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Iterators

Iterators. "First things first, but not necessarily in that order " -Dr. Who. Iterator Interface. An iterator object is a “one shot” object it is designed to go through all the elements of an ADT once

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Iterators

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  1. Iterators "First things first, but not necessarily in that order " -Dr. Who

  2. Iterator Interface • An iterator object is a “one shot” object • it is designed to go through all the elements of an ADT once • if you want to go through the elements of an ADT again, you have to get another iterator object • Iterators are obtained by calling the iterator method CS 221 - Computer Science II

  3. Iterator Interface Methods • The Iterator interface lists 3 methods: boolean hasNext() //returns true if this iteration has more elements T next() //returns the next element in this iteration //pre: hasNext returns true void remove() /* Optional method. Removes the last element returned by the Iterator. pre: Can only be called only once for each call to next. After calling, must call next again before calling remove again. */ CS 221 - Computer Science II

  4. Question 1 • Which of the following will not compile? ArrayList<String> list = new ArrayList<String>(); Iterator<String> it1 = new Iterator(); // I Iterator<String> it2 = new Iterator(list); // II Iterator<String> it3 = list.iterator(); // III A. I B. II C. III D. I and II E. II and III CS 221 - Computer Science II

  5. Question 1 • Which of the following will not compile? ArrayList<String> list = new ArrayList<String>(); Iterator<String> it1 = new Iterator(); // I Iterator<String> it2 = new Iterator(list); // II Iterator<String> it3 = list.iterator(); // III A. I B. II C. III D. I and II E. II and III CS 221 - Computer Science II

  6. Visualizing an Iterator • Imagine a fence made up of fence posts and rail sections rails fenceposts CS 221 - Computer Science II

  7. Fence Analogy • The iterator lives on the fence posts • The data in the collection are the rails • Iterator created at the far left post • As long as a rail exists to the right of the iterator, hasNext() is true iterator object CS 221 - Computer Science II

  8. Fence Analogy ArrayList<String> names = new ArrayList<String>(); names.add(“Jan”); names.add(“Levi”); names.add(“Tom”); names.add(“Jose”); Iterator<String> it = names.iterator(); int i = 0; “Jan” “Levi” “Tom” “Jose” CS 221 - Computer Science II

  9. Fence Analogy while( it.hasNext() ) { i++; System.out.println( it.next() ); } // when i == 1, prints out Jan first call to next() moves iterator tonext post and returns “Jan” “Jan” “Levi” “Tom” “Jose” CS 221 - Computer Science II

  10. Fence Analogy while( it.hasNext() ) { i++; System.out.println( it.next() ); } // when i == 2, prints out Levi “Jan” “Levi” “Tom” “Jose” CS 221 - Computer Science II

  11. Fence Analogy while( it.hasNext() ) { i++; System.out.println( it.next() ); } // when i == 3, prints out Tom “Jan” “Levi” “Tom” “Jose” CS 221 - Computer Science II

  12. Fence Analogy while( it.hasNext() ) { i++; System.out.println( it.next() ); } // when i == 4, prints out Jose “Jan” “Levi” “Tom” “Jose” CS 221 - Computer Science II

  13. Fence Analogy while( it.hasNext() ) { i++; System.out.println( it.next() ); } // call to hasNext returns false // while loop stops “Jan” “Levi” “Tom” “Jose” CS 221 - Computer Science II

  14. Typical Iterator Pattern Iterator<T> it = list.iterator(); while( it.hasNext() ) { T temp = it.next(); // do something with temp } CS 221 - Computer Science II

  15. Question 2 • What is output of the following code? ArrayList<Integer> list; list = new ArrayList<Integer>(); list.add(3); list.add(3); list.add(5); Iterator<Integer> it = list.iterator(); System.out.println(it.next()); System.out.println(it.next()); A.3 B. 5 C. 3 3 5 D. 3 3 E. 3 5 CS 221 - Computer Science II

  16. Question 2 • What is output of the following code? ArrayList<Integer> list; list = new ArrayList<Integer>(); list.add(3); list.add(3); list.add(5); Iterator<Integer> it = list.iterator(); System.out.println(it.next()); System.out.println(it.next()); A. 3 B. 5 C. 3 3 5 D. 3 3OR E. 3 5OR F. 5 3 CS 221 - Computer Science II

  17. remove method • An Iterator an be used to remove things from an ADT • Can only be called once for each call to next public void removeWords(Iterable<String> myList, intlen) { Iterator<String> it = myList.iterator while( it.hasNext() ) { String temp = it.next(); if(temp.length() == len)it.remove(); } } Given list= [“dog”, “fish”, “cat”, “gerbil”], what is resulting list after call to removeWords(3)? CS 221 - Computer Science II

  18. Question 3 public void printTarget(ArrayList<String> names, intlen) { Iterator<String> it = names.iterator(); while( it.hasNext() ) if( it.next().length() == len ) System.out.println( it.next() ); } Given names = [“Jan”, “Ivan”, “Tom”, “George”] and len = 3, what is output by the printTarget method? • Jan Ivan Tom George • Jan Tom • Ivan George • No output due to syntax error • No output due to runtime error CS 221 - Computer Science II

  19. Question 3 public void printTarget(ArrayList<String> names, intlen) { Iterator<String> it = names.iterator(); while( it.hasNext() ) if( it.next().length() == len ) System.out.println( it.next() ); } Given names = [“Jan”, “Ivan”, “Tom”, “George”] and len = 3, what is output by the printTarget method? • Jan Ivan Tom George • Jan Tom • Ivan George • No output due to syntax error • No output due to runtime error CS 221 - Computer Science II

  20. The Iterable Interface • An interface implemented by an ADT • One method in the interface: public Iterator<T> iterator() • Can use typical iterator pattern to access all elements in ADT: public void doSomething(Iterable<T> list) Iterator<T> it = list.iterator(); while( it.hasNext() ) { T temp = it.next(); // do something with temp } } CS 221 - Computer Science II

  21. Using IterableADTs • But if you simply want to go through all the elements of an Iterable ADT, use for-each loop • hides creation of the Iterator public void doSomething(Iterable<T> list) { for(T temp : list) { // do something with temp } } CS 221 - Computer Science II

  22. Using IterableADTs • Can also use typical for loop public void doSomething(Iterable<T> list) { for (Iterator<T> itr = list.iterator(); itr.hasNext(); ) { T temp = iter.next(); // do something with temp } } CS 221 - Computer Science II

  23. Implementing an Iterator • Nested / Inner Class • Example of encapsulation • Key Implementation Details • does your ADT need an Iterator? • keeping track of “location” in ADT • checking whether ADT has been modified since Iterator has been instantiated • checking precondition on remove method CS 221 - Computer Science II

  24. Comodification • If a ADT with an Iterator is changed after it’s been instantiated, a ConcurrentModificationException will be thrown the next time call next or remove methods are called ArrayList<String> names = new ArrayList<String>(); names.add(“Jan”); Iterator<String> it = names.iterator(); names.add(“Andy”); it.next(); // exception will occur here CS 221 - Computer Science II

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