Coming%20up

1
Coming up

• Implementation vs. Interface
• The Truth about variables
• Comparing strings
• HashMaps

2
Lecture 9

• Shhh, no talking in the library

OO
3
Coming up

• Implementation vs. Interface
• The Truth about variables
• Comparing strings
• HashMaps

4
Implementation and interface

• The API describes
• what a class does (interface of the class)
• not how a class does it (implementation of a
  class)
• When you write classes, you write the
  implementation of those classes

5
Library

• The java library is full of helpful classes
• Like ArrayList
• What does the inside of an ArrayList look like?
• How does it handle the resizing?
• How does it know when the throw an error?
• How does it handle renumbering when removing
  elements?

Implementation
6
Interface
OO

• All we need to know to use the ArrayList class,
  and the other library classes is what their
  interface is
• Interface is how we interact with a class
• what methods to call
• what they will return
• etc

7
Importing

• Library classes must be imported using an import
  statement
• except classes from java.lang.
• i.e. String look it up on API
• import java.util.ArrayList
• public class myClass
• ArrayListltStringgt arrl
• arrl new ArrayListltStringgt
• public static void main(String args)

8
Importing packages

• Classes are organised in packages.
• Single classes may be importedimport
  java.util.ArrayList
• Whole packages can be importedimport
  java.util.

9
Strings

• Strings are actually objects
• Did you notice we always use a capital S like
  other classes?
• You dont need to import them they are from the
  automatically imported from java.lang.
• As is System as in System.out.println()

10
Coming up

• Implementation vs. Interface
• The Truth about variables
• Well, an abstract version of the truth anyway
• Comparing strings
• HashMaps

11
Have you ever..
OO

• Accidently tried to output (print) an object?
• int array new int10
• System.out.println(array)
• Youll get something like
• I_at_955cd5

12
Whats that?
OO

• This is the address in memory that the computer
  uses to store objects
• in some other langauges (i.e. C) you can
  manipulate those addresses and their contents
  directly
• Its time to look into the variables as cups
  anaology in a bit more detail

13
So far...

• Weve said
• A variable is a cup
• A primitive fits into a cup
• A remote control to an object fits into a cup

14
The Truth

• The cup is a bit of storage on a silicone chip
  in the computer somewhere
• By giving the storage cups names, the computer
  associates the name of the cup with the address
  in memory where the storage is.
• int age 7 really means
• when the user types age they really mean the
  contents of memory area 01011101101000101011100111
  0101010

15
The Truth

• A bit pattern goes into the storage
• 0100100111110101110101000011011000010011
• This bit pattern can either represent
• A number, character or other primitive value
• The address in memory of something else, an
  object

16
So what?
OO

• Why is this important?
• Its important to know for comparing Strings.

17
Coming up

• Implementation vs. Interface
• The Truth about variables
• Comparing strings
• HashMaps

18
A word about comparing Strings
Tests Identity

• if(input hello")
• //code here
• if(input.equals(hello"))
• //code here
• Strings should always be compared with .equals

Tests Equality
OO
19
identity vs equality
OO

• name1name2
• False, different addresses
• name1.equals(name2)
• True, same value

• name1name2
• True, same address
• name1.equals(name2)
• True, same characters

Memory

• name1 name2
• False, different addresses
• name1.equals(name2)
• False, different value

Harry
Ron
Harry
20
Coming up

• Implementation vs. Interface
• The Truth about variables
• Comparing strings
• HashMaps

21
HashMap

• One of the most useful classes in the library is
  the HashMap
• It
• takes an object (The key)
• looks at the internal properties within it
• uses algorithms to compute a number based on
  those properties. This is the objects hash code
• stores the object in something like an array in
  the index given by the hash code

22
Then...

• Then the HashMap associates the second object
  (the value) with the key object
• This is useful

23
Maps
407351
Alfie

• Maps are a collection type that map a key to a
  value
• add(Alfie,407351)
• and so on

Memory
Map Name -gt Phone number
763412
Jenny
24
Lookup

• Lookup is the act of supplying the key and having
  the value returned
• String num myHashMap.get(Alfie)

Memory
myHashMapltString, Integergt
407351
Alfie
25

• HashMaps are used in upcoming labs...
• you will come across Sets. They are a collection
  like any other, you can use the for-each loop
  with them etc
• What happens if you add a key and a value, and
  then add the same key again, but with a different
  value?
• Check out HashMaps in the API

26
Covered today

• Implementation vs. Interface
• The Truth about variables
• Comparing strings
• HashMaps