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Collection, Iterable, and Iterator Interfaces

Collection, Iterable, and Iterator Interfaces. The Collection Interface and its Hierarchy The Iterable and Iterator Interfaces For-each Loops with Iterable Collections Reading: L&C 3 rd : 15.1-15.2 2 nd :3.2 . 1. Collections. A collection is a typical example of Abstract Data Type.

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Collection, Iterable, and Iterator Interfaces

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  1. Collection, Iterable, and Iterator Interfaces • The Collection Interface and its Hierarchy • The Iterable and Iterator Interfaces • For-each Loops with Iterable Collections • Reading: L&C 3rd: 15.1-15.2 2nd:3.2 1

  2. Collections • A collection is a typical example of Abstract Data Type. • A collection is a data type that contains and allows access to a group of objects. • The Collection ADT is the most general form of ADTs designed for containing/accessing a group of objects. • We have more specific forms of Collection ADTs which describe the access “strategy” that models that collection: • A Set is a group of things without any duplicates • A Stack is the abstract idea of a pile of things, LIFO • A Queue is the abstract idea of a waiting line, FIFO • A List is an indexed group of things

  3. The Java Collections API • The classes and interfaces in the Java Collections Library are named to indicate the underlying data structure and the abstract Data type. • For example, the ArrayList we studied in CS110 uses an underlying array as the data structure for storing its objects and implements its access model as a list • However, from the user’s code point of view, the data structure is hidden by the API.

  4. The Collection Interface • Any class that implements the Collection interface can contain/access a group of objects • The type of objects that a Collection class contains can be specified using generics <T> • ArrayList class implementsCollection • Hence, polymorphism allows us to do these: ArrayList<String> a = new ArrayList<String>(); Collection<String> c = a; // widening ArrayList<String> d = (ArrayList<String>) c; 4

  5. 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() 5

  6. Iterator iterator() boolean remove(T o) boolean removeAll(Collection c) boolean retainAll(Collection c) int size() Object [] toArray() <T> T[] toArray(T[] a) Collection Interface Methods 6

  7. 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

  8. Java Collection Hierarchy Collection AbstractCollection Set List AbstractSet AbstractList TreeSet HashSet ArrayList Abstract Class Interface Class

  9. List Methods • boolean add(T elem) void add(int index, T elem) boolean addAll(int index, Collection c) T get(int index) T remove(int index) T set(int index, T element) int indexOf(Object o) int lastIndexOf(Object o) • Indexing is NOT a feature of all collections • It is a feature of the List ADT 9

  10. ArrayList methods • ArrayList is a class that implements the methods of the List interface. Therefore, all the these methods can be called on an ArrayList object. Ex: ArrayList<String> list = new ArrayList(); list.add(“ABC”); list.add(“EFG”); System.out.println(list.get(1)); System.out.println(list.indexOf(“ABC”)); System.out.println(list.indexOf(“HJK”));

  11. ArrayList methods • ArrayList is an actual class that implements the methods of the List interface. Therefore, all the these methods can be called on an ArrayList object. Ex: ArrayList<String> list = new ArrayList(); list.add(“ABC”); list.add(“EFG”); System.out.println(list.get(1)); System.out.println(list.indexOf(“ABC”)); System.out.println(list.indexOf(“HJK”));

  12. The Collection Interface Hierarchy <<interface>> Iterable <<interface>> Collection <<interface>> List <<interface>> Set <<interface>> SortedSet <<interface>> Queue 12

  13. Iterating over a Collection • Many times, we need to write code that retrieves all the elements of a collection one at a time in order to process them. • We call this iterating over the collection. • Java library has a way to conveniently accomplish the process of accessing (visiting or retrieving) each element of a collection once. • The core of this approach is based on two interfaces: • Iterable<T> • Iterator<T> 13

  14. Iterator<T> • An iterator over a collection allows programs to conveniently access each element of a collection once and only once without having to keep track of anything related to the process of iteration (visiting of each element) [an example of use of encapsulation]. • Methods of Iterator<T> interface: • boolean hasNext() • T next() • void remove()

  15. Iterator Interface • We don't create an object of type iterator by itself, but we ask the collection object to return us one: ArrayList<Book> shelf = new ArrayList<Book>(); … Iterator<Book> iter = shelf.iterator(); while(iter.hasNext()){ Book book = iter.next(); System.out.println(book.toString()); } (use simple for loop)

  16. Iterable<T> Interface • An object of type Iterable allows you obtain an Iterator object to retrieve its elements. It has only one method: Iterator<T> iterator() returns an Iterator object to access to the elements of this Iterable group of objects. • Collection interface extends the Iterable interface: Collection<T>extendsIterable<T> (which is another interface) 16

  17. 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. • 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: 17

  18. 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); 18

  19. 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() method but does not implement Iterable. while (bookList.hasNext()) System.out.println (bookList.next()); 19

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