CS3006N Mass Storage Section 11

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CS3006N Mass Storage Section 11

• James King
• 12 August 2008

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Mass storage - Arrays, Strings and Vectors
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Arrays

• Mass Storage

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Arrays

• Arrays allow large numbers of simple data types
  such as int or instances of classes to be easily
  accessed.
• int arraynew int10
• array04
• array91234
• System.out.println(array0)

indicates that the array will hold 10 items
indicates variable is an array
accessing an element of an array also uses
arrays start at 0
this is the last element of the array
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Arrays holding Objects

• Arrays can also hold instances of a class or any
  of its subclass
• Monster arraynew Monster10
• array0new Monster()
• array9new Dragon()
• array0.take_damage()

indicates that the array will hold 10 instances
of the Monster class or subclass
Dragon is a subclass of Monster so this is fine
array0 is an instance of Monster so we can call
take_damage on it
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Dangers of Arrays of Objects

• with an array of simple types such as ints every
  element in the array exists even if we dont
  assign a value to them
• int arraynew int2
• System.out.println(array0)
• with an array of Objects elements are null until
  we assign an instance to them
• Monster arraynew Monster2
• array0.take_damage()

no problem!!
null pointer exception!
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Initialising Object Arrays

• unlike arrays of simple types arrays of any type
  of object require each element to be instantiated
  and inserted into the array...
• Monster arraynew Monster2
• array0new Monster()
• array1new Monster()
• array0.take_damage()

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Loops and Arrays

• loops are natural ways to perform the same
  operation on each element of an array (or a sub
  set). For example
• int arraynew int10
• int index
• for (index0indexlt10indexindex1)
• arrayindex0

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Loops and Dangers

• int arraynew int10
• int index
• for (index0indexlt10indexindex1)
• arrayindex0
• There is a problem with this code. Suppose I
  change the array to only have 5 elements... as
  soon as the index is 5 and tries to assign 0 to
  the element in position 5 Java will issue an
  array out of bounds exception

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Finding the number of elements in an array

• Fortunately we can ask the array how long it is
  using .length
• int arraynew int10
• int index
• for (index0indexltarray.lengthindexindex1)
• arrayindex0
• Now we can make the array bigger or smaller and
  the code will not crash and will initialise the
  entire array

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Problems with Arrays I

• The single biggest problem with an array is that
  it is not variable length. You have to declare
  the length of the array as a constant and you can
  not tag on extra elements
• int arraynew array20
• array301234

array out of bounds exception!
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Problems with arrays II

• You also can not remove elements in an array even
  if they are no longer needed. For instance
  suppose I have an array of 4 Monsters which the
  Hero is fighting. If the Hero kills 2 it would be
  nice to remove two from the array... the only
  thing you can do is to replace the dead instance
  with null.

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Strings

• Mass Storage

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Strings

• The String class allows you to store and
  manipulate sequences of characters
• Like chars, Strings are case sensitive so hello
  is not the same as Hello
• Strings are in fact Objects
• The length method tells you the length of a
  string in chars e.g. hello.length()5
• You can get a copy of the character at any
  position in a string using charAt()
• hello.charAt(0)h
• hello.charAt(4)o

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Relationship between String and Arrays

• It is possible to get a copy of the contents of a
  String as a char array
• String s"hello world"
• char connew chars.length()
• s.getChars(0,s.length(),con,0)

make sure the array is large enough to store the
entire String
Starting char in the String
ending char 1 in the String
starting position in the array
array to copy chars into
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Relationship between String and Arrays

• It is also possible to create a string from a
  char array
• String s"hello world"
• char connew chars.length()
• s.getChars(0,s.length(),con,0)
• con0'H'
• snew String(con)
• System.out.println(s)
• However, manipulating char arrays is not easy so
  in general it is better to manipulate the String
  by keeping it in its String format

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Manipulating Strings

• There are many manipulation functions defined for
  the string object one of the most useful is
  subString
• You can get a copy of part of a string using
  subString
• hello.subString(0,2)hel
• hello.substring(2,3)ll

ending index
starting index
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Adding to your String

• Unlike arrays, Strings can be appended and
  pre-pended to easily
• String sworld
• shello s
• ss!
• This gives the String hello world!

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Seeing if Two Strings are Equal using

• For simple types you can use and ! to see if
  something is equal or not equal.
• and ! do not work correctly for objects
• String shello there
• String s1hello there
• ss is true
• s1s1 is true
• ss1 is false!!!!!!!!!!!!!!
• This is because for objects and ! check where
  the instance is in memory and not the contents of
  the instances

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Seeing if Two Strings are Equal using equals

• To see if two Strings are identical you need to
  use equals() or equalsIgnoreCase()
• String shello there
• String s1hello there
• s.equals(s) is true
• s.equals(s1) is true
• s.equals(s1) is true
• This is because equals and equalsIgnoreCase check
  the contents of the instances and not their
  locations in memory

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Converting other types into Strings

• Java will convert almost any simple type into a
  String for you
• String s
• int i166
• si
• This does not work for Objects because Java can
  not work out how to convert an object into a
  String

is an empty String
converts a copy of i into a String
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Converting Strings into other types.

• Java provides static methods in classes called
  Integer, Long, Float and Double to help you
• int iInteger.parseInt("100")
• long lLong.parseLong("100")
• float fFloat.parseFloat("100")
• double dDouble.parseDouble("100")
• If and only if the entire String looks like
  another type can the conversion take place.
• int iInteger.parseInt(hello 100")

Exception
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More String Functions

• There are many more String manipulation functions
  than we have time for to look at here
• Take a look at the Java documentation for the
  String class and have some fun!

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Vectors

• Mass Storage

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Vectors

• Vectors offer an alternative from arrays that
  allow you to insert, delete and add extra
  elements.
• Unfortunately Vectors can only store objects so
  you can not create a Vector of simple data types
  such as int.... but there is nothing stopping you
  putting a single int into a class and putting
  that class into the Vector...
• Vectors are in the package java.util and so
  require importing
• import java.util.

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Vectors

• Unlike arrays that can only store a single class
  and its subclasses Vectors can store anything in
  any position... Its up to you to remember what is
  in each element...
• Vector vnew Vector()
• v.insertElementAt(new Monster(),0)
• v.insertElementAt(new Hero(),1)

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Vectors getting the value of an element

• Unlike arrays elementAt is used to access an
  element of the Vector
• However, element at returns a generic Object
  which must be type cast to be used
• Vector vnew Vector()
• v.insertElementAt(new Monster(),0)
• Monster m(Monster)v.elementAt(0)
• m.take_damage()

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Vectors and invalid type casts

• If you use elementAt and type cast into the wrong
  type of object you will get a invalid type cast
  exception
• Vector vnew Vector()
• v.insertElementAt(new Hero(),0)
• Monster m(Monster)v.elementAt(0)
• m.take_damage()

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Finding the number of elements in a Vector

• The vector is actually a class and we can ask it
  how many elements are in it using the size method
• Vector vnew Vector()
• v.insertElementAt(new Hero(),0)
• v.insertElementAt(new Dragon(),1)
• int iv.size()

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Removing items from Vectors

• You can remove a single element from the Vector
  (all the high numbered elements are moved down to
  fill the gap)
• Vector vnew Vector()
• v.insertElementAt(new Hero(),0)
• v.insertElementAt(new Dragon(),1)
• v.removeElementAt(0)
• The elementAt(0) is now an instance of Dragon

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Removing All the items from a Vector

• You can remove all the items from a vector
• Vector vnew Vector()
• v.insertElementAt(new Hero(),0)
• v.insertElementAt(new Dragon(),1)
• v.removeAllElements()