COP3502- Objects and Classes

COP3502- Objects and Classes

1. Objects

   • Attributes & actions
     • Attributes are "fields"
     • Actions are "methods" or "functinos"
       • Define actions that a class's objects can do
         • returnType methodName (listofArguments)
           • double getSalary()
             • getSalary does not accept arguments
           • int negate (int number)
             • accepts an argument
           • Address getAddress()
             • returns an Address object
           • int add(int a, int b)
             • following add method takes two ints and return an int
         • return type of method can be primitive, an object or void.
           • void: method returns nothing.
       • main: provides entry point into an application
         • application usually has many classes and only one of the classes needs to have a main method.
           • public static void main (String[] args)
             • signature of main method
           • java className arg1 arg2 arg3 ...
             • by passing arguments to main when using java to run a class
             • all arguments must be passed as string.
               • java Test 1 safeMode
   • To create, you need "classes"
     • Class name
       • must have at least one constructor to construct an object
         • does not have a return value, even when void
           • constructorName (listof Arguments)tab {enter, tab [constructor body] enter}
             • must have the same name as class
             • can be used to initialize the fields in the object.
               • • with this constructor, you can assign values to age and position
                   • new Employee (20, 90000.00)
       • Class Members- UML allows one to include field types and method signatures
         • • method's agrument list is in parenthesis and return type is written after a colon
     • the field
       • Variables that can be primitive or refer to objects.
         • Primitive: Age, salary
         • Objects: such as Address which represents a street address
       • should follow camel naming convention
     • the methods
     • class className { enter, tab [class body] enter, tab }
       • class Employee { int age; double salary; }
     • Class definition: must be saved in a file that has the same name as the class name and extension: Employee class must be saved as Employee.java
       • • in UML diagrams, a class contains three parts: 1. top: class name, 2. middle: fields and 3. bottom: methods
     • creating an object from a class
       • "new" keyword is followed by the constructor of the class to be instantiated
         • new Employee () ;
           • Employee employee = new Employee () '
             • declares an object reference with the same type as the object.
             • employee is an object reference of type Employee
       • upon creating an object, you call its methods and access it field by:
         • objectReference.methodName objectReference.fieldName
           • Employee employee = new Employee (} ; employee.age = 24; employee.salary = 50000;
       • null keyword: when reference variable doesnt have a value.
         • Book book; // book is null
         • Book book = null ;
         • test by using":
           • if (book == null) { book = new Book () ; } System.out.printIn(book.title);

2. Naming & Conventions

   • Pascal: class names capitilize the initial of each word: Employee, DateUtility,. RegularRateCalculator
   • Camel: capiitallize the initia of each word, except for the first one

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