A Reality Check a sort of quiz

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A Reality Checka sort of quiz

• Dave Elliman

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A Story with a Moral My brother
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What Is a Computer?
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What Is a Computer Program?
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What Bits and Pieces Must a Computer Have?
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What is the Point in Having Programming Languages
like Java?
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What does javac do?
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What does java do?
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What is the trick that enables java to run on any
computer?
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There are two fundamental types of data object in
Java what are they?
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A variable in java has the name fred. What does
the java run time need to know about fred?
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I enter 1.2345 at the keyboard. What type am I
creating?
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A byte in computer memory contains00010101What
does it represent?
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A Simple Java Program
public static void main(String args)
System.out.println( I Love G51PRG)
What is the meaning of the word static in this
program?
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What does new really do in Java?
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What is output by this program?
public static void main(String args) String
fred fred fred fred.toUpperCase()
System.out.println(fred)
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What is the value of x?
int x x 2 3 4 / 2 2
Why?
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A Proper Object
import java.io. public class ReadDouble
double readDouble() String aLine
"" double d 0.0 boolean
failed false BufferedReader input
input new BufferedReader(new
InputStreamReader(System.in)) try
aLine input.readLine()
catch (Exception e) failed true
try d Double.parseDouble(aLine)
catch (Exception e) failed
true if(failed)
System.out.println("Invalid Coefficient!")
System.exit(-1) return
d
what filename should be used?
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• myArray has room for 8 elements
• the elements are accessed by their index
• in Java, array indices start at 0

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Declaring Arrays

• int myArray
• declares myArray to be an array of integers
• myArray new int8
• sets up 8 integer-sized spaces in memory,
  labelled myArray0 to myArray7
• int myArray new int8
• combines the two statements in one line

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Assigning Values

• refer to the array elements by index to store
  values in them.
• myArray0 3
• myArray1 6
• myArray2 3 ...
• can create and initialise in one step
• int myArray 3, 6, 3, 1, 6, 3, 4, 1

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Iterating Through Arrays

• for loops are useful when dealing with arrays
• for (int i 0 i lt myArray.length i)
• myArrayi getsomevalue()

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Arrays of Objects

• So far we have looked at an array of primitive
  types.
• integers
• could also use doubles, floats, characters
• Often want to have an array of objects
• Students, Books, Loans
• Need to follow 3 steps.

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Declaring the Array

• 1. Declare the array
• private Student studentList
• this declares studentList
• 2 .Create the array
• studentList new Student10
• this sets up 10 spaces in memory that can hold
  references to Student objects
• 3. Create Student objects and add them to the
  array studentList0 new Student("Cathy",
  "Computing")

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Java Methods Classes
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Classes ARE Object Definitions

• OOP - object oriented programming
• code built from objects
• Java these are called classes
• Each class definition is coded in a separate
  .java file
• Name of the object must match the class/object
  name

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Simple class

• class Fruit
• int grams
• int cals_per_gram

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Methods ...

• Class Fruit
• nt grams
• int cals_per_gram
• int total_calories()
• return(gramscals_per_gram)

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Another Class

• public class Point public double x,
  y private attribute
• public Point() x 0 y 0 size 1
• public double getSize() return size
• public void setSize(int newSize) size
  newSize

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Source Files

• Put
• public class Fred
• IN Fred.java
• A Source file can have only one public class in it

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Methods

• A method is a named sequence of code that can be
  invoked by other Java code.
• A method takes some parameters, performs some
  computations and then optionally returns a value
  (or object).
• Methods can be used as part of an expression
  statement.
• public float convertCelsius(float tempC)
• return( ((tempC 9.0f) / 5.0f) 32.0 )

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Method Signatures

• A method signature specifies
• The name of the method.
• The type and name of each parameter.
• The type of the value (or object) returned by the
  method.
• The checked exceptions thrown by the method.
• Various method modifiers.
• modifiers type name ( parameter list ) throws
  exceptions
• public float convertCelsius (float tCelsius )
• public boolean setUserInfo ( int i, int j, String
  name ) throws IndexOutOfBoundsException

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Using objects

• Here, code in one class creates an instance of
  another class and does something with it
• Fruit plumnew Fruit()
• int cals
• cals plum.total_calories()
• Dot operator allows you to access (public)
  data/methods inside Fruit class

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Public/private

• Methods/data may be declared public or private
  meaning they may or may not be accessed by code
  in other classes
• Good practice
• keep data private
• keep most methods private
• well-defined interface between classes - helps to
  eliminate errors

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Creating objects

• Following code creates an instance of the Fruit
  class
• Fruit plum
• defines the plum object
• plum new Fruit()
• creates it in memory
• the content of the Fruit class must be defined in
  another file Fruit.java

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Constructors

• The line
• plum new Fruit()
• invokes a constructor method with which you can
  set the initial data of an object
• You may choose several different type of
  constructor with different argument lists
• eg Fruit(), Fruit(a) ...

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Overloading

• Can have several versions of a method in class
  with different types/numbers of arguments
• Fruit()grams50
• Fruit(a,b)
• gramsacals_per_gramb
• By looking at arguments Java decides which
  version to use

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Object Oriented Programming
instance variables
methods
messages
break()
speed 45.7 gear 3
changeGears(g)
an object
a program
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The three principles of OOP

• Encapsulation
• Objects hide their functions (methods) and data
  (instance variables)
• Inheritance
• Each subclass inherits all variables of its
  superclass
• Polymorphism
• Interface same despite different data types

car
Super class
auto- matic
manual
Subclasses
draw()
draw()
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Example Russian Roulette
5/6
1/6
4/6
2/6
3/6
3/6
2/6
4/6
1/6
5/6
2
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Web of message calls...
revolver
load()
model
trigger()
trigger()
player1
player2
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Roulette
Classes
instances
Model
model
new
new
new
player1
Player
player2
Revolver
revolver
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Inheritance ...

• Important feature of OOP - new classes can be
  based on existing classes eg. Could define a
  specialized type of Fruit class called Citrus
• Has all methods of Fruit plus possibly some new
  ones eg
• class Citrus extends Fruit
• void squeeze().

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Inheritance II

• How to use
• eg.
• Citrus lemon new Citrus()
• lemon.squeeze()
• lemon.total_calories()
• old methods exist alongside new methods

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Overriding

• Even more powerful concept of OOP
• can override the functionality of one method in a
  descendant class
• eg. Add method peel() to Fruit class. Since
  Citrus extends Fruit this method will also be
  available to an instance of Citrus
• But can redefine content of peel() inside of
  Citrus - the new definition hides the earlier ...
  

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Libraries

• Java comes with libraries for creating GUIs and
  network applications and for embedding in Web
  pages- java.applet.Applet
• eg import java.awt.
• compile to byte code - can be run by any system
  with a Java interpreter - portable!
• Relatively robust and secure

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Interface vs. implementation
User only has to be familiar with the interface
of an object, not its implementation
Objects hide their functions and data
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Where to Revise

• Eckel, B. Thinking in Java
• Free online book
• http//www.ibiblio.org/pub/docs/books/eckel/
• - Sun Tutorials
• http//java.sun.com/docs/books/tutorial/index.html