F21SF Software Engineering Foundations

1
F21SFSoftware Engineering Foundations

• L10 More on Arrays
• arrays Cars, 2 D, operations
• ArrayList
• StringBuffer class
• (nothing to do with arrays!)

Monica Farrow EM G30 email
monica_at_macs.hw.ac.uk Material available on Vision
and my website
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An array of Cars

• Arrays can store any data type, so lets create
  an array of 3 Cars
• private Car allCars
• private static final int NUM_CARS 3
• allCars new Car NUM_CARS
• allCars0 new Car("Ford Ka",
• 40, 33.6, Jo Low")
• allCars1 new Car("Mercedes Benz E280",
• 80,22, Tom Grey")
• allCars2 new Car("VW Sharan",
• 60, 29, Tim White")

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Finding the Car which goes the furthest

• //assume first can go furthest
• Car furthestCar allCars0
• //for each car, compare with the 'furthest' one
• //Should this index start at 0 or 1?
• for (int index1 indexltNUM_CARS index )
• if (furthestCar.estimateDistance() lt
• allCarsindex.estimateDistance())
• furthestCar allCarsindex

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The main method

• Now we can understand the parameter in the main
  method
• public static void main (String args)
• The parameter is an array of Strings
• We probably wont use the parameter much if at
  all in this module
• However, it is there, and could be used for
  providing runtime data

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main with parameter

• public class Greetings
• public static void main (String arg)
• System.out.println(Hello
• 
  args0 args1)
• gtjavac Greetings.java
• gtjava Greetings Sarah Jane
• Hello SarahJane

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Multiple arrays

• You can have arrays of many dimensions. The next
  example shows 2D arrays
• Consider the game of noughts and crosses

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2 D arrays notation

• Declare a 2D array with the rows first, then the
  columns
• int grid new char 33

0 1 2
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2D arrays contd

• Each element needs to have the row and the column
  specified
• grid20 X
• Use nested loops to traverse complete array
• for(int row0 rowlt3 row)
• for (int col 0 collt3 col)
• myGridrowcol ' '
• Use meaningful indexes e.g. row, col rather than
  i and j.

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Arrays (summary)

• An array in java is a special kind of object,
  with a unique syntax
• An array is a fixed-length sequence of values of
  the same type
• Arrays are good for
• Fixed length sequences
• Finding or altering elements according to their
  index position
• An array is a kind of list. Lists are good for
• Holding elements in order (ideally order of
  adding)
• Processing each element in turn, by traversing
  the list

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Problems with arrays

• An array is created with a fixed size.
• If our list is variable in size,
• We need to make sure that there is room to add
  items
• We need to keep a note of how many items are in
  the array, and alter it if the number of items
  changes
• Adding and deleting items from the middle of the
  array is time-consuming
• You shouldnt usually leave gaps in the array, so
  all the elements which come after the deleted
  item have to be shuffled down, one by one
• Likewise to add in the middle, all the elements
  which will come after the inserted element have
  to shuffled up, one by one (starting at the end)

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Arrays contd

• Using arrays involves a block of code for other
  manipulations such as
• Deleting an item
• Sorting the array
• Searching the array for a value
• Deleting an item

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ArrayList

• The ArrayList class in the Java Collections
  Framework stores items in an array and provides
  methods for adding, removing and getting
• You can use reliable methods for these operations
• The size of the array is automatically enlarged
  if necessary
• There is a size() method so you dont have to
  keep track of how many items you have.
• It is in the java.util package
• Use an import statement.

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Generic types

• lt..gt encloses the class of objects to be stored
  in the collection
• private ArrayList ltCargt carList
• Checks at compile time that classes match
• Whatever goes into this ArrayList must be a Car.
• Whatever comes out will be a Car.
• New in java 5. Earlier versions of java did not
  allow the programmer to specify the content type
  of an ArrayList . Generic types are an
  improvement.

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Creating an ArrayList of Cars

• We have added an ArrayList of cars to the
  CarComparison class used in L6
• In the constructor, an empty list is created

import java.util.ArrayList public class
CarComparison private ArrayList ltCargt
carList public CarComparison() carList
new ArrayListltCargt ()
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Populating the arraylist

• We use the ArrayList add method to provide Cars
  to be added to the list

public void createCarList() Car myCar new
Car("Ford Ka", 40, 33.6) carList.add(myCar)
for (int i 1 i lt3 i) Car anotherCar
this.getCarFromUser() carList.add(anotherCa
r)
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Traversing the array

• Note the use of the ArrayList get method, using
  the required index.
• There is a special for-each loop for
  collections
• for (Object-type object-name collection-name)
• For each item in the collection, perform the loop
  body, referring to the object as object-name

//initially assume first can go furthest Car
furthestCar carList.get(0) //for each car,
compare with the 'furthest' one for (Car
currentCar carList) if (furthestCar.estimat
eDistance() lt currentCar.estimateDista
nce()) furthestCar currentCar
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Why Arrays

• Arrays
• Some programming languages only provide arrays as
  a form of list
• Fast for accessing elements directly
• Slow for adding and deleting elements if we want
  to keep the list in a sorted order
• There are lots of alternative ways of storing
  lists
• More about all this later in the module

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String Implementation

• String is a class
• Strings are objects not variable
• Once created the characters of a String object
  cannot be changed
• String wStr"The weather"
• wStr wStr" is cold"
• wStr is actually detached from its first
  instance. A new instance is created leaving the
  old instance 'dereferenced
• This is not very efficient.

wStr
wStr
The weather
The weather is cold
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The StringBuffer class - optional

• The StringBuffer class can be used in preference
  to the String class in applications requiring
  frequent changes to the sequence of characters
• To create a StringBuffer object containing some
  text
• StringBuffer textBuf new StringBuffer(mytext
  )
• Or with no text
• StringBuffer textBuf new StringBuffer()

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The StringBuffer class - optional

• To add some text
• textBuf.append(more text)
• To return the String that is the same as the
  StringBuffer
• String text textBuf.toString()

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Familiar methods in the StringBuffer class

• StringBuffer has most of the methods familiar
  from the String class
• subString, charAt, length
• and they have similar behaviour
• indexOf only has version for String parameter
• And some StringBuffer methods
• append, replace, insert, delete