Objects and Classes

Objects and Classes

1. What is an object and a class?

   • Objects have attributes (fields) and can perform actions (methods)
     • Fields are variables that describe objects. OR they can refer to another object
       • Example: an Employee class can have an address field of type Address, which is a class that represents a street address: Address address
       • Objects can contain other objects if they relate to each other
       • Syntax: Every word besides the first word is capitalized
         • Example: age, maxAge, numberOfRows
     • A method has a declaration part (or signature) and a body
       • Declaration part has a return value, method name, and a list of arguments
         • To declare a method: returnType methodName (listOfArguments)
         • 3 return types: a primitive, an object, void
           • Void = return nothing
         • Methods with multiple arguments are separated by a comma
           • Example: int add(int a, int b)
       • The body contains code that can perform the action
       • A main is a special method that serves as the entry point to an application
         • An application can have many classes but only one class needs to have a main method
           • Declaration point of a the main: public static void main(String[ ] arga)
   • Classes are the blueprint needed to create objects. The class determines what objects you can have.
     • 3 necessary components: the class name the fields, and the methods
       • UML Class Diagrams summarizes all the fields and methods
         • Can include field types and method signatures
         • The field and its type is separated by a colon
         • Method's argument list is presented in parenthases and its return type is written after a colon
     • Declare a class by using keyword 'class" followed by the class name
       • Example: class className {[class body]}
       • Class names capitalize the initial of each word
         • Example: Employee, DateUtility, RegularRateCalculator
     • Class definitions must be saved in a file with the same name as the class name and also have the java extension
       • Example: Employee class is saved as Employee.java
     • Each class must have one constructor
       • No return value (even void) and must have the same name as the class
         • constructorName {listOfArguments
       • May have a zero argument, called a no-argument (or no-arg) constructor
       • Can be used to initialize the fields in the object

2. Creating objects in Java

   • Most common: use keywrod 'new' collowed by the constructor of the class you want to create
     • Example: new Employee();
     • Note: assign the created object to an object variable so you can manipulate it later
       • To do this: declare an object reference with the same type as the object
         • Example: Employee employee = new Employee();
     • Once an object is created, you can call its methods and access its fields by using the object reference that was assigned to the object
       • Use a period to call a method or field
         • Example: objectReference.methodName
   • Reference variables refer to objects but when they don't have a value, it's considered a null value
     • Example: the class level reference variable is of type Book but hasn't been assigned a value: Book book; book is null
     • If you declare a local reference variable in a method but don't assign an object, need to assign null to satisfy the compiler
       • Example: Book book = null;
     • Can test if a reference variable is null by using the == operator