Algorithms

1
Algorithms

• Series of mathematical or variable manipulations
• Integer a,b,c
• a 12
• b 22
• c a b (what is c?) 12 22 264

2
An example.

• Describe how to tie a shoe
• How to dial a phone
• Give a pill to a dog

3
Programs Complicated, Concatenated Algorithms

• integer Total, LoopCounter
• Total 0
• LoopCounter 0
• Start
• Add 2 to Total
• Add 1 to LoopCounter
• Go Back to Start Until Total 12
• What is LoopCounter?

Total 2 4 6 8 10 12
LoopCounter 1 2 3 4 5 6
done
test for Total12, done here
4
Written in Code

• int LoopCounter, Total
• LoopCounter 0
• Total 0
• while (Total lt 12)
• Total Total 2
• LoopCounter LoopCounter 1

semicolon
no semicolon (because brackets follow)
open bracket
odd-looking algebra
close bracket
5
A computer Program
public class Lab2MainClass public static void
main( String args ) . your
code here .

• A computer file.
• In your Lab work area.
• Written in an editor.
• filename.java
• gt mkdir Lab2
• gt cd Lab2
• /Lab2gt pico Lab2MainClass.java

6
Unix vs. Java

• Unix
• - At the terminal prompt
• Conversations with the computer to build files
• clear
• ls
• set prompt gt
• mkdir
• cd
• cd directoryName
• pico filename
• rm filename

• Java
• A program that runs
• In a file
• not Unix
• public class name
• public static void main
• - brackets

7
Basic Java Program Framework

• public class Lab2MainClass
• public void someBehavior( input variables )

your main program name
brackets ALWAYS paired
8
Comments

• included in your program
• read by humans, not the computer
• used to explain what you intend the code to do

9
Commenting code

• / anything between delimiters /
• // anything after two slashes
• Header - comments at the beginning of a program
• // one line only
• /
• CS113
• Author Harry Truman
• Date September 7, 2010
• /

10
Readability

• Indentation
• Skipped lines
• Brackets
• Comments
• Pages Functional paragraphs
• Meaningful variable names

11
Java Commands

• go in the brackets
• tells the computer what to do, one step at a time
• mathematical, algorithmic
• follows a strict syntax

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example

• public class MyProgram
• public void test( )
• int X
• X 14
• System.out.println( X )
• on the computer
• gt javac MyProgram.java
• gt java MyProgram
• 14
• gt

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Display

• System.out.println( anything in quotes )
• System.out.println( anyVariable )

14
The Assignment Statement

• public void name ( )
• int A
• A 4
• NOT A equals 4
• instead A is assigned the value 4

semicolon, brackets, main must be Java
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the Assignment

• int x
• x 14
• x x 1
• what is x?
• ( same as x which is a java shortcut )

Java
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Keywords vs. myWords

• Keywords are Java-defined
• public
• static
• void
• main
• String
• args
• class
• int
• double
• many more

• variables, file names are your choice
• new class names
• new method names
• greeting
• Lab2MainClass
• x, y, z
• myName
• Convention
• lowerUpper

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Structure of a class (well stop calling it a
program)

• // introductory comments
• public class className extends Parent
• public outputs name ( inputs )
• // variables
• // algorithms
• // outputs (i.e. System.out.println( )
  )
• // end method
• // end class

18
A very simple program contained in file
AnyName.java

• public class AnyName
• public static void main(String args )
• System.out.println("The main method in the
  AnyName class")

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Writing a Java Class

• Write the main class, in a file with the same
  name
• public class AnyName in AnyName.java
• must contain a
• public static void main (String args)
• Compile it javac AnyName.java
• produces AnyName.class
• 4. Run the program java AnyName

20
Run Java Programs within a terminal window
21
Alternation if, else

• Integer x
• String y
• If (x 0)
• y X is zero
• else
• y X is not zero

no semicolon must be English
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Proper Java

• int x
• String displayText
• if (x 0) // note comparison equal
• displayText X is zero
• else
• displayText X is not zero
• // end if x is zero
• System.out.println( displayText )

optional
23

• public class Factorialpublic static void main(
  String args ) long Product 1 //
  holds the running factorial int FactNum
  25 // number to find the factorial of int
  x 0 // increment variable while ( x lt
  FactNum ) x x1 Product
  Product x System.out.println( x "
  factorial " Product ) if (Product lt
  0) System.out.println("Err
  or! Number too large.") System.exit(
  0 ) // end while loop
  // end main // end class

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New stuff

• long Product 1
• - bigger int
• System.out.println( x " factorial " Product
  )
• - concatenate Strings
• System.exit( 0 ) - stop the program

25
casting

• We want to avoid mixing math operations between
  incompatible types
• What happens when we WANT to multiply a double
  an int?
• Cast
• ( double ) x changes x temporarily to a
  double
• ( int ) product changes product temporarily to
  an int

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public static

• public can be used throughout the program
• static guaranteed unique name

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A Java Method

• Methods are often called routines, subroutines,
  functions or procedures
• A single function from start to finish
• Surrounded by curly-brackets ...
• Can be used many times (called) from all over
• Allows intelligent Transfer Of Control
• Many methods and variables are collected into a
  class
• A Class is a Java program that does one job, and
  draws on many methods and variables to accomplish
  it. Classes eventually become objects

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The main method

• public class Lab2MainClass
• public static void main( String args)
• String Greeting
• Greeting Hello
• System.out.println( Greeting )

29
public static void ExampleMethod( )
System.out.println(Hello)
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Using (Calling) a method

• in main ExampleMethod( )
• public class Lab2MainClass
• public static void main( String args)
• ExampleMethod( )
• public static void ExampleMethod( )
• System.out.println(Hello)

Note parenthesis
31
Our First Complete Java Class (i.e. collection of
Methods and variables).
class
public class SimpleClassProgram public
static String Greeting public static
void main( String args )
sayHello( ) System.out.println(
done ) public static void
sayHello( ) Greeting
Hello System.out.println( Greeting )

public variables
method1main
call
return
method2
32
public static

• variables that are public static can be used by
  any method.
• methods that are public static can be called from
  any method (not just main), and can be used by
  other programmers.

33
using another authors public class and method

• import javax.swing.JOptionPane
• // JOptionPane has many useful methods
• in main
• String userInput
• userInput JOptionPane.showInputDialog( type
  something)

used by method
sent from method
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Exchanging values with main

• methods can return the result of a calculation
  (as a service to main)
• public static int addNumbers( ) // not void
• int x, y, z
• x 1
• y 2
• z x y
• return( z )

35
Calling a method that returns a value

• public static void main( String args)
• int b
• b addNumbers( ) // note b
• System.out.println( b )

36
Java Dot Notation

• Used to separate larger categories from detailed,
  smaller instances (more when we study Objects)
• System.out.println( )
• System is a library of code someone else wrote
  (imported), a large collection of classes and
  methods
• out is a subset of System, as opposed to in, a
  class
• println is a specific method in the out subset
  of the System library of programs

37
import statement

• import javax.swing.JOptionPane
• public class myClass
• .....
• javax - a library of programs
• swing - a library within a library
• JOptionPane - a class (has methods we can use)

38
Terms

• Class a type, like int or boolean or float,
  that designates a conglomerate of other types,
  variables, code snippets (method), anything,
  always regarded as a whole. A collection of
  methods and variables (best studied after
  Objects)
• Public within a class, free and unrestricted
  access
• Static within a class, there will only be one
  method or variable with this name.
• Void a method with no output back to the main

39
More Terms

• main always the beginning of a (concatenated)
  program. The computer runs this as the first
  method of a program.
• String args human readable stuff (more later)

40
Structure of a Variable Type

• type VariableName
• examples
• int x,y,z
• String Greeting
• used by all methods within a class
• public static double myAccountBalance

41
Structure of a Method

• domain static return type Name ( )
• / useful code here /
• // matching, closing brackets
• example
• public static void main (String args )

42
How to call a method

• Calling a void method
• MyMethod( )
• Calling a method with a return int variable -
• i.e. public static int MyMethod( )
• return(6)
• from main
• int LocalVariable
• LocalVariable MyMethod( )

43
The main method

• The computer only runs the main method.
• All support methods must be called from main.
• The computer jumps to the support methods as they
  are called, runs them, then returns.
• At the closing bracket of main, the computer
  stops.

44
Example in file NewClass.java

• public class NewClass
• public static String signOn
• public static boolean testFlag
• public static void main( String args )
• testFlag MyMethod( )
• if (testFlag true)
• System.out.println(signOn)
• System.out.println(testFlag)
• public static boolean MyMethod( )
• signOn Please Log In
• return( true )

45
Compiling and running

• gt javac NewClass.java
• gt ls
• NewClass.class NewClass.java
• gt java NewClass
• Please Log In
• true

46
Summary

• Three operations (so far)
• Concatenation
• Alternation if / else
• Transfer of Control methods
• one to go Iteration
• Methods and Variables