Today

1
Todays Class

• Recap - simple Java program
• Revised history of Java
• Overview of Statements and Variables
• Simple types
• Printing out text
• The nuts and bolts of compiling programs

2
Java Program

• Always in a file
• Starts with public class
• Usually contains a public static void main
• We compile it to make it work

3
Sample Java Program

• public class hello
• public static void main (String args)
• System.out.println(Hello, world!)

4
Observations

• Case sensitive match cases exactly
• Program is worked in order from start to end
• Sequences are fundamental to computing
• Curly braces mark nested components
• Hierarchies are fundamental to computing
• Learn the patterns of typical programming tasks
• Setting up the program
• Doing output

5
The Big Picture

• CPU and RAM
• Programs are sequences of statements
• Or commands or instructions (pick a word)
• The CPU has a built in numeric machine language
• Java is write once, run anywhere
• The trick is the Java Virtual Machine.

6
Compiling - the mechanics

• Should work the same on PCs and Macs
• You have to compile programs to run them
• javac hello.java
• You have to use the JVM to run them, too
• java hello
• Hello, world!

7
My History of Java

• Interpreting expressions (1940s)
• Sort/Merge Generator
• A2 Compiler
• Fortran
• Algol
• C
• Then, Objects happened

8
Objects

• Programming where the data is logically
  associated with its own software
• Attributes - specific pieces of data associated
  with an object
• Methods - software procedures associated with
  objects
• A Vehicle object
• Attributes location, direction, speed
• Methods start, stop, accelerate, checkSpeed,
  maxCapacity
• Inheritance
• You can define new types of objects in terms of
  old ones
• New types inherit objects/methods of the old
  one
• New type can be tailored cars vs trucks

9
Enough background.

• Back to Programming
• Printing out text
• Statements and Variables

10
Printing out text

• System.out.print - print out text
• System.out.println - print out text w/newline
• Text strings bounded by quotes
• \ escape character for specials
• \n new line starts new line
• \r carriage return new line on Mac
• \t tab character depends on tab sets
• \\ puts a backslash in a text string

11
Example

• Print one two three
• On different lines
• On the same line
• On different lines using \n
• What does println do that print doesnt?
• Whats the difference?

12
Statements and Variables

• Statements well use
• Wrappings like class and static void
• Always followed by a pair of curly braces
• Other statements are followed by a
• Using methods or procedures
• Assignment statements
• Variable declarations
• Variables
• Storage cells in your program
• Each one is like a spreadsheet cell,
• only dumber

13
Variable Naming

• Naming Structure
• First character A-Z, a-z, _ or
• Remaining same plus digits
• Style rules
• Descriptive names
• Usually start with lowercase letter
• Camelback use capitals to mark words
• itemsOrdered, totalPayment
• DO NOT USE to start a name

14
Types of variables

• First use you say the type
• int integer
• signed, no decimal part, less than 2G
• String character string
• Usually captured in quotes
• Note that String is capitalized
• Examples
• int sampleInt 12
• sampleInt 25
• System.out.println(sampleInt)

15
Printing Variables

• Our print and println methods
• Will print out integers and strings
• Combine strings with
• Combine a String and int with

16
Example

• String myname Joe
• int age22
• System.out.println(My name is
• myname)
• System.out.println(I am age
• years old.)

17
Another Example

• String myname Joe
• int age 25
• System.out.print(My name is myname . I am
  age .)
• myname Barb
• age 26
• System.out.print(My name is myname . I am
  age .)

18
Lab Problem

• Given the text, write print and/or println method
  calls to print it out.
• Replace the name (Jon) and pronoun (he) with
  string variables.
• Set the variables to different values and produce
  the results
• NameSally, pronounshe
• NameFrank, pronounhe

19
The Text

• Jon went to the store. When he arrived, he got a
  shopping cart.
• Then he walked to the produce section. Jon was
  surprised. The
• raspberries were fresh, so he picked up two
  boxes. Then he went
• to the cereal section and picked up three boxes.
  Jon really
• likes cereal. After he had visited the whole
  store, Jon pushed
• the cart to the front. There, he paid for the
  groceries. It
• took six bags to carry all of Jon's groceries.
• The End.

20
Suggestion

• Do the first line, compile it, run it.
• Fix what doesnt work, try again till its right.
• Add another line, compile, run.
• Etc.
• Find and fix problems incrementally.
• Its much easier than trying to fix 30 different
  errors.