CST252 Network Software Design

1
CST252 Network Software Design

• Lecture 2 Fundamental Programming Structures
  Java Syntax

2
Resources

• Java Tutorial (java.sun.com)
• Basic structures trail
• Course Website
• users.wmin.ac.uk/lancasd/CCTM91
• Book
• First chapters of any Java book
• or C. Horstmann and G. Cornell. Core Java
• Volume 1 chapter 3

3
Lecture 1 Review

• Course Structure
• Java in General
• About Java, hype and reality
• Compiling and Interpreting
• HelloWorld
• next slide
• Introduction to Syntax
• basic types, variables (constant), operators,
  casts/conversions, comments, Strings

4
Review HelloWorld

• Case Sensitive
• Name of class name of file.java
• File contains
• public class ....
• Class contains
• public static void main(String args) ....
• Note semicolons, braces, comments
• Cycle Edit, javac, java

5
Quiz

• Which lines are OK, which are not?
• A) boolean done 0
• B) float x 97.3
• double x2 x
• x2 x
• C) static final double PI 3.145
• D) byte _Flag 253
• E) short 9ofSpades

6
Lecture 2 Overview

• Applications not Applets (L9)
• Fundamental Programming Structures
• Flow Control if, then etc.....
• Reading Input
• More Syntax
• Arrays, More about Strings
• Elementary classes
• methods, instance fields, test classes

7
Control Flow

• Block Scope
• Conditional
• if if, else
• Loops
• while
• do while
• for
• Switch and break

8
Block Scope

• public static void main(String args)
• int n
• ...
• int k
• k n
• ....
• // k only defined up to here
• public static void main(String args)
• int n
• ...
• int k
• int n // error cant redefine n
• ....

9
Conditional Statement

• if (YourSales gt Target)
• performance OK
• bonus 100

NO
Sales gt target
YES
performanceOK
bonus 100
10
If/Else conditional

• if (YourSales gt Target)
• performance OK
• bonus 100
• else
• performance bad
• bonus 0

NO
YES
Sales gt target
performanceOK
performancebad
bonus 100
bonus 0
11
While Statement

• while (balance lt goal)
• balance payment
• double interest balance 4.2/100
• balance interest
• years
• System.out.println(years)

NO
balance lt goal
YES
update balance
years
print years
12
Do/While

• While loop tests at the top, if you want the test
  at the end of the loop - so that it executes at
  least once - use do/while
• do
• balance payment
• double interest balance 4.2/100
• balance interest
• years
• // ask if ready to retire - get input
• while (input.equals(N))

13
For loop

• For loop executes a fixed number of times
  determined by a counter
• for (int i 0 i lt 10 i ) // common syntax
• System.out.println(i)
• Or
• int i
• for (i 0 i lt 10 i )
• System.out.println(i)
• // in this case i is still defined after loop
  (i10)

14
Other Control Statements

• Switch for multiple selections - like many
  if/else statements, as in C
• Breaking control flow - to jump out of a loop
• break
• labelled break
• continue

15
Screen Input/Output

• Output to terminal window is easy
• System.out.println(s)
• (actually - formatting issues - look in a book)
• Input - not so simple - 3 ways (more)
• From command line arguments
• Direct coding uses ugly I/O classes (can hide by
  creating a special class Console.java)
• A pop up window (InputTest.java)
• JOptionPane.showInputDialog(prompt)

16
Input of Numbers

• Output of numbers is easy - the system takes care
  of changing int, float etc to String
• But irrespective of how input is done, it is
  understood as a String and the user must turn it
  into a number type
• Changing String input to numbers
• int age Integer.parseInt(inputstring)

17
Examples

• InputTest.java (uses popup window)
• Console input (ugly)
• Loops
• Retirement.java
• Retirement2.java
• Lab session (calculate VAT)

18
Arrays

• A way of storing a bunch of values of the same
  data type
• Eg an array x of int, access the ith integer as
  ai
• Must know the size, how many items are to be
  stored
• if you dont - use an ArrayList or some other
  collection data structure that automatically grows

19
Creating Arrays

• Use new to create empty arrays
• int a new int20
• Loops
• for(int i0 ilt20 i)
• ai 43 2i
• If you try and access a25 you will get an error
• The size of the array is a.length so often see
  for(int i0 ilta.length i)
• An alternative syntax for arrays
• int a instead of int a

20
Filling Arrays

• Can create and initialise in one step
• int smallprimes 2, 3, 5, 7, 11, 13
• Copying
• int luckynumbers smallprimes
• luckynumbers5 12 //now smallprimes5 is
  also 12
• Both variables refer to the same array
• For a real copy, must copy each element or use
  System.arraycopy(from,fromIndex,to,toIndex,count)
• Java has no C-like pointer arithmetic

21
Command line parametersan example of an Array

• public static void main(String args)
• Then compile and run with the command line
• java myclass -d gray 34
• args0 -d
• args1 gray
• args2 34
• The args parameter is an array of Strings

22
Arrays continued

• Sorting arrays
• Arrays.sort(a)
• This sorts according to numerical size or
  alphabetically depending on what type is stored
  in the array
• LotteryDrawing.java code. Put 1,2,3..40 into one
  array, pick 5 (distinct) entries and place into
  another array. Sort and print.

23
Multidimensional Arrays

• Arrays can themselves hold other arrays - so you
  can have multidimensional arrays
• final int NROWS 8
• final int NCOLUMNS 8
• char chessboard new charNROWSNCOLUMNS
• char06 Q // Queen here
• char32 // Blank (not occupied)

24
Strings

• Double quotes
• String greeting Hello
• Concatenation
• String s2 greeting David
• Equality
• s.equals(t)
• Empty String
• String empty //Not a blank space
• Length (brackets cf arrays)
• int len s2.length()

25
String Methods

• boolean equals(String)
• boolean equalsIgnoreCase(String)
• boolean startsWith(String)
• String subString(int,int)
• Need documentation to lookup all this
• Lab exercise

26
Documentation

• On-line Java API documentation (intranet copy)
• Created from Sun Java source implementation using
  javadoc
• Recall javadoc comments
• /
• This class prints HelloWorld
• /
• Comment your own code the same way

27
Simple Classes

• Think of classes as software packages that
  contain data and methods
• HelloWorld is not the common kind of class
  because it only contains a special method - the
  main() method
• A more typical class with similar function is
  Greeter

28
Greeter.java

• public class Greeter
• public String sayHello()
• String message Hello World
• return message

29
Testing classes

• Can compile Greeter.java
• gt javac Greeter.java
• But cant run it by itself since it doesnt have
  a main method
• Some development environments allow you to check
  classes by calling their methods
• We create a test class - the purpose is just to
  test the Greeter class - very common technique

30
GreeterTest.java

• public class GreeterTest
• public static void main(String args)
• Greeter wG new Greeter()
• System.out.println(wG.sayHello())

31
Testing

• GreeterTest creates an object of type Greeter,
  invokes its sayHello() method and prints the
  result on the console
• Running
• gt javac Greeter.java
• gt javac GreeterTest.java
• gt java GreeterTest

32
Instance fields

• The Greeter class isnt very interesting - it
  contains a method but no data
• Modify it to contain an instance field - data or
  state that the class saves
• This field is labelled private, so external
  classes cannot access it directly - only through
  methods

33
Greeter.java (Modified)

• public class Greeter
• public Greeter(String aname)
• name aname
• public String sayHello()
• String message Hello name
• return message
• private String name

34
GreeterTest.java (Modified)

• public class GreeterTest
• public static void main(String args)
• Greeter wG new Greeter(World)
• System.out.println(wG.sayHello())
• Greeter dG new Greeter(David)
• System.out.println(dG.sayHello())

35
Palaver

• This example may seem like a long way of doing
  something very simple
• In C, might just call a function
• In Object Oriented programming methods are
  contained in classes - which may also contain
  data (instance fields) - these data are not
  directly accessible from outside
• This control is useful in big software projects

36
Summary L2

• Syntax
• if/else, loops, arrays, Strings
• Inputting information
• Elementary classes (cover again with a slightly
  more complicated eg next week)
• methods and how to call them
• instance fields, private
• Test class to create and invoke methods