Elements of a Java Program

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Elements of a Java Program

• Bina Ramamurthy
• SUNY at Buffalo

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Introduction

• A programming language is used to translate an
  algorithmic solution into a computer program.
• In this discussion we will study words, symbols,
  simple statements, and rules for constructing a
  simple Java program.

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Topics of Discussion

• Problem Analysis
• class
• Program structure
• method, main method
• Comments
• Identifier, reserved words, and literal
• Standard output
• Compilers and interpreters
• Errors
• Summary

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Out of Their Minds

• In general whatever youre trying to learn, if
  you can imagine trying to explain it to a
  computer, then you learn what you dont know
  about the subject. It helps you ask the right
  questions. It is the ultimate test of what you
  know.
• ---Donald E. Knuth

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Problem Analysis - Case 1

• Problem Design and build a car.
• Solution Lets follow D.E.Knuth and describe a
  car to a computer.
• Step 1 List the parts / attributes /
  properties. Ex Number of doors, Color...
• Step 2 List the functionality. Ex Drive, honk,
  brake..
• Step 3 Design a blue-print to put the
  partsfunctions together.
• Step 4 Build the car and test it.

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Problem Analysis - Case 2

• Problem Design a counter to keep track of scores
  in any field games such as football, hockey,
  basketball.
• Step 1 List parts / properties of Counter data
  container to hold score.
• Step 2 Functionality initialize, increment,
  decrement, display.
• Step 3 Build and use.

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Java Class

• Properties and functions together can be used to
  specify a class of objects class of cars, class
  of counters.
• In Java, classes are means for describing the
  properties and capabilities of objects in real
  life that a problem has to deal with.
• Properties are referred to as data
  declarations or simply declarations.
• Capabilities are referred to as methods

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Class examples

• Example class of cars, class of trees
• Class car
• Properties / attributes/parts of a car color,
  number of cylinders, make
• Capabilities/ methods/member function
  acceleration, anti-lock brake

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Elements, Syntax, Semantics

• Elements The words, symbols, basic structures
  that form the vocabulary of a programming
  language. Ex verb, noun
• Syntax Rules of a language that specify how the
  elements of a language can be put together to
  form a statement in the language. Ex English
  grammar
• Semantics It defines what will happen if a
  statement is executed. It defines the meaning.

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Program Structure

• A simple Java program is a class with at least
  one method called main
• main method must always be defined using the
  words public, static, and void.
• main method is where the processing begins in a
  Java program.
• main contains statements to be executed.
• File name a program class name should be same.

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Example (on the overhead)

• class CountDown
• public static void main (String args)
• System.out.print (Three)
• System.out.print (Two)
• System.out.print (One)
• System.out.println (LiftOff )

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Comments

• Comments are used for internal documentation of
  your program.
• Single line comments start with //
• Multiple line comments start with / and end
  with /
• Use comments efficiently Express clearly what
  you to say in minimum number of words.
• Do not crowd the program with too many comments
  Ex one comment/line

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Java Syntax

• Page. 651, Appendix N (quite complex for a
  beginner)
• Here is an informal syntax for a simple program
  structure
• class ClassName
• public static void main (String args)
• // main method add statements here
• We will add to this definition as we learn more.

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More Details About Class

• Java is case-sensitive. Ex Rings and rings two
  distinct names.
• Semi-colon is a terminator of a statement.
• A class represents a class of objects.
• A class contains the data declarations (parts)
  and methods (behaviors or capabilities ).
• Declarations are answers to What is it made of?
  (It has a ____, ____, etc.)
• Methods are answers to What can it do?

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Identifiers

• All the entities used in a program construction
  should have a name.
• These names known as identifiers.
• An identifier can be composed of letters,
  underscore ( _ ), digits and dollar symbol (),
  but it cannot begin with a digit.
• Example (correct) label7, next_stock, sys,
  Ex_9
• Example (incorrect) 3rd_rock, coinvalue
• Style ThirdRock, NextStock, CoinValue

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Identifiers (contd.)

• Identifiers can be of any length.
• Reserved words have special meaning in a
  programming language and cannot be used as
  identifiers.
• This is similar to English language reserving
  what, when, who , to mean a question or
  interrogation. You dont use these for names of
  objects or people?!
• See the list of reserved word on page.41 of blue
  book.

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Primitive Data Types

• Every data is used in a program should have Name
  (identifier) and Type.
• Basic data types supported by Java are
• byte, short, int and long to represent whole
  numbers.
• float and double to represent real numbers
  (numbers with fractional components).
• char to represent single character data.
• boolean to represent conditions.
• void - no type

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Variables

• A variable is an identifier (name) of data whose
  value may change during the execution of a
  program.
• A variable refers to the memory location where
  the data is stored.
• Syntax for a variable declaration
• DataType VariableName
• DataType VariableName InitialValue
• DataType VName1, VName2, VName3 / multiple
  variables of the same type/

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Variables Example

• Data Number of cars in the parking lot.
• int NumCars
• Data Salary of an employee.
• float Salary 25000.00 //signing bonus
• First and last initials of a person.
• char FirstInitial, LastInitial
• Raining?
• boolean raining
• How about distance between stars? How about
  distance between two atoms in a crystal?

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Constants

• Constants are identifiers that hold a particular
  value for the duration of their existence.
• Syntax
• final DataType ConstantName ConstantValue
• Example
• final double Pi 3.14159
• final is a reserved word for indicating that
  the value is final or constant.

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Assignment Statement

• Syntax
• Variable expression
• Semantics
• Evaluate the expression on the right hand side
  (RHS) of the and assign the result to the
  variable on the left hand side (LHS).
• The expression on the RHS should be consistent
  with the type of the variable on the LHS.
• Expression represents a formula. It could be a
  constant, variable or combination of variables,
  constants and operators.

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Assignment Statement Example

• int Score1, Score2, Score3
• int Total
• Total Score1
• Total Total Score2
• Total Total Score3

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A Complete (but still simple) Program

• Class Circle
• static final double PI 3.14
• public static void main (String args)
• double Radius 4.6
• double Area, Circumference
• Area PI Radius Radius
• Circumference 2.0 PI Radius
• System.out.println(Area Area)
• System.out.println(Circumference
  Circumference)

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Summary

• We learnt about
• class, method and declarations
• main method
• Program structure
• Identifiers, variable and constants
• Assignment statement