Transcript presentation source(Office 97)

Elements of a Java Program
Bina Ramamurthy
SUNY at Buffalo
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 you’re trying to
learn, if you can imagine trying to
explain it to a computer, then you learn
what you don’t 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
parts+functions 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:

–
–
B.Ramamurthy
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, coin#value
 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 don’t 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.01/26/98
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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
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