Begin Java

1
Begin Java

• Pepper

2
Objectives

• What is a program?
• Learn the basics of your programming tool BlueJ
  
• Write a first Java program and see it run
• Make some changes and see them run
• Understand basic program flow - Create two
  methods in one program

3
What Is Programming?

• Computers cannot do all the wonderful things that
  we expect without instructions telling them what
  to do.
• Program a set detailed of instructions telling
  a computer what to do
• Programming designing and writing computer
  programs
• Programming language a language used to express
  computer programs. (Syntax Vocab)

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A Very Simple Java Program
Open braces mark the beginning
Class header
Method header
public class JavaWorld public static void
main(String args) System.out.println ("T
his is my first Java program.")
statements
Close braces mark the end
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A First Program What Does It Do?
System.out.println ("This is my first Java
program.")
Prints the message This is my first Java program.
Ends the line
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Structure of Java programs

• public class ltnamegt
• public static void main(String args)
• ltstatementgt
• ltstatementgt
• ...
• ltstatementgt
• Every executable Java program consists of a class
• that contains a method named main
• that contains the statements (commands) to be
  executed

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Steps to create a program
Design
Code Compile
Run
Test
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Make the Program Run

• Compile (translate to java byte code)
• Run (interpreter for that OS reads java byte code
  and translates for machine)

9
Development Environment

• Blue Jay
• Lets you enter code and run it to get results
• Download at
• http//www.bluej.org/download/download.html
• Instructions on syllabus

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Exercise

• Add the line Welcome to Java
• When you run the program, you will see
• This is my first Java Program. Welcome to
  Java.
• Extra
• Info print() does not go to the next line
• Try splitting Welcome to Java into two
  statements Welcome and then to Java.

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Comments

• // -gt comment line ex // this is a comment
• / xxx / ? comment between marks ex
• / these are a bunch of comments
• xy
• that line above is meaningless /
• Space liberally
• For you to do Add some comments to your program
  and see it makes no changes

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Add a method

• What a set of statements that has a name you can
  call.
• Why
• deal with complex code
• repeatedly call the same statements
• Divide responsibility among programmers
• How
• Add a method header with following
• Ex public static void method1()
• Write the statements inside the .
• Call the method a few times from your main
  routine
• ex method1()

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Example of a new method

• public class JavaWorld
• public static void main(String args)
• System.out.println
• ("This is my first Java program.")
• method1()
• method1()
• public static void method1()
  System.out.println
• (I love Java.")
• YOU TRY add method1 to your program and have
  your main method call method1 a few times. Have
  it call method1 before and after the This is my
  first java program line. Add another print
  statement to method1 and see what happens.

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Escape Characters 1

• BUT, I really want a double quote inside my
  string!
• \ is -? abc\def -? abcdef
• \\ is \ -? abc\def -? abc\def

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Escape Characters 2

• How do I get new lines and tabs?
• \n new line (go to beginning of next line)
• \r carriage return (go to beginning of this
  line)
• \t tab (go to next tab stop)

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Escape characters in your program

• Insert \t inside the quotes of a print statement
  in your program ? what prints?
• Insert a \\ inside the quotes of a print
  statement in your program ? what prints?
• Now remove the \ and see that the program wont
  even compile.

17
Your toolset

• Basic setup
• Create and call methods
• Print to screen including escape commands
• Comments
• Here are the programming steps you explored