Objects and Classes

Objects and Classes

1. Every object can have fields(attributes) and/or methods(actions).

2. Class: Blueprint for the object

   • Needs: Class name, fields, methods
     • Class declaration: must use keyword class
       • {} are used after class for the body
       • Every class must have at least one constructor
         • Looks like a method
         • Does not have a return value
         • Must have same name as class
         • constructorName (listofArguments) { [constructor body] }
           • Used to initialize fields in object
             • new Employee(20, 90000.00);
           • public className (int arg1, method arg 2)
   • Class definition must be saved in file with same name as class name with .java

3. Fields are variables for the class in question

   • Fields can refer to another object as well
     • Follow camel naming convention (add,dataType,etc.)

4. Methods define actions of a object

   • returnType methodName (listofArguments)
     • double getSalary() returns a double with no accepting arguments
     • int add(int a, int b) accepts two arguments and adds them
   • Main provides entry point to an application
     • One of the classes in an application needs to have a main method
       • public static void main(String[] args)
       • Can pass arguments to main when using java to run a class
         • java className arg1 arg2 arg3...

5. Assign created object to an object reference variable

   • objectReference.methodName
   • objectReference.fieldName
   • When reference variable does not have value, it has a null value
     • If (book == null){ book = new Book(); }