Java Introduction

1
Java Introduction

• If you understand this, you will be able to
• Outline 4 key features of java
• Use simple control structures
• Declare variables
• Understand simple class declarations

2
Key Features

• Portability
• Garbage collection
• No memory leaks - no need to free objects
• Support for concurrent programming
• Threads
• Secure
• Applets run in a sandbox,
• Supports digital signatures
• Many additional packages
• Communication, user-interface, etc

3
Java Virtual Machine
Java Source
Class file (byte code)
Compiler
Java Application
Java Application
Windows JVM
Linux JVM
Linux
Windows
4
Portability

• Uniform run-time system
• Java Virtual Machine
• Same interface on different processors
• Interpreted assembly language
• Compiler generates instructions for JVM
• No implementation dependencies
• e.g. define size of types
• C int could be 32 or 64 bits
• In Java size of int is 32 bits on every machine
• A program will give the same results on any
  machine

5
Robust

• Simple language
• Cut out C features
• No pointers - no direct memory access
• Strong typing checked at compile time
• Run-time bounds cast checking
• Exceptions
• Automatically jump to handler code on error
• Ensure programmer handles faults

Cant access beyond the end of an array
6
Java Basics

• Syntax control structures
• if, for, while, do while () like C
• Primitive data types
• int, char, short, long, float, double like C
• Also byte, boolean
• All initialised to defined default values
• Compound data types
• Class e.g. to represent a person age, name,
• Strings a normal class holding characters
• Arrays a normal class holding a collection of
  items

7
Example Parity Calculation

• Parity check to detect errors
• Add extra parity bit to 7 data bits
• Ensure that total number of ones is even
• An error will make the total odd
• On receipt, count the number of bits
• If odd, there has been at least one error
• If even, assume no error
• Cannot detect even number of errors

8
Parity Calculation overview
Set up using System.in like cin

• // initialisation
• String inputData formattedInput.readLine()
• int pos 0
• int parityBit 0
• / Calculate the parity bit /
• if (inputData.length() ! 7)
• System.out.println("There should be 7 bits
  of input")
• else
• System.out.println("Result
  "inputDataparityBit)

String is built into Java. A string object can
tell you its length and return individual
characters.
System.out is like cout
9
Parity Calculation main body
A getter function

• while (pos lt inputData.length())
• char current inputData.charAt(pos)
• pos pos1 // current
  position for user (start at 1)
• switch (current)
• case '0'
• break
• case '1'
• parityBit 1 - parityBit // invert
  parityBit
• break
• default System.out.println("Invalid
  input "current" at "(pos))

while, switch, , if are the same as in C
10
Classes

• A class defines a type of object
• An object is an instance of a class
• Data and function members
• Data members belong to an object
• Member functions affect current object
• Static data and static function members
• One per class rather than one per instance

isLeapYear()
nextDay()
Day
Month
Year
11
Class Example Position

• class Position
• float x
• float y
• Position(int nx, int ny) x nx y
  ny
• public String toString() return "("x",
  "y")"
• public double distFrom(Position p)
• return Math.sqrt((p.x-x)(p.x-x)
  (p.y-y)(p.y-y))

Data Member
Constructor
Method
12
Class Example Position

• public class Coordinates
• public static void main(String args)
• Position a new Position(3,4)
• Position b new Position(6,8)
• System.out.print(a" - "b"
  "a.distFrom(b))
• try System.in.read()
• catch (Exception ex) System.out.println(e
  x.toString())

Start point like C
Just to pause
13
Notes

• A class has data members functions
• Constructor
• Creates an object
• Position a new Position(3,4)
• Use as distFrom() method
• This can access as x and y data members
• Automatically use toString() method

14
Example Value Reference

• int x 5
• int y 2
• x y
• String Sx new String ("five")
• String Sy new String (two")
• Sx Sy

5
2
2
Garbage cant be reached from the program
could be returned to the run-time system
15
Reference Value Variables

• Primitive variables hold the value
• int, boolean variables, float
• Class variables hold a reference to an object
• String, Object, Position
• Primitive types have corresponding classes
• Boolean, Integer, Float
• Provide useful methods

16
Inheritance
The child is like the base With extra facilities
Base class
Child Class

• A class automatically has the methods and
  properties of its ancestor (base class)
• Define new class starting from the ancestor
• Can add data members
• Can add methods
• Can change implementation of methods
• A class always inherits from 1 ancestor
• Except for the Object class

17
Declaring a Descendent Class

• class GridReference extends Position
• GridReference(int nx, int ny)
• super(nx,ny)
• public String toString()
• return "("x" East, "y" North)"

Position
GridReference is like Position with a different
toString()
GridReference
GridReference has x and y data members by
inheritance
GridReference has a distFrom() method by
inheritance
GridReference overrides toString()
18
Using the Descendent

• GridReference grA new GridReference(4,5)
• GridReference grB new GridReference(16,10)
• System.out.println(grA" - "grB"
  "grA.distFrom(grB))

19
What Java Hasn't Got

• Constants
• Use 'final' variables - can't be changed
• Structures
• Combine related values (e.g. name, age, address)
• Use classes instead
• Pointers
• However, objects use the reference model
• A field in a object can refer to another object
• Single byte characters
• All characters are Unicode (2 byte)

20
Garbage Collection

• Memory management - major cause of bugs
• Forget to release memory - lose resources (a
  leak)
• Use memory after release - unpredictable contents
• Release twice confuse the memory allocator
• C
• Explicitly release allocated memory with delete
• Java
• Run-time system scans memory
• (Periodically or when a memory request cant be
  met)
• Release blocks not referenced by program

21
Summary of Java

• Great similarities with C
• Uses reference variables not pointers
• Classes
• Group data functions together
• Inheritance
• Can define new classes by extension
• Portability through Virtual Machine
• Concerned with safety garbage collection