Object-Oriented Programming (OOP)

Object-Oriented Programming (OOP)

1. Objects

   • Has attribute(s) (fields) and can perform action(s) (methods)
   • To create objects, you need classes
   • Creating objects
     • Instantiate: instantiating the Employee is the same as creating an instance of Employee
     • Most common way to create an object is by using the "new" keyword
       • Example: to create an Employee object
     • Assign the created object to an object variable (or a reference variable) to manipulate the object later
     • You can call an object's methods and access its fields by using the object reference that was assigned the object. You use a period (.).

2. Classes

   • Determines what kind of objects you get
   • Tool that enable programmers to create the abstraction of a problem
     • Abstraction is the act of using programming objects to represent real-world objects
   • Blueprint/template to create objects of identical type
   • ALL program elements in Java must reside in a class
   • Consider 3 things when writing a class:
     • 1. the class name
     • 2. the fields
     • 3. the methods
   • General Syntax
     • • Use Pascal naming convention (ex: Employee, DateUtility, RegularRateCalculator)

3. Fields

   • Variables
   • Can be primitive or references to objects
   • Can refer to another object
     • Example: an Employee class may have an address field of type Address, which is a class that represents a street address
   • Follows the camel naming convention (ex: maxAge, age, numberOfRows)

4. Methods

   • Define actions that a class's objects (or instances) can do
   • Consists of a declaration part and a body
     • Declaration part consists of a return value, the method name, and a list of arguments
     • Body contains code that perform the action
   • Declaring a method: syntax
   • If a method takes more than one argument, two arguments are separated by a comma
     • Example: the following add method takes two ints and return an int

5. The Method main

   • Special method called main provides the entry point to an application
   • An application normally has many classes; only one of the classes needs to have a main method
   • The signature of the main method:
   • You can pass arguments to main when using Java to run a class:
     • • All arguments MUST be passed as strings

6. Constructors

   • Every class must have at least one constructor
     • If your class does not explicitly define a constructor, the compiler adds one for you
   • Used to construct an object
   • Does not have a return value, not even void
   • Must have the same name as the class
   • Syntax
   • no-argument (no-arg) constructors have zero argument

7. Class Members in UML Class Diagrams

   • A field and its type is separated by a colon
   • A method's argument list is presented in parentheses and its return type is written after a colon

8. The null Keyword

   • When a reference variable does not refer to an object, it has a null value
   • If you declare a local reference variable within a method but do not assign an object to it, you will need to assign null to it to satisfy the compiler
     • Class-level reference variables will be initialized when an instance is created, therefore you do not need to assign null to them
   • Trying to access the field or method of a null variable reference raises an error
     • You can test if a reference variable is null by using the == operator

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