Collections

1. Collections A First Glimpse

2. Collections • A collection is a structured group of objects • An array is a kind of collection • A Vector is a kind of collection • A linked list is a kind of collection • Java 1.2 introduced the Collections Framework and provided many great implementations • Vectors have been redefined to implement Collection • Trees, linked lists, stacks, hash tables, and other classes are implementations of Collection • Arrays do not implement the Collection interfaces

3. Types of Collection • Java supplies several types of Collection: • Set: cannot contain duplicate elements, order is not important • SortedSet: like a Set, but order is important • List: may contain duplicate elements, order is important • Java also supplies some “collection-like” things: • Map: a “dictionary” that associates keys with values, order is not important • SortedMap: like a Map, but order is important

4. Collections are ADTs • I’m not going to cover Collections just yet, but I want to use them as an example • Collections are one of the best-designed parts of Java, because • They are elegant: they combine maximum power with maximum simplicity • They are uniform: when you know how to use one, you almost know how to use them all • You can easily convert from one to another

5. Uniformity through interfaces • Much of the elegance of the Collections Framework arises from the intelligent use of interfaces • For example, the Collection interface specifies (among many other operations): • boolean add(Object o) • boolean isEmpty() • boolean remove() • int size() • Object[] toArray() • Iterator iterator()

6. Vectors • The class Vector has been retrofitted to implement the Collection interface • Vector supplies add(Object) and iterator() methods (among others) • Let’s look at creating a Vector and iterating through the elements

7. The Iterator interface • interface iterator { // java.lang.util • boolean hasNext(); • // Returns true if the iteration has more // elements. • Object next(); • // Returns the next element in the // interation. • void remove(); • // Removes from the underlying collection // the last element returned by the // iterator (optional operation).

8. Using a Vector • Collection numerals = new Vector(); numerals .add("one"); numerals .add("two"); numerals .add("three"); • Iterator iter = numerals.iterator(); while (iter.hasNext()) { System.out.println(iter.next());} • Results: one two three

9. Using a TreeSet • Collection numerals = new TreeSet(); numerals .add("one"); numerals .add("two"); numerals .add("three"); • Iterator iter = numerals.iterator(); while (iter.hasNext()) { System.out.println(iter.next());} • Results: onethree two

10. Conversions • Vector v = new Vector(numerals); • TreeSet ts = new TreeSet(v);

11. Back to arrays • Arrays are not part of the new Collections Framework, but they haven’t been ignored • Java 1.2 introduced the new Arrays class, with some useful operations, for example: • static void sort(Object[] a) • static int binarySearch(Object[] a, Object key) • static boolean equals(Object[] a, Object[] a2) • static void fill(Object[] a, Object val) • static void fill(Object[] a, Object val, int from, int to) • static List asList(Object[] a)

12. What to remember • We haven’t really begun to study the Collections framework yet, but— • You should learn to use the Iterator interface • You should examine and learn more about the Arrays package

13. The End