Iteration Abstraction

SWE 619 - Software ConstructionFall 2009

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Common Collections List (Interface)

AbstractList ArrayList, Linked List, Vector

Set (Interface) AbstractSet

HashSet, TreeSet Map (Interface)

AbstractMap HashMap, TreeMap

Arrays Stack -subtype of Vector (Anomaly?)

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Visiting each element of a collection Scenario: A common container has

several objects stored Task: To visit each element in the

container HOW?

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Approaches IntSet choose() method?

Remove – choose combination What is the problem here?

Return the collection? Why is this bad?

Return a clone? Is this better?

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Approaches Custom built method?

Close, but not it Problem? Not general We want uniformity

Iteration Abstraction Client code is simplified, using is easy Implementation may be complex, but

standard

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Iterator Interfacepublic Interface Iterator {

public boolean hasNext();public Object next() throws NoSuchElementException;public void remove() throws IllegalStateException;

} Liskov has only first hasNext() and next() To satisfy the compiler, you need to

override/implement remove() too

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Specificationpublic boolean hasNext() ;// Effects: Returns true if there are more elements

to yield else returns false

public Object next();// Modifies: this// Effects: If there are more results to yield, returns

the next result and modifies the state of this to record the yield. Otherwise, throws NoSuchElementEx.

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DefinitionsAn iterator is a procedure that returns a generator. A

data abstraction can have one or more iterator methods, and there can also be standalone iterators.

A generator is an object that produces the elements used in the iteration. The generator’s type is a subtype of Iterator.

The specification of an iterator defines the behavior of the generator; a generator has no specification of its own. The iterator specification often includes a requires clause at the end constraining the code that uses the generator.

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Examples: Poly and IntSetpublic Iterator terms() // Effects: Returns a generator that will produce exponents // of nonzero terms of this (as Integers) up to the degree, // in order of increasing exponent

public Iterator elements() // Effects: Returns a generator that will produce all the

elements // of this (as Integers) each exactly once, in arbitrary order // Requires: this must not be modified while the // generator is in use

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Example: PolyPoly p … // p = 2 + 3 x2+ 4 x5

Iterator itr = p.iterator(); // UsualIterator itr = p.terms(); // Liskov’s itr = [0,2,5]itr.hasNext() // return true, itr = [0,2,5]itr.next() // return 0, itr = [2,5]itr.next() // return 2, itr = [5]itr.hasNext() // return true, itr = [5]itr.next() // return 5, itr = []itr.hasNext() // return false, itr = []itr.next() // return NSEE, itr = []

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Abstraction Function It’s the current list of things that you

are going to send back Very close to a Stack

top = next() element remove() adds complexity If multiple interfaces, iterator may

not know about removed items! Only mutable data types are affected

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Af(c) for Poly IteratorPoly p … // p = 2 + 3 x2+ 4 x5

AF(itr.hasNext()) = [0,2,5] //trueAF(itr.next()) = [2,5] //0AF(itr.next()) = [5] //2AF(itr.hasNext()) = [5] //trueAF(itr.next()) = [] //5AF(itr.hasNext()) = [] //falseAF(itr.next()) = [] //NSEE

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Implementation (Fig 6.8)public class Poly{

// Rep …public Iterator terms() {return new PolyGen(this);}// inner classprivate static class PolyGen implements Iterator { private Poly p; // the Poly being iterated

private int n; // the next term to considerPolyGen (Poly it){ //Requires: it !=null p = it; if(p.trms[0] == 0) n=1; else n= 0;}

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Implementation (contd.)public boolean hasNext() {return n<= p.deg;}

public Object next () throws NSEE{for(int e = n; e <= p.deg; e++) { if (p.trms[e] != 0) {

n= e+1;return new Integer(e);

}}throw new NSEE(“Poly.terms”);

} // end PolyGen}

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Inner Class private class visibility only inside the class where

defined no outside code can see/instantiate it if it has public methods && an

instance available, outside code can call it

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State for iterator How to figure out the state? Same way as AF(c) for Data

Abstraction Ask yourself: What do I need to send

back to the client? Example Rep state: [2,0,3,0,0,4]

What if PolyGen was immutable?

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Another example: PrimesGenprivate static class PrimesGen implements Iterator{

private Vector ps; // primes yieldedprivate int p; // next candidate to tryPrimesGen () { p =2 ; ps = new Vector();} //constructorpublic boolean hasNext() {return true;} // always truepublic Object next() throws NSEE {if (p==2) {p=3; return 2;}for (int n=p; true; n = n+2){… //Prime number generation}}

}// end of PrimesGen

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Abstract State for PrimesGen?Iterator itr = num.allPrimes();

AF(c) = [2,3,5,7,11,13,17,19, …]

No end? Can we figure out the length of the tail? What does hasNext() have to do in this case?

[2,3,5,7,9,…]Integer x = (Integer) itr.next(); [3,5,7,9,11,..]Integer y = (Integer) itr.next(); [5,7,9,11,13,17,…]..

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Exercises What if there is an upper bound on

the prime numbers? Suppose primes <100. What will be AF(c) be like? What will hasNext() implementation

do? How will implementation of

PrimesGen change?

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Another Exercisepublic Interface TwoWayIterator {

Object next ();Object previous ();boolean hasNext();boolean hasPrevious();

Suppose we want to go back AND forward What does the abstraction function look like?

Still a stack? What other state information is needed?

How to implement this for Poly?

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What about supporting remove()? The contract for remove():Removes from the underlying collection the last element

returned by the iterator (optional operation). This method can be called only once per call to next. The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method.

This is complex! What is the new abstract state?

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Iterable vs. Iterator

Only one method required: public Iterator<T> iterator();

Allows very nice code:// Note: that Collection implements Iterable// Side note: String does NOT implement IterableSet<String> mySet = new HashSet<String>();// populate mySet with various Stringsfor (String s : mySet) { // auto invocation of iterator(), next() // do something with s}