OO Concepts, Terms and Definitions

• type

    category of data
    included int, long, float, double, char, String, boolean
    every variable must be declared to be some type
  

• class

    is a pattern or template
    exists at compile time
    java source files
    analogy: recipe for a chocolate cake
  

• object

    a wrapper around data and functions that operate on that data
    exists at run time
    created by the java machine when you run a program
    made as instances of a class definition
    analogy: the chocolate cake itself... 
             many cakes can come from one recipe
  

• methods

    the OO name for the functions defined in a class definition
    these functions will be callable in objects made from that class definition
    let's say a class called "Employee" has a method called "reportHours()"
    let's say we have this in the Java program:
       Employee emp1;
       emp1 = new Employee();
       double hours;
    this means we have made an object "emp1" that is of type/class "Employee"
    then we can run the method "reportHours" with this syntax:
       hours = emp1.reportHours();
  

• fields

    the OO name for data (variables) in a class definition
    a field is a variable that is declared in a class, but not in a method
  

• constructor

    special method(s) in a class that is used to create a new object and
      initialize the fields of the object
  

• data hiding

    the software engineering principle that says information should not
    be visible outside an object unless it is necessary.
  

• public interface

    the collection of methods in an object that are visible outside the 
    object, and are therefore callable from other objects.
  

Basic Java

• executable java program

     one or more class definitions
     at least one class definition has a method with the signature
        "public static void main (String[] args) "
  

• all variables must be declared (given a type) before they are used

     variables declared in a class but not in a methods are "fields"
        also called "instance variables"
        each object created from the class has separate instance variables
        a field in one object is different storage from that field name
           in another object of the same class
     variables can be declared in a methods... "local variable"
        local variable comes into existence when the method is called,
        goes away when the method execution ends.
     variable can be declared as a parameter to a method.
        these variables get initial values when the method is called
     a variable declared as a field, but with the keyword "static"
        this is a "class variable"... it is one storage location,
           shared by all objects of that class.
        example: static int _pid;
     a variable declared with the keyword "final" is a constant.
        example: final int PI = 3.1415927 ;
  

• built-in types:

     int       integer, 16 bits
     long      integer, 32 bits
     float     real number, 32 bits
     double    real number, 64 bits
     boolean   logical value, 1 bit 
     char      single character, 16 bits (a character)
     String    quoted sequence of characters
  

• classes are types too

     declare a variable to have some class type, it will contain an object
  

• Inside a method (function), you write code in Java pretty much as you did for JavaScript. Java has the same 6 main concepts as JavaScript, with similar syntax:

  assignment    speed = 45.71;
  
  expressions   System.out.println("Results are: " + (speed*5.6/factor) );
  
  loops         int i;  // loop index
                for (i=0; i < maxVal; i++) {
                   x = x * i;
                }
  
                int k=0;
                while (k < 10) {
                   speed = speed + k*factor;
                   k = k+1;
                }
  
  conditional   if (speed > threshold) {
                   adjustment = speed * 0.10;
                   speed = speed - adjustment;
                } else {
                   speed = speed * 1.05;
                } 
  
  functions     double increaseSpeed (double speed, int factor) {
                   return speed*factor;
                }
  
  data structure
  (array)       double[] grades;
                grades = new double[maxGrades];