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

2 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 ArrayList class implements Collection Hence, polymorphism allows us to do these: ArrayList a = new ArrayList (); Collection c = a; // widening ArrayList d = (ArrayList ) 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()

4 Collection Interface Methods Iterator iterator() boolean remove(T o) boolean removeAll(Collection c) boolean retainAll(Collection c) int size() Object [] toArray()

5 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

6 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.

7 The Collection Interface Hierarchy > Collection > List > SortedSet > Iterable > Set > Queue

8 The Collection Class Hierarchy > AbstractList > AbstractCollection > Collection > AbstractSet > AbstractQueue ArrayList > List

9 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

10 Iterable Objects and Iterators An Iterable object allows you obtain an Iterator object to retrieve objects from it Iterator 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

11 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:

12 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 itr = bookList.iterator(); while (itr.hasNext()) System.out.println (itr.next()); for (Book myBook : bookList) System.out.println (myBook);

13 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());

14 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

15 Use of Exceptions Hence, a method of a collection object may have a throws clause in its header T 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."); }

16 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 N 1/2 xN 1/2 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.

17 Introduction to Exercise 1 In Exercise 1, you need to implement: –An Iterable class to contain the N 2 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 N 1/2 xN 1/2 boxes –An isSolution method that iterates over the puzzle and determines if each row, column, and N 1/2 xN 1/2 box is valid The Iterator will return each cell three times: –In its row –In its column –In its N 1/2 xN 1/2 box

18 UML Class Diagram for Project 1 SudokuValidator + main (args: String [ ] ) : void - isSolution(puzzle : Sudoku) : bool > Iterable + iterator() : Iterator > 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 > - puzzle : Cell [ ][ ] - cursor : int {See assignment text} - SudokuIterator () + SudokuIterator (puzzle : Cell [ ][ ])

19 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(n 2 ) for n = size

20 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 N 2 times The next() method returns N 2 times - each time with another Cell from the NxN array Hence, it is O(N 2 ) with respect to N

21 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(N 2 ) again

22 Introduction to Project 1 For a “rows only” SudokuIterator class: Class Header could be: public class SudokuIterator implements Iterator { private Cell [][] puzzle; private int SIZE; private int cursor; Constructor could be: public SudokuIterator(Cell [][] puzzle) { this.puzzle = puzzle; SIZE = puzzle.length; cursor = 0; }

23 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; }