Creating a Program

1
Creating a Program

• Chapter 1
• Dr. James Jiang

A First Program Using Java
2
Programming

• Program set of instructions that tell a
  computer what to do
  
• High-level programming language
• Syntax rules of the language
• Logic produce desired results
• Compiler/interpreter/assembler translates
  high-level language to machine code
  

3
Programming Approaches
Procedural programming
Object-oriented programming
4
Procedural Programming

• Create memory locations ? variables
• Write operations to manipulate variable values
• Operations are grouped into logical units ?
  procedures
  
• Procedures are called or invoked

5
Object-Oriented Programming

• An extension of procedural programming
• Program components are objects
• OOP objects are manipulated
• DOS vs Windows
• Objects are made up of states and methods
• States describe characteristics (attributes) of
  an object

6
Class
Inheritance

• A collection of attributes methods
• An object derived from an existing class is an
  instance of the class
  
• Goldie, the retriever, instance of class dogs
• Inheritance ability of an object to take on
  characteristics of base class
  
• Apply general knowledge of class to individual
  members of the class
  

7
Methods

• Code segments used to perform a task
• Attributes and methods are encapsulated into
  objects

Encapsulation
8
Encapsulation

• Act of assembling attributes, methods, other
  elements into a single self-contained unit
  
• Encapsulated objects hide low-level details (how
  attributes are set how methods are executed)
  
• Attributes and methods are executed through an
  interface
  

9
Java

• Sun Microsystems
• General-purpose business applications
• Interactive WWW-based programs
• Runs on any platform (architecturally neutral)
• Bytecode compiled form of Java that can run on
  any platform with a Java interpreter
  
• Interpreter program that translates code
• Java Virtual Machine (JVM) the Java language
  interpreter
  
• Java eliminates difficult features found in C

10
First Java Program

• public class First
• public static void main(String args)
• System.out.println(First Java program)

11
Syntax

• All statements must end with a semicolon ( )
• Literal strings must appear between double
  quotation marks
  
• Arguments to methods must appear in parentheses
• Argument information that methods require
• println() method
• prints a line of output on the screen
• positions the cursor on the next line

12
Syntax

• out is an object that represents the screen
• print() method does not advance cursor
• System is a class (case sensitive)
• Periods ( . ) separate class, object, method
• System.out.println()

Everything is an object and every object is a
member of a class

13
Creating a Class

• public class First
• Defines a class named First
• Creating class names
• Must begin with a letter of the alphabet
• Only digits, letters, underscores,
• No reserved words
• Not true, false, null
• Begin class names with uppercase letters

14
Syntax

• public keyword is an access modifier
• Defines how a class may be accessed
• Enclose contents of all classes within curly
  brackets
  
• Methods require a set of curly brackets
• Brackets must be used in pairs
• Opening bracket ?
• Closing bracket ?

15
The main() Method Header

• First line of a method
• All Java applications require a main() method
• The main() method always executes first
• Example (explanation p. 11)
• public static void main(String args)

16
Shell Output Program

• public class First
• public static void main(String args)
• /insert code here/

17
Writing a First Program

• Class name must be the same as the classs
  filename
  
• Java is case sensitive
• Program comments
• Line comments ? //
• Block comments ? / /

18
Running a Program

• Compile the program (source code) into bytecode
• javac Hello.java
• ( see Appendix A)
• Results
• Bad command or file name
• Program language error messages
• No messages

Java is case-sensitive