Objects and Classes

Objects and Classes

1. Class

   • "A blueprint or template to create objects of identical type"
   • "Fundamental building blocks of a Java program"
   • class className { [class body] }
     • Class names use the Pascal naming system - capitalize the first initial of each word.
   • UML Class Diagram
     • Top: Class name, Middle: list of fields, Bottom: list of methods
       • Fields and types are separated by a colon.
       • A method's arguments are presented in parentheses and its return value is written after a colon.
     • Provides a summary of all fields and methods
   • Object
     • Fields
       • Variables - can be primitives or references to objects
     • Methods
       • Defines actions that objects or instances can do
       • Declaration Part
         • Return Value
           • Can be a primitive, object, or void
             • return type void means method returns nothing
         • Method Name
         • List of Arguments
           • If a method takes more than one argument, separate arguments with a comma.
         • Also known as the signature of the method
       • Body
         • Contains code that performs the action
       • returnType methodName (list of arguments)
       • Method main
         • Provides the entry point to an application
         • public static void main (String [ ] args)
         • Can pass arguments to main when using Java to run a class
           • To pass arguments, type after the class name
             • java className arg1 arg2 arg3 ...
     • Method and field names use the camel naming system - capitalize the first initial of each word except the first word.
     • Also known as an instance
     • A new keyword can be used to create an object.
       • new is followed by the constructor of the class that is being created.
       • Assign the created object to an object or reference variable with the same type as the object to manipulate the object.
       • Once an object is created, methods can be called and fields can be accessed.
         • Use a period (.) to call a method or field.
       • When an object is created, the JVM performs initialization to assign default values to fields.
     • Null keyword
       • Reference variable has no value.
       • When declaring a local reference variable within a method without assigning an object, assign null to satisfy computer.
       • Null does not need to be assigned to class-level reference variables as they will be initialized when an instance will be created.
       • Trying to access the field or method of a null variable reference will raise an error.
       • Test to see if a reference variable is null by using the == operator.
   • Constructor
     • Every class must have a constructor.
       • If a class does not explicitly define a constructor, the compiler will automatically add one.
         • no-arg constructor
           • Constructors with zero argument
         • Addition will be implicit and not displayed in source file.
       • Must have same name as the class
     • Used to construct an object
     • Also known as constructor method; looks like a method
       • Unlike methods, constructors do not have a return value.
     • constructorName (listOfArguments) { [constructor body] }

2. Object-Oriented Programming (OOP)

   • Models application on real-world objects