Objects

Objects

1. How are they represented in life?

   • Any thing you can think of: abstract or concrete
   • Ex: people, animals, streets, houses, apples, love, trees, laws

2. What features do they have?

   • Attributes and actions they can perform
     • Attributes are descriptors
       • Ex for a car: color, # of tires, plate number, # of valves
     • Actions they can perform
       • Ex for a car: run, brake

3. How are they represented in Java?

   • actions are called methods
     • Both are optional. Object may have one, both, or neither of a method or field.
   • attributes are called fields

4. How do you create Objects in Java?

   • Classes!
     • "A class is a blueprint or a template to create objects of identical type"
       • To create "Employee" objects, you must first create an "Employee" class
         • Ex: Creating an "Employee" class that has fields for age and position, then all "Employee" objects created out of this class will have those fields
     • Classes are the abstraction of a problem, with the objects being representative of real life objects
       • All program elements must reside in a class in Java
     • 3 things to consider when writing a class
       • the class name
       • The Fields
         • Fields are variables
           • Can be primitives or references to objects
             • Ex of reference to another object: Address address;
               • in which an "Employee" class has an "address" field of type "Address"
         • initial of each word after the first is capitalized
           • Ex: validAddress, numberOfRows
       • The Methods
         • Define actions that a class's objects (instances) can do
         • syntax
           • a method has a declaration part and a body
             • declaration part consists of: a return value, method name, and a list of arguments.
               • AKA "signature"
             • the body contains code to perform the action
           • • Ex of a getSalary method that returns a double:
               • double getSalary ( )
                 • "the getSalary method does not accept arguments" (I am confused)
             • Ex of a method that returns an Address object"
               • Address getAddress ( )
             • Ex of a method that accepts an argument:
               • int negate (int ~an actual number~)
             • Ex of a method that takes more than one argument (separate them by a comma):
               • int add (int a, int b)
                 • this "add" method will take 2 "ints" and return an "int:
         • the return type of a method can be primitive, an object, or void
           • return type "void" means that the method returns nothing
         • The Method main
           • provides an entry point to an application. An application only needs one class to have a main method
           • the signature goes:
             • public static void main (String [ ] ~args~)
           • can pass arguments to "main" when using "java" to run a class by typing them after the class name and separating them with a space
             • java ~className~ ~arg1~ ~arg2~ ~arg3~ ...
               • Ex passing two arguments "1" and "safeMode" when running Test class:
                 • java Test 1 safeMode
     • the syntax
       • • Example for an employee
       • class names capitalize the initial of each word
         • Ex: DateUtility
       • class definitions saved in a file with the same name as the class name
         • Ex: Employee.java
           • • "age" and "salary" are fields
     • Constructors
       • Used to construct an object and looks like a method
       • Every class needs at least one constructor
       • does not have a return value
       • a constructor must have the same name as the class
       • syntax
       • Ex adding two constructors to Listing 4.1:
         • • second constructor passes the values at the same time you create the object
             • instead of:
   • Creating Objects AKA constructing/instantiating instances (wow!)
     • using the "new" keyword, followed by a constructor of the class to be instantiated
       • Ex to create an "Employee" object
         • new Employee ( ) ;
       • Usually want to assign the new object to an object/reference variable
         • Ex:
           • Employee employee = new Employee ( ) ;
             • where employee is an object reference of type Employee
         • once you have object, can call its methods and access its fields through the assigned object reference. Use a period to call a method or field
           • • Ex creating an "Employee" object and assigning values to its "age" and "salary" fields
         • Reference variables can also not contain values AKA not reference an object
           • Ex of type "Book" not assigned a value
             • Book book; // Book is null
           • Ex of local reference variable within a method without an assigned object:
             • Book book = null
           • Can’t access the field or method of a null variable, leads to error.

5. Notes

   • can not italicize individual words on here
     • ~'s surrounding a word means to italicize it
   • white colored node contains code

6. AKA instances

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