible  protected: access for subclasses only In Java  each member is prefixed with a keyword In C++  public, private, and protected

sections

OOP Slide 15

Inheritance

   Feature that allows a class to be defined based on another class  methods and attributes are inherited Java and C++ difference   Java: public class A extends B { … } C++: class A: public B { … } (different types of inheritance) Multiple inheritance possible in C++, not in Java  But in Java, one may implement several interfaces OOP Slide 16

 

Objects and Identity

Questions:   How/when are objects created?

What is the relationship between a variable and an object?

Difference between Java and C++   distinction between primitive (built-in) type variables and variables for objects reference relationship between variable and actual object OOP Slide 17

Variables for Built-in Types

 Variables for built-in types (C++ and Java) X int x; … x = 5; X

5

OOP Slide 18

Reference Variables (in Java)

 Reference type variables X BankAccount x; … X

BankAccount Object

100 x = new BankAccount(100.00); OOP Slide 19

Variables That “hold” Objects (in C++)

 Declaration of an object variable allocates space for the object X BankAccount x(100.00); 100 OOP Slide 20

Methods

   A method describes a specific behavior applicable to objects of a class A method defines a sequence of instructions (or statements) to be carried out when that method is called A method is called or invoked on an object of the class  Carried out through the dot operator ( e.g., x.deposit( 1000.00 ); ) OOP Slide 21

Pointers (in C++)

 Variables can be explicitly declared as pointers to objects X BankAccount *x; … X x = new BankAccount(100.00);

BankAccount Object

100 OOP Slide 22

Disposing of Allocated Memory

  In Java, garbage collection is automatic   Memory allocated objects are reclaimed when no variables refer to them Need to set reference variables to null when the object is no longer needed In C++, object destruction is the programmer’s responsibility using the delete keyword OOP Slide 23

delete in C++

  There should be a delete for every new   SomeClass *x = new SomeClass(…); // … use object pointed to by x  delete x; // done using object Memory leak    Occurs when you forget to delete Wasted memory Can this occur in Java?

OOP Slide 24

Object Construction

   Constructor  place where you include code that initializes the object Constructor without parameters  “default” constructior  no additional info required Constructor with parameters   with parameters that specify initial values or sizes Example: public BankAccount( double initBal ) { balance = initBal; } OOP Slide 25

Constructors in Java and C++

  In Java,   a constructor is invoked only through the new keyword recall that all object variables are references In C++,   a constructor is called upon variable declaration, or explicitly through new with pointers, or in other situations other types of constructors OOP Slide 26

Next…

      More advanced OOP features in Java and C++ Arrays Destructors Operator overloading Static vs Dynamic Binding Many others OOP Slide 27