Object-oriented Programming (OOP)

Object-oriented Programming (OOP)

1. Java Object

   • Fields (Attributes)
     • Methods (Actions)

2. Java Classes

   • Classes create abstractation of problem
     • When writing a class remember:
       • Class name
         • Fields
           • Methods

3. Fields

   • Are variables - primitives or ref. to objects
     • Object can contain other objects
       • Field names follow camel naming convention

4. Methods

   • Define actions that a class's objects can do
     • Declaration part + body
     • Return can be primitive, an object, or void
     • Two arguments can be separated by comma

5. The Method main

   • Main provides entry point to application
     • Main method allows class containing it to be invoked
       • Arguments must pass as strings

6. Constructors

   • Every class must have one
     • Used to construct object
       • No return value, including a void
   • Constructor can have 0 argument (no-arg)
     • Used to initialize fields in object
   • W/two constructors, you can pass values at same time as creating object

7. UML Class Diagrams

   • Provides summary of all fields & methods

8. Creating Objects

   • Can create object using the new keyword
     • Use new followed by constructor of class
   • assign created object to object variable
     • Declare an object reference w/same type as object
   • If you have object, you can call its methods and access its fields
     • Use object reference that was assigned to object

9. The null Keyword

   • Sometimes reference variable has no value - called null value
   • If declaring local reference variable within method w/no object asssigned:
     • Must assign null
   • Can test for null with == operator