Advanced Review

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Advanced Review
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Advanced Review

• Time classes
• Date Classes
• File input/output
• Packages

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Multiple dimensions

• No reason cant create 4,5,6 dimension arrays
• Gets hard to manage
• Better idea
• Think about another way of representing the data
• Often creating an object is a better approach

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Arrays further

• Need to explicitly copy contents of arrays when
  resizing arrays with temp one
• Better solution
• ArrayList
• Vector
• Full object versions of arrays
• Capacity can grow over time
• Useful methods bundles with them

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code

• Create a new class with a main called VectorTest
• Create a vector
• Put in some stuff
• Print them out
• Replace something in the middle
• Print it out
• Clear the vector
• Print it out

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Default values

• Should be aware if you forget to set values
• Might mess up your logic
• Think of multiplying a bunch of numbers and not
  setting one of them
• Compiler/IDE will let you know if you forgot to
  set values (warning)

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Time

• Next lets discuss how time is handled in Java
• Java.util.Date is the way java represents a point
  in time
• It is measured in milliseconds
• Time 0 what does that mean?

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Date

• Time 0 first millisecond in 1970
• Historical reasons for this
• long is the type it uses
• Range to 9,223,372,036,854,775,807
• Dont memorize that
• Lets look at the API
• What is a deprecated method ??
• Example getDay() ??

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Why Date

• Why are we wrapping the time long number?
• "I'll see you on 996,321,998,346." doesnt really
  work
• Also allows you to easily order and compare
  different points in time.meaningfully

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Change over time (no pun)

• Nothing you program should be set in stone
• Sometimes your design changes
• Need to go back and change your code
• Deprecated methods are those which have been
  replacedbut left in place so your old stuff
  shouldnt crash if you try to recompile with
  latest jdk

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Back to time

• System.currentTimeMillis()
• Returns the current time on the system
• As a Date Object

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Ideas

• Although would like to represent a point in time,
  usually time is associated with other
  measurements
• Month
• Year

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The GregorianCalendar Class

• The Date class doesn't measure months, weekdays,
  etc.
• That's the job of a calendar
• A calendar assigns a name to a point in time
• Many calendars in use
• Gregorian
• Contemporary Hebrew, Arabic, Chinese
• Historical French Revolutionary, Mayan

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Relationships
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Next step

• Lets design a new class to represent a day
• Today is Tuesday
• Day today new Day()
• Today.add(1) //should give us Wednesday

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Goal of Day Class

• Answer questions such as
• How many days are there between now and the end
  of the year?
• What day is 100 days from now?
• How many days till my birthday (Ive always
  wanted a _____________)

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Designing the class

• Lets have a method
• daysFrom it computes number of days between two
  days
• int n today.daysFrom(birthday)
• addDays computes a day that is some days away
  from a given day
• Day later today.addDays(999)
• Mathematical relationship
• d.addDays(n).daysFrom(d) n
• d1.addDays(d2.daysFrom(d1)) d2

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• Constructor Date(int year, int month, int date)
• Will need the following methods
• getYear
• getMonth
• getDate

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Implementation

• Straightforward which member will need
• private int year
• private int month
• private int date
• addDays/daysBetween tedious to implement
• April, June, September, November have 30 days
• February has 28 days, except in leap years it has
  29 days
• All other months have 31 days
• Leap years are divisible by 4, except after 1582,
  years divisible by 100 but not 400 are not leap
  years
• There is no year 0 year 1 is preceded by year -1
• In the switchover to the Gregorian calendar, ten
  days were dropped October 15, 1582 is preceded
  by October 4

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Day Code

• public Day(int aYear, int aMonth, int aDate)
• year aYear
• month aMonth
• date aDate
• private int year
• private int month
• private int date
• private static final int DAYS_PER_MONTH
• 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30,
  31
• private static final int GREGORIAN_START_YEAR
  1582
• private static final int GREGORIAN_START_MONTH
  10
• private static final int GREGORIAN_START_DAY
  15
• private static final int JULIAN_END_DAY 4

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Day Code

• private Day nextDay()
• 112
• 113 int y year
• 114 int m month
• 115 int d date
• 116
• 117 if (y GREGORIAN_START_YEAR
• 118 m GREGORIAN_START_MONTH
• 119 d JULIAN_END_DAY)
• 120 d GREGORIAN_START_DAY
• 121 else if (d lt daysPerMonth(y, m))
• 122 d
• 123 else
• 124
• 125 d 1
• 126 m
• 127 if (m gt DECEMBER)
• 128
• 129 m JANUARY

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• private static int daysPerMonth(int y, int m)
• int days DAYS_PER_MONTHm - 1
• if (m FEBRUARY isLeapYear(y))
• days
• return days
• private static boolean isLeapYear(int y)
• if (y 4 ! 0) return false
• if (y lt GREGORIAN_START_YEAR) return true
• return (y 100 ! 0) (y 400 0)

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Tester

• 01 public class DayTester
• 02
• 03 public static void main(String args)
• 04
• 05 Day today new Day(2001, 2,
  3)//February 3, 2001
• 06 Day later today.addDays(999)
• 07 System.out.println(later.getYear()
• 08 "-" later.getMonth()
• 09 "-" later.getDate())
• 10 System.out.println(later.daysFrom(today)
  )// Prints 999
• 11
• 12

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Notice

• Private helper methods
• Notice all the work to increment a day

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Another idea

• This is for illustration, dont need to code
• For greater efficiency, we can use Julian day
  number
• Used in astronomy
• Number of days since Jan. 1, 4713 BCE
• May 23, 1968 Julian Day 2,440,000
• Greatly simplifies date arithmetic

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Code

• public Day(int aYear, int aMonth, int aDate)
• //notice we are calling a private
• //helper function
• julian toJulian(aYear, aMonth, aDate)
• //thats all we need
• private int julian

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Helper function

• private static int toJulian(int year, int month,
  int date)
• int jy year
• if (year lt 0) jy
• int jm month
• if (month gt 2) jm
• else
• jy--
• jm 13
• int jul (int) (java.lang.Math.floor(365.25
  jy)
• java.lang.Math.floor(30.6001 jm) date
  1720995.0)
• int IGREG 15 31 (10 12 1582)
• // Gregorian Calendar adopted Oct. 15, 1582
• if (date 31 (month 12 year) gt IGREG)
• // Change over to Gregorian calendar
• int ja (int) (0.01 jy)
• jul 2 - ja (int) (0.25 ja)

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Any other ideas?

• So you see that using the class doesnt change
  even though we totally changed how the inside
  works
• Called encapsulation
• So its easy to move up or down the calendar
• Add subtract
• Where would it cost us ??

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Switch gears

• Lets talk about how to use files
• Your program starts in main, computes, then maybe
  prints out something before closing up
• Would be great if can save results somewhere
• Hey lets use files ?

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File manipulations

• Working with files
• Reading files
• Writing files

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Please note

• One of the great things about file manipulation
  on java is that it is the same on
• Windows
• Linux
• Mac
• If done right

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File

• Basic object is called File
• File data new File(test.txt)
• It will look in the same directory the java file
  is sitting in for the test file
• Calling data.getAbsolutePath()
• Will print out the local version of the full path
  to the file

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Directories

• If your File object is a directory
• The list method returns an array of String file
  names,
• listFiles returns an array of File objects

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limitations

• The File object has a limited number of useful
  methods
• None which can actually write something to it
• Need to use higher level class to work with file
  contents

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Dealing with text

• When dealing with text output/input can use
• PrintWriter to write to a file

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code

• PrintWriter pw new PrintWriter(new
  FileWriter(new File("Test.txt")))
• What is FileWriter ?
• Lets pull up the API

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• Helper class to help make sure things are written
  efficiently
• Dont forget to close the file handle when done
• And flush if using a buffered handle

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• Ok lets write some code
• Main program
• Will write a 3 line poem (yes you need to write
  one now) to a test.txt file
• Notice how your have to add try catch to handle
  certain declared exceptions ??

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Run code

• Confirm that the file has been created
• Now write another class to read the file

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• How would you adopt the reader to find something
  in the file ??

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• For each line read
• Look for something
• See String API for helper methods
• Now write the code ?

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Next up

• Interfaces
• Inheritance
• Abstract Classes
• Polymorphism
• Generics

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Two dimensions

• You can also initialize the inner array as a
  separate call.
• Doesnt have to be congruous memory locations
• int example new int5
• for (int i0ilt5i)
• examplei new inti1

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Interface

• An interface is a special class that defines the
  behavior of other classes
• Example
• How many mp3 players are on the market ?

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Mp3 players

• No matter what type of mp3 player you buy you
  expect certain behavior
• Play
• Forward(next song)
• Rewind(last song)
• Random
• Think of your own

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• If I want to program a bunch of mp3 players and
  want to force all of them to have some minimum
  behavior I would encode that as an interface
• Here is an example

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code

• public interface mp3player
• public boolean play()
• public boolean rewind()
• public boolean forward()
• public int getSongCount()
• public boolean deleteAll()

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analysis

• Basically am defining ideas
• Would add comments to each what they are supposed
  to be doing (expected behavior
• Then any class which wants to be nice, would
  agree to behave this way

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Code IpodMP3

• Say we want to create a really cool mapple ipod
  player
• Want to stick to the mp3 behavior
• public class Ipodmp3 implements mp3player ..
• This means that we need to define those methods
  for this specific class

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Note

• Can implement as many interfaces as you like
• Will notice them in the api when you look at the
  standard libraries
• Compiler will check and complain if you dont
  implement all the methods you are promising in
  the interface you impliment
• Nothing actually checks that you are doing the
  correct thing.that is up to the programmer

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GASP!

• Question so what does Ipodmp3 player1 new
  Ipodmp3 ()player1.play()actually do ??
• Answer Maybe erase your hard drivelook at the
  source code ?

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Iterators

• Many of the classes in Java represent a
  collection of items
• Some have order
• Some dont have order
• All the students in one room
• Alphabetical list
• Bag of names (raffle)

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Two types

• Here is a general Iterator idea
• Get the iterator object from the class
• Can ask the iterator object if it has any more
  stuff
• Get the next thing

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Inheritance

• You have a really useful class
• Want to specialize it in more than one way
• Can either copy paste a bunch of time and tweak
  each copy
• Problem if we find a bug, will need to run to all
  copies and fix everything

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Easier way

• Look at all the classes that are related, what
  ideas do they all share
• Example what is common between all kinds of cars
• Make that idea the base class, and each
  specialization a sub class
• Known as inheritance!

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Inheritance

• Use the extends keyword
• Allows you to reuse objects
• Some issues
• When do you extend?
• When do you implement an interface?

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Example

• Geneal idea of an Animal
• Can code it as class Animal
• Then have specific code
• class Horse extends Animal
• class Whale extends Animal
• What ever methods are the same in both would be
  coded in the Animal class, specific methods
  (numberLegs() would be defined in the specific
  subclass)

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Super constructors

• When refering to parent class (Animal)
• Use super keyword in subclass constructor
• public Horse(String aName)
• super(aName) // calls superclass constructor
• //rest of your stuff here
• Call to super must be first statement in subclass
  constructor
• If subclass constructor doesn't call super,
  superclass must have constructor without
  parameters

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Polymorphism

• This is just fancy word that tells you java will
  figure out the following code
• Animal A new Horse(tom)
• So although A is really a horse, it can be
  refered to by parent handle.
• This would be illigal
• Horse A new Whale()
• Why ?

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Next

• Imagine you have a class which has variable
  members of a specific type
• Example
• Have a list of numbers, say a list of zip codes
  which you have sent packages in the last X months
• What if you want to change zip codes to Strings
  because you are now sending packages to canada

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2 solutions

• Copy paste code and replace all ints with Strings
• Debug like crazy
• .or use Generics

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Basic idea

• Tell java you are dealing with some type without
  know what it is
• Can set some rules on it
• When you create an instance of the object you
  will tell java what you want

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Code

• public class shoppingListltTgt
• private ArrayListltTgt theList
• shoppingList()
• theList new ArrayListltTgt()
• public void addItem(T item)
• theList.add(item)
• public int getNumberOnList()
• return theList.size()
• public T getItemAt(int location)
• return theList.get(location)

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Code 1

• //once you get previous screen running at the
  following method
• public void showType()
• System.out.println("Type of T is "
• ob.getClass().getName())

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Usage

• shoppingListltIntegergt e1
• e1 new shoppingListltIntegergt
• e1.add(15)
• int v e1.getItemAt(0)
• Write a main to test the idea of generics

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Example2

• class Example2ltT, Vgt
• T ob1
• V ob2
• Example2(T o1, V o2)
• ob1 o1
• ob2 o2
• T getob1()
• return ob1
• V getob2()
• return ob2

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Example 3

• class Example3ltQ extends Numbergt
• Q nums // array of Number or subclass
• Stats(Q o)
• nums o
• double sum()
• double sum 0.0
• for(int i0 i lt nums.length i)
• sum numsi.doubleValue()
• return sum

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Usage

• Double dnums 1.1, 2.2, 3.3, 4.4, 5.5
• Example3ltDoublegt dob new Example3ltDoublegt(dnums)
  
• double w dob.sum()
• System.out.println("dob sum is " w)

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Example4

• class TwoD
• int x, y
• TwoD(int a, int b)
• x a
• y b
• class ThreeD extends TwoD
• int z
• ThreeD(int a, int b, int c)
• super(a, b)
• z c
• class FourD extends ThreeD
• int t
• FourD(int a, int b, int c, int d)

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• class CoordsltT extends TwoDgt
• T coords
• Coords(T o) coords o

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• static void showXY(Coordslt?gt c)
• System.out.println("X Y Coordinates")
• for(int i0 i lt c.coords.length i)
• System.out.println(c.coordsi.x " "
• c.coordsi.y)
• System.out.println()

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• static void showXYZ(Coordslt? extends ThreeDgt c)
  
• System.out.println("X Y Z Coordinates")
• for(int i0 i lt c.coords.length i)
• System.out.println(c.coordsi.x " "
• c.coordsi.y " "
• c.coordsi.z)
• System.out.println()

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• static void showAll(Coordslt? extends FourDgt c)
• System.out.println("X Y Z T Coordinates")
• for(int i0 i lt c.coords.length i)
• System.out.println(c.coordsi.x " "
• c.coordsi.y " "
• c.coordsi.z " "
• c.coordsi.t)
• System.out.println()

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Using in methods

• Just learn how to read the following
• static ltT, V extends Tgt boolean isIn(T x, V y)
  
• for(int i0 i lt y.length i)
• if(x.equals(yi)) return true
• return false

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• Hope you had fun learning this!