Chapter 4

Question 1

Class Libraries

Answer 1

Java calls it “libraries”, packages.

Grouping objects is a recurring requirement

The java utility package contains classes for doing this

Question 2

Requirement Group Objects

Answer 2

-The collection abstraction becomes a class

-The operations are methods of that class

-A particular collection like music collection would be an instance of that class

-Items stored in the instance are object

Question 3

Collections

Answer 3

We specify:
-The type of collection: ArrayList
-The type of object it will contain:<String>
-Private
ArrayList<String>files;
we say "ArrayList of String"</String></String>

Question 4

Importing

Answer 4

-How we gain access to lib classes
-Import statements must be placed before class distinctions

Question 5

Creating an ArrayList Object

Answer 5

-In versions of Java prior to version 7: -files=new ArrayList<String>();</String>

-Java 7 introduced diamond notation:
-Files=new ArrayList<>();

The type of parameter can be inferred from the variable being assigned to a convenience

Question 6

Features of the Collection

Answer 6

-Increases it capacity as necessary.
-Keeps a private count:
*size() accessor
-Keeps objects in order
-Details of how this is done is hidden

Question 7

Using the Collection

Answer 7

public class MusicOrganizer & private ArrayList<String>files;
...</String>

public void add file (string filename) & files.add(filename):
*For adding a new file

…

public int setnumberofFiles() &
return files.sizes():
*For returning of files (Delegation)

Question 8

Generic Classes

Answer 8

-Collections are known as parameterized or generic types

-ArrayLIst implements list functionality:
*add, set, size, etc

Question 9

One type parameter says what we want a list of:

Answer 9

-ArrayList<Person>
-ArrayList<TicketMachine>
-Etc</TicketMachine></Person>

Question 10

Index Number

Answer 10

-Position of an object in a collection = index

Starts at 0 & last size is -1

-Remove method can change the index value of remaining objects to fill in the gap

Question 11

The General Utility of Indices

Answer 11

Using integers to index collections has a general utility
-‘next’ is: index +1
-‘previous’ is: index -1
-last’ is: list.size() -1

‘The first three’ are the items @ indices 0,1,2…
We could also think about accessing items in sequence:0,1,2….

Question 12

Iteration Fundamentals

Answer 12

-We often want to perform some actions an arbitrary number of times
-Most programming languages include loop statement to make this possible
-Java has several sorts of loop statements

Question 13

For- each loop pseudo code

Answer 13

I.e
for(ElementType element: collection) (loop body)

• for each element in collection, do the things in the loop body
• Type of loop variable must be the same as the declared element type of the collection we’re going to use

Question 14

Critique of for-each loop

Answer 14

• only use for processing whole collection - definite iteration
• can’t stop part way

Question 15

For- Each loop- search tasks are indefinite

Answer 15

• can’t predict, how many places we will have to look
• although there may be an absolute limit (like checking every possible location)
• infinite loops are also possible- through error or the nature of the task

Question 16

While loop syntax

Answer 16

while(the keys are missing){
look in the next place;
}
or
while(not the keys have been found){
look in the next place;
}

Question 17

While loop

Answer 17

• uses a boolean condition to decide rather or not to continue

Question 18

For-each vs while loop

Answer 18

For-each:
- easier to write
- safer, guaranteed to stop

while:
- Don’t have to process whole collection
- doesn’t have to be used with a condition
- could be infinite

Question 19

Searching a collection

Answer 19

• fundamental activity
• applicable beyond collections
• necessarily indefinite iteration
• must code for both success & failure- exhausted search
• each of the finishing criteria must make the loop’s condition false to stop loop
• collection might be empty
• for each loop inappropriate for searching

Question 20

Phishing a search

Answer 20

• either there are no more files to check:
  index >= files.size()
• or the item has been found
  found == true
  found
  ! searching

Question 21

Continuing a search

Answer 21

• with a while loop we need to state the condition for continuing
• so the loop’s condition will be opposite of that for finishing:
  index < files.size() && : found
  index < files.size() && searching
• NB: ‘or’ becomes ‘and’ when inverting everything

Question 22

Moving away from string

Answer 22

• our collection of string objects for music tracks is limited
• no separate ID of artist, title, etc

Question 23

Index vs iteration

Answer 23

Ways to iterate over a collection
- for- each loop - use if we want to process every element
- while loop - if we want to stop partway through - and for repetition that doesn’t involve a collection
- iterator object - to stop partway through - often used with collections where indexed access is not very efficient or possible - use to remove from a collection

Question 24

Iterator & iterator()

Answer 24

• collections have an iterator() method
• this returns an Iterator object
• Iterator <E> has 3 methods:
  1. boolean hasNext()
  2. E next()
  3. void remove()</E>
• remove method for Iterator :
  it.remove();

Question 25

null

Answer 25

• used with object types
• used to indicate, ‘no object’
• test to see if an object holds the null value:
  if(highestBid == null)…
• used to indicate ‘no bid yet’

Question 26

anonymous objects

Answer 26

• objects are often created and handled on elsewhere immediately:
  Lot furtherLot = new Lot(nextLotNumber, description);
  lots.add(furtherLot);
  we don’t really need furtherLot:
  lots.add(new Lot(…));

Question 27

Chaining method calls

Answer 27

• each object in the chain is called on the object returned from the previous method call in the chain
• methods often return objects
• we often immediately call a method on the returned object
  Bid bid = lot.getHighestBid();
  Person bidder = bid.getBidder();
• we can use the anonymous object concept and chain method calls
  lot.getHighestBid().getBidder()