Interfaces, Polymorphism and Iteration. Lecture 7a: OOP, autumn 2003

Transcription

1 Interfaces, Polymorphism and Iteration Lecture 7a: OOP, autumn 2003

2 Interfaces Interfaces = abstract classes, where Only public abstract methods No attributes => no state, no instances May have constants ( public static final attributes in Java) Interfaces vs. abstract classes abstract classes may have implementations interfaces can t abstract classes can be implemented only by subclasses any class obeying the interface contract can implement an interface Hard to retrofit existing classes to extend a new class Very easy for any class to implement any interfaces Interfaces support multiple inheritance by extends

3 Interfaces UML example public interface Composer { public interface SingerComposer extends Composer, Singer { Public class Assistant extends Teacher, implements Singer { Person «interface» Composer «interface» Singer Teacher Student +WriteSong(in MusicStyle) «implements» +Sing(in Song) «implements» +report() +takeexam() Skeletal ComposerImpl «interface» SingerComposer Assistant Lecturer Singer implemetation

4 Iteration (1) How to iterate over arbitrary collections in a uniform way for all collections independent of the collection implementation Typical use: for all elements of a collection do action E.g.: sum all elements of an IntSet Such operations called aggregation Solutions: Materialize each collection as array, use for loops Return the internal representation, use for loops Add indexing to all collections (e.g. get(int idx) methods) Add aggregation methods directly to containers (e.g. sum, max) Needed some mechanism that abstracts iteration from collections aggregations

5 Iteration (2) General idea Produce all items in a collection sequentially Use a standard way to do this: for each element e in C perform some_action on e Suitable for both indexed and non-indexed collections Direct implementation of an iteration interface Limiting - allows only one loop at a time E.g.: two nested aggregations over same collection Need special objects that store current state - generators Need methods to produce generators - iterators Take care of simultaneous access to same collection

6 Iteration in Java public interface Iterator { public boolean hasnext(); public Object next() throws NoSuchElementException(); Iterator g = personset.elements(); while (g.hasnext()) { Teacher t = (Teacher) g.next(); t.fire();

7 Implementing iterators and generators public class IntSet { private int[] contents; public Iterator iterator() { return new IntGen(); Inner class private class IntGen implements Iterator { long currentindex = 0; public boolean hasnext() { public Object next() {

8 Enhanced for in JDK 1.5 void cancelall(collection c) { for (Iterator i = c.iterator(); i.hasnext(); ) { TimerTask tt = (TimerTask) i.next(); tt.cancel(); The same method with an "enhanced for" statement: void cancelall(collection c) { for (Object o : c) { ((TimerTask)o).cancel();

9 Polymorphic abstractions (1) poly = many, morphos = shape Abstractions need to be parameterized Collections: IntSet, PersonSet vs. Set Methods: studentcompare, teachercompare vs. compare => same abstraction redefined many times Generalize abstractions to work with many types procedures, data abstractions, iterators Use type hierarchy and the substitution principle to make abstractions polymorphic

10 Polymorphic abstractions (2) Polymorphic data abstractions Set s = new Set(); s.insert(new Teacher( teacher name )); s.insert(new Student( student name )); Iterator gen = s.iterator(); while(gen.hasnext()) { String name = ((Person) gen.next()).getname(); Problem: type safety, downcasts

11 Java generics in JDK 1.5 (templates) Source: Idea: use types as parameters to generate families of abstractions In C++ called templates Parameterised types: class Hashtable<Key, Value> {... class Hashtable<Key, Value> { Value get(key k) {... Hashtable<Integer, String> ht = new Hashtable<Integer, String>(); ht.put(new Integer(0), "value"); Parameterised methods: class Utilities { <T extends Object> public static List<T> make(t first) { return new List<T>(first); Utilities.make(Integer(0)) ;