Object-Oriented Programming Objects and Classes

Object-Oriented Programming Objects and Classes

1. Object/Instance

   • Both fields and methods are optional for Java objects.
     • Attributes / Field (Java) car example
       • color
         • number of tires
           • plate number
             • number of values
     • Actions / Methods (Java) car example
       • run
         • brake
     • Most common way to create object is new keyword. new is followed by constructor of class
       • new Employee;
         • Want to assign object to an object variable to manipulate later
           • ObjectName Object reference = new Object()
             • Employee employee = new Employee()
               • can call its methods and fields but using a period (.)
                 • objectReference.methodName objectReference.fieldName
   • Classes
     • A class determines what object you get. It is a template used to create OBJECTS of identical type
       • All program elements in Java must reside in a class
         • Class Name
           • class file name must have a java extension
             • class className { [class body] }
               • class Employee { int age; double salary; }
           • Pascal naming convention used. where the initial of each word is capitalized
         • Fields
           • Camel naming convention used, where the initial of each word is capitalized except the first word
           • Can be primitive of references to objects. Can also refer to another object. (object can contain other objects)
             • Employees may have address field of type Address which is a class
               • Address address;
             • Null reference variable
               • A reference variable of an object that does not have a value
                 • if you declare a local reference within a method but assign it no object, need to assign null
                   • Book book = null;
         • Methods
           • Camel naming convention used, where the initial of each word is capitalized except the first word
           • Define actions that a class's objects can do
             • declaration / signature
               • consists of a return value, method name, and list of arguments
                 • return can be primitive, object, or void.
                   • void means method returns nothing
             • body
               • contains code to preform the action
             • returnType methodName (list of arguements)
               • if method take more than one argument: int add ( int a, int b)
           • The Method Main
             • provides entry point to an application that consists of multiple classes. only on class needs to have main method.
               • public static void main ( String [] args)
                 • can pass arguments to main to run a class
                   • java className arg1 arg2 arg 3 ...
     • Constructors
       • Every class must have at least one constructor. Looks like a method but does not have a return value.
         • Must have same name as the class
         • looking at the elements of a method, a constructor may have zero argument which is called nonarguement
         • constructorName (listOfArguments) { [constructor body] }
           • with second constrictor can pass values at same time to create object
             • new Employee (20, 90000.00)
     • UML Class diagram example with 5 fields and one method.
       • field and type separated by :
         • method's arguments in parentheses and return is written after :

Bilder und Dateien der Mindmap