Objects and Classes

Objects and Classes

1. what is an Object?

   • Objects are everywhere, can be concrete (things) or abstract (ideas and knowledge)
   • has attributes (fields), that can perform actions (methods)
     • created by using "new" and the keyword of the object
       • once the object is created it can be used to call methods and access fields
   • an object can contain other objects
   • when there is not a value associated it is considered null

2. What is a Class?

   • To create an object we need the blueprint for that object, a class
   • THE FUNDAMENTAL BUILDING BLOCK OF JAVA!
     • the declaration of class must use the word "Class" followed by the class name and the body must use brackets
       • a class must also have a class definition that is saved in a file that has the same name as the class
   • field
     • also called attribute, are variables that could be primitive data or objects
   • name
   • method
     • actions that a class's object can do
       • the return can be primitive, an object, or void
       • the method main is the entry point to an application
         • needed by only one class
   • Can be represented by a UML class diagram: top is the class name, middle are the class attributes (fields), bottom is the class method
   • EVERY class must have a CONSTRUCTOR
     • used to construct an object
     • usually does not have a return value
     • must have the same name as the class