Java Class Libraries

• Java the platform contains around 4,000
• classes/interfaces
• Data Structures
• Networking, Files
• Graphical User Interfaces
• Security and Encryption
• Image Processing
• Multimedia authoring/playback And more...
•  All neatly(ish) arranged into packages (see API docs)
• Important chunk of the class library
• A collection is some sort of grouping of things (objects)
• Usually when we have some grouping we want to go through it (“iterate over it”)
• The Collections framework has two main interfaces: Iterable and Collections. 
• They define a set of operations that all classes in the Collections framework support
• add(Object o), clear(), isEmpty(), etc.
• <<interface>> Set
• Like a mathematical set in DM 1
• A collection of elements with no duplicates
• Various concrete classes like TreeSet (which keeps the set elements sorted)
•  <<interface>> List
• An ordered collection of elements that may contain duplicates
• ArrayList, Vector, LinkedList, etc.
• <<interface>> Queue
• An ordered collection of elements that may contain duplicates and supports
• removal of elements from the head of the queue
• PriorityQueue, LinkedLIst, etc.
• <<interface>> Map
• Like relations in DM 1, or dictionaries in ML
• Maps key objects to value objects
• Keys must be unique
• Values can be duplicated and (sometimes) null.
• for loop
• LinkedList list = new LinkedList();
• for (int i=0; i<list.size(); i++) {
• Object next = list.get(i);
• }
• foreach loop (Java 5.0+)
• LinkedList list = new LinkedList();
• for (Object o : list) {
• }
• What if our loop changes the structure?
• for (int i=0; i<list.size(); i++) {
• If (i==3) list.remove(i);
• }
• Java introduced the Iterator class
• Iterator it = list.iterator();
• while(it.hasNext()) {Object o = it.next();}
• for (; it.hasNext(); ) {Object o = it.next();}
• Safe to modify structure
• while(it.hasNext()) {
• it.remove();
• }