1

Collection, Iterable, and Iterator Interfaces

• The Collection Interface and its Hierarchy

• The Iterable and Iterator Interfaces

• For-each Loops with Iterable Collections

• Introduction to Exercise 1

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The Collection Interface

• Any class that implements the Collection interface can contain a group of objects

• The type of objects that a Collection class contains can be specified using generics <T>

• ArrayList class implements Collection• Hence, polymorphism allows us to do these:

ArrayList<String> a = new ArrayList<String>();Collection<String> c = a; // wideningArrayList<String> d = (ArrayList<String>) c;

3

Collection Interface Methods

boolean add(T o)

addAll(Collection c)

void clear()

boolean contains(T o)

boolean containsAll(Collection c)

boolean equals(Object o)

int hashCode()

boolean isEmpty()

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Collection Interface Methods

Iterator iterator()

boolean remove(T o)

boolean removeAll(Collection c)

boolean retainAll(Collection c)

int size()

Object [] toArray()

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ArrayList Unique Methods• Any methods involving indexing:

void add(int index, T o)

boolean addAll(int index, Collection c)

T get(int index)

int indexOf(T element)

int lastIndexOf(T element)

T remove(int index)

T set(int index, T element)

• Indexing is NOT a feature of all collections

• It is a unique feature of the ArrayList class

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The Collection Interface Hierarchy

• Typically, the Collection interface is not implemented directly by a class

• There is a hierarchy of interfaces that extend the Collection interface

• Each subclass of the Collection interface is designed to support a specific model for access to the contents of the collection– List, Set, Stack, Queue, etc.

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The Collection Interface Hierarchy

<<interface>>Collection

<<interface>>List

<<interface>>SortedSet

<<interface>>Iterable

<<interface>>Set

<<interface>>Queue

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The Collection Class Hierarchy

<<abstract>>AbstractList

<<abstract>>AbstractCollection

<<interface>>Collection

<<abstract>>AbstractSet

<<abstract>>AbstractQueue

ArrayList

<<interface>>List

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Iterating over a Collection

• Many times, we need to write code that retrieves the elements of a collection in a fashion according to its access model

• In Java, we refer to this as iterating over the collection

• Collection extends Iterable (which is another interface) so let’s look at what that means

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Iterable Objects and Iterators

• An Iterable object allows you obtain an Iterator object to retrieve objects from itIterator<T> iterator() returns an Iterator

object to access this Iterable group of objects

• An Iterator object allows you to retrieve a sequence of T objects using two methods:boolean hasNext() returns true if there are more

objects of type T available in the group

T next() returns the next T object from the group

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Iterable Objects and Iterators

• Classes in the Java standard class library that implement the Collection interface are Iterable OR you can implement Iterable in a class that you define (Project 1)

• If bookList is an object of an Iterable class that contains Book objects, we can retrieve all the available Book objects in either of two ways:

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Iterable Objects and Loops• We can obtain an Iterator object from an

Iterable object and use it to retrieve all the items from the Iterable object indirectly:

• The Java 5.0 for-each loop simplifies the repetitive processing of the items available from an Iterable object

Iterator<Book> itr = bookList.iterator();while (itr.hasNext()) System.out.println (itr.next());

for (Book myBook : bookList) System.out.println (myBook);

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Iterators and Loops• If bookList is an object of an Iterator

class that contains Book objects, you can access all the available objects directly:

• You can not use a “for-each” loop on an object of a class that only implements Iterator but does not implement Iterable

while (bookList.hasNext()) System.out.println (bookList.next());

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Use of Exceptions

• In some cases, there may be constraints that prevent the execution of a method for a specific instance of a collection object

• For example, a remove operation can not be performed on a collection that is empty

• A remove method for a set collection may be coded to check for an empty set

• If an empty set is found, it may throw an EmptySetException

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Use of Exceptions

• Hence, a method of a collection object may have a throws clause in its headerT remove() throws EmptySetException

• To throw an exception, the method uses:throw new EmptySetException(“string”);

• The exception itself is defined as a class:public class EmptySetException

extends RuntimeException{public EmptySetException (String set) { super ("The " + set + " is empty."); }}

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Introduction to Exercise 1

• Sudoku puzzles are quite the rage today

• Solving a Sudoku puzzle involves filling a set of numbers into an NxN array so there is exactly one number of each value 1-N in each row, column, and N1/2xN1/2 box

• You won’t need to write a program that solves Sudoku puzzles!

• In Exercise 1, you will write a program that validates the solution of a Sudoku puzzle.

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Introduction to Exercise 1

• In Exercise 1, you need to implement:– An Iterable class to contain the N2 cells of a Sudoku

puzzle and a Cell class to represent each cell– An Iterator class that returns arrays of cells in the order

of their rows, columns, and N1/2xN1/2 boxes– An isSolution method that iterates over the puzzle and

determines if each row, column, and N1/2xN1/2 box is valid

• The Iterator will return each cell three times:– In its row– In its column– In its N1/2xN1/2 box

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UML Class Diagram for Project 1

SudokuValidator

+ main (args: String [ ] ) : void - isSolution(puzzle : Sudoku) : bool

<<interface>>Iterable

+ iterator() : Iterator

<<interface>>Iterator

+ hasNext() : bool+ next() : Object+ remove() : void

Sudoku

- puzzle : Cell [ ] [ ]

- Sudoku()+ Sudoku(file : Scanner)+ toString () : String

SudokuIterator

Cell

- value : int

- Cell()+Cell(value : int)+setValue(value : int)+getValue() : int

<<instantiates>>

<<instantiates>><<uses>><<uses>>

<<uses>> - puzzle : Cell [ ][ ] - cursor : int{See assignment text} - SudokuIterator ()+ SudokuIterator (puzzle : Cell [ ][ ])

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Introduction to Project 1

• To loop through a two dimensional array using for statements is relatively intuitive:Cell [][] puzzle = new Cell[size][size];

...

for (int i = 0; i < size; i++)

for (int j = 0; j < size; j++)

//statements using puzzle[i][j]

• It is obvious that this algorithm is O(n2) for n = size

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Introduction to Project 1• If an Iterator on a collection with an internal

NxN array returns one element each time, the algorithm appears to have only one loop and be O(n), but it is not!

• The code using the Iterator object has:while (iterator.hasNext()) //statements using iterator.next()

• The hasNext() method returns true N2 times• The next() method returns N2 times - each

time with another Cell from the NxN array• Hence, it is O(N2) with respect to N

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Introduction to Project 1

• However, in Project 1 with an NxN array:

• hasNext( ) returns true 3xN times

• next( ) returns a one dimensional Cell [ ] of length N for N rows, N columns, and N boxes

• The iteration process g(N) is 3xN and is O(n)

• However, the code calling the hasNext( ) and next ( ) methods processes N elements each time for overall g(N) = 3NxN or O(N2) again

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Introduction to Project 1

• For a “rows only” SudokuIterator class:• Class Header could be:

public class SudokuIterator implements Iterator<Cell[]>{private Cell [][] puzzle;

private int SIZE;private int cursor;

• Constructor could be:public SudokuIterator(Cell [][] puzzle){this.puzzle = puzzle;

SIZE = puzzle.length;cursor = 0;

}

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Introduction to Project 1• For a “rows only” Interator class:• Code in Iterator hasNext method could be:

public boolean hasNext(){ return cursor < SIZE;}

• Code in Iterator next method could be:public Cell [] next(){ value = puzzle[cursor]; cursor++; return value; }