Definition     A COMPUTER is a general purpose machine which can be made to act like a special purpose machine by means of a program

  REMARK   We want to write programs in Java to render the computer a special purpose problem solver.  

The Approach We Will Adopt We will write Java programs (texts) by means of a ``template based slot & filler'' scheme noted below  

/ /General Information   / / ---------------------------------------------------     / /File:   CLASS.java   / /Type:   java application file   / /Date:   Wed Aug 28,  2000   / /Name:   AUTHOR   / /Line:   LINE     / /Application Description   / / ---------------------------------------------------     / \*   DESCRIPTION   \* /     / /Required Packages   / / ---------------------------------------------------     import blue.io.   \*;   NEEDS     / /Application Class   / / ---------------------------------------------------     class CLASS   {   static public void main (String args [])   {   PROGRAM   }   }     / /Demo   / / ---------------------------------------------------     / \*   DEMO   \* /

 

Definition   A JAVA APPLICATION TEMPLATE is a text which is a Java program except for ``slots'' which must be ``filled''.   These templates contain five textual sections:   General Information,  App Description,  Required Packages,  Application Class,  Demo.

REMARK A concise way of presenting a program in class is simply to display slots & fillers - since most of the other stuff is invariant technical information.  

A presentation of ``Hello World'' using slots & fillers.  

  Class - > HelloApp
  Author - > Craig Graci
  Line - > A Classical First Program
  Description - > Description
  Needs - > Classes needed (i.e.   java.lang.object.   \*)
  Program - > IO.println ( ``Hello World'');
 

REMARK One can interpret the line IO.println ( ``Hello World'');  in the following way:

• IO is an object.
  
• println ( ``Hello World'') is a message
  

REMARK The basic mechanism of computation in object-oriented programming is the sending of messages to objects which respond with some form of behavior.  

REMARK In the sole executable instruction of the ``Hello World'' program...   The message println ( ``Hello World!   '' ) is sent to the object IO which responds behaviorally by printing the string ``Hello World!   '' to the standard output file.  

Definition   OBJECT-ORIENTED programming is a style of programming which features the modelling of objects in a hierarchical fashion and the sending of messages to appropriately behaving objects.

Definition   A MESSAGE SEND is a construct of the form < object >.   < message > which tells the computer to ``send the message to the object (which knows how to process it) .''

Hypothetical message sends

• hand.display ()
  
• dime.area ()
  

A Java program to read two integers and display their sum.  

Program - >

/ /Read the two integers and store them away   int n1 = IO.read_int ();   int n2 = IO.read_int ();     / /Compute the sum   int sum = n1 + n2;     IO.print ( ``The sum is:   ``);   IO.print (sum);   IO.println ();

 

Demo javac SumOfTwoIntsApp.java java SumOfTwoIntsApp 12 7 The Sum is:19  

Environment (bindings in memory) n1- > 12 n2- > 7 sum- > 19 There are 5 message sends to IO.  

Definition   A VARIABLE is a name (identifier) which represents some concept and which may denote a value.

Example temp- > 79 suppose this represents the current temperature.  

Example classCount- > 137  

REMARK In Java all variables must be introduced by specifying the type of value they can denote.  

Definition   a VARIABLE DECLARATION is the introduction of a variable into a program which specifies the type of value which the variable can denote.

Specification of Variable Declaration < type > < identifier > Example int n1 = IO.read_int ();  - this is a variable declaration