Java Objects

Java Objects

1. They all can have 2 features

   • Attributes/Fields
     • A car has attributes
       • color
       • # of tires
       • plate #
   • Actions/Methods
     • A car has methods too
       • accelerate
       • Steer
       • brake
   • Both are optional

2. Java Class: blueprint required to create an object

   • template/cookie cutter used to create objects of the same type
   • Allow people to create abstraction of a problem
     • abstraction = using programming objects to represent real world objects
     • Abstraction only includes details of a real world object that are relevant
   • "Employee" Class creates any number of Employee objects with different values plugged into fields and methods
     • Assume class has fields for age, position, and salary
       • all employees must have an age
       • all employees must have a position
       • all employees must have a salary
   • Class definition
     • 3 features of a class
       • class name
         • class names capitalize the initial of each word
           • PostOffice instead of Postoffice, postOffice, or postoffice
       • Fields
         • They're variable
         • can be primitive or references to objects
           • "Employee class may have "address" as a field of type "Address", which is a class that represents a street address
           • Object A can contain object B if object A's class has variables that reference to object B
         • field names follow "camel" convention
           • maxAge
           • address
           • numberOfRows
       • Methods
         • Define actions that a class's objects can do
         • methods have 2 parts
           • Declaration
             • has a return value, the method name, and a list of arguments
               • commas used to separate more than 1 argument
             • ex: returnType methodName (listOfArguments)
               • returnTypes:
                 • primitives
                 • objects
                 • void
                   • keyword for when a method isn't supposed to output anything
               • getAddress returns an "Address" object ex: Address getAddress()
           • Body
             • contains code that performs the action
     • Class declaration: must use keyword "class" followed by desired class name, a class has a body within braces
       • class ClassName { [class body] }
     • Must be saved in a file with same name as ClassName with java extension
       • Employee class = Employee.java
   • Method Main
     • special method that provides entry into application
     • only 1 class in an app needs the main method, which allows the class containing it to be invoked
     • ex: public static void main (String [ ] args)
   • Constructors
     • every class needs at least 1 constructor so it can create objects
     • A constructor looks like a method but isn't one because it doesn't return a value and doesn't use void
     • constructor must have the same name as the class
       • ex: constructorName (list of inputs) { [constructor body] }
     • constructors may not have inputs/arguments
     • constructor arguments can be used to initialize the fields in the object
       • like default settings/state