Object Oriented Programming: Java

Object Oriented Programming: Java

1. Class

   • Fields
     • Attributes/ variables within class
       • camel naming: ex. 'firstLetters'
         • Can be primitives or references to objects
           • If variable is not referencing an object, the value 'null' must be assigned to avoid error
             • '==' can be used to test if variable value is null
   • Methods
     • Define actions or functions
       • To declare a method:
         • returnType
           • methodName
             • (list of arguments)
               • Can be blank, or have one or more (comma separated) arguments within parenthesis
             • camel naming: 'firstLetter'
           • Either a primitive, object, or void
         • The method body is the code that performs the action
     • Main
       • Entry point to application (allows class to be evoked)
         • Only one class needs a main method per application
           • 'java' keyword is used when running class to pass arguments to main
             • Arguments (strings; space separated) are declared after className
               • java className arg1 arg2 arg3 ...
         • To declare method main:
           • public static void main (string[] args)
   • Coded template to create objects
     • 'class' keyword is used to declare a class
       • The name of the class (Pascal naming: ex. 'FirstLetters')
         • Braces flanking class body
     • class definition file must be saved by ClassName.java
   • Constructor
     • constructs an object
       • Looks like method w/o return value
         • Must have same name as the class
           • Compiler adds a constructor to the class if there are none.
             • Constructor arguments can be used to initialize the fields in the object
       • Each class must have at least one constructor
         • Constructor may have some, one, or no arguments
       • 'new' keyword followed by the constructor of the class used to create object
         • Created objects are often assigned to variables
     • Object
       • An objects methods and fields can be called by the reference variable and the method of field name connected with a period (.)
         • objectReference.methodName