1. Lecture 15: More Iterators
CSE 116/504 – Intro. To Computer Science for Majors II
Lecture 15: More Iterators

2. Announcements Project phase #1 due TODAY at 11:59PM
Submit in Autolab; please list partners in submission(?)
Self & peer evaluations this week in your recitations
I can override individual scores, but will need evidence
Test #1 in 8 days (Tuesday, October 10th)
The first test will cover material through this week
Let me know of conflicts – make-up is following day

3. Implementing the Iterator
Iterator class needs access to ADT’s internal fields
Implementation-less ADT concept must be violated
Code specific to implementation and not reusable
All good OO design concepts get thrown out window

4. Implementing the Iterator
Iterator class needs access to ADT’s internal fields
Implementation-less ADT concept must be violated
Code specific to implementation and not reusable
All good OO design concepts get thrown out window

5. Which Is Iterator’s Purpose?
1-by-1 access to data independent of source
Simplify coding using for-each loops
Find all non-null elements in Collection
Access all elements in a Collection 1

6. Which Is Iterator’s Purpose?
1-by-1 access to data independent of source
Simplify coding using for-each loops
Find all non-null elements in Collection
Access all elements in a Collection
Convince your neighbor your answer is correct

7. Which Is Iterator’s Purpose?
1-by-1 access to data independent of source
Simplify coding using for-each loops
Find all non-null elements in Collection
Access all elements in a Collection 1

8. Which Is Iterator’s Purpose?
1-by-1 access to data independent of source
Simplify coding using for-each loops
Find all non-null elements in Collection
Access all elements in a Collection
Nope; for-each uses Iterable (and that is mostly side effect)

9. Which Is Iterator’s Purpose?
1-by-1 access to data independent of source
Simplify coding using for-each loops
Find all non-null elements in Collection
Access all elements in a Collection
null is valid element; Iterator just provides access

10. Which Is Iterator’s Purpose?
1-by-1 access to data independent of source
Simplify coding using for-each loops
Find all non-null elements in Collection
Access all elements in a Collection
Not just Collections; Iterator can be created for anything

11. Which Is Iterator’s Purpose?
1-by-1 access to data independent of source
Simplify coding using for-each loops
Find all non-null elements in Collection
Access all elements in a Collection
Iterator.next()
returns next item from source;
Iterator can be written to draw from any source

12. Iterable holds data processed with for-each loop
Iterable Key Concept #1
Iterable holds data processed with for-each loop

13. Iterator accesses data BUT separate class
Iterator Key Concept #1
Iterator accesses data BUT separate class

14. Algorithm For ALL Iterators
Algorithm hasNext() if cursor will access a valid location return true else return false endif end

15. Algorithm For ALL Iterators
Algorithm hasNext() if cursor will access a valid location return true else return false endif end

16. What is Role of Cursor? int equal to array index
Way to access data to be returned by next()
Data which was just returned by next()
Array which holds the data we are iterating over 1

17. Convince your neighbor your answer is correct
What is Role of Cursor?
int equal to array index
Way to access data to be returned by next()
Data which was just returned by next()
Array which holds the data we are iterating over
Convince your neighbor your answer is correct

18. What is Role of Cursor? int equal to array index
Way to access data to be returned by next()
Data which was just returned by next()
Array which holds the data we are iterating over 1

19. What if data is not in an array?
What is Role of Cursor?
int equal to array index
Way to access data to be returned by next()
Data which was just returned by next()
Array which holds the data we are iterating over
What if data is not in an array?

20. Value accesses data returned by NEXT call to next()
What is Role of Cursor?
int equal to array index
Way to access data to be returned by next()
Data which was just returned by next()
Array which holds the data we are iterating over
Value accesses data returned by NEXT call to next()

21. May not be an array; Cursor used to access data from source
What is Role of Cursor?
int equal to array index
Way to access data to be returned by next()
Data which was just returned by next()
Array which holds the data we are iterating over
May not be an array; Cursor used to access data from source

22. Cursor used to access data; Cursor details depend on data source
What is Role of Cursor?
int equal to array index
Way to access data to be returned by next()
Data which was just returned by next()
Array which holds the data we are iterating over
Cursor used to access data; Cursor details depend on data source

23. Iterator’s cursor used to access next value
Iterator Key Concept #2
Iterator’s cursor used to access next value

24. Iterator Principles Independent element-by-element data access
next() returns an element; each call provides new value
Using Iterator methods, data source unimportant
public interface Iterator<E> { boolean hasNext(); E next() throws NoSuchElementException; void remove() throws UnsupportedOperationException; }

25. Iterator Principles Independent element-by-element data access
next() returns an element; each call provides new value
Using Iterator methods, data source unimportant
public interface Iterator<E> { boolean hasNext(); E next() throws NoSuchElementException; void remove() throws UnsupportedOperationException; }

26. Algorithm For (almost) ALL Iterators
Algorithm next() if hasNext() then E retVal = value at cursor’s location Advance cursor to refer to next location return retVal else // Panic – we cannot do this! endif end

27. Algorithm For (almost) ALL Iterators
Algorithm next() if hasNext() then E retVal = value at cursor’s location Advance cursor to refer to next location return retVal else throw new NoSuchElementException(); endif end

28. 2 Varieties of Exceptions
Checked Exception
Unchecked Exception
Use for fixable errors
Java forces methods to consider them
Only useful if possibly fixable
Subclass of Exception
throws must list uncaught
Will crash program if uncaught
Use when royally screwed
Java lets methods ignore these if they want
Helps when error cannot be fixed
Subclass of RuntimeException
Can be listed in throws
Will crash program if uncaught

29. 2 Varieties of Exceptions
Checked Exception
Unchecked Exception
Use for fixable errors
Java forces methods to consider them
Only useful if possibly fixable
Subclass of Exception
throws must list uncaught
Will crash program if uncaught
Use when royally screwed
Java lets methods ignore these if they want
Helps when error cannot be fixed
Subclass of RuntimeException
Can be listed in throws
Will crash program if uncaught

30. If Exception Thrown, What Type?
double sumPos(Iterator<Double> it){ double sum = 0; while (it.hasNext()) { if (it.next() > 0) { sum += it.next(); } } return sum; }
Checked Exception
Unchecked Exception
Throwable Exception
Thread Exception
Do this using a show of hands – the question is NOT in TopHat (it could be, but not for time purposes) 1

31. If Exception Thrown, What Type?
double sumPos(Iterator<Double> it){ double sum = 0; while (it.hasNext()) { if (it.next() > 0) { sum += it.next(); } } return sum; }
Cannot be fixed
Checked Exception
Unchecked Exception
Throwable Exception
Thread Exception

32. If Exception Thrown, What Type?
double sumPos(Iterator<Double> it){ double sum = 0; while (it.hasNext()) { if (it.next() > 0) { sum += it.next(); } } return sum; }
Cannot be fixed & no throws provided
Checked Exception
Unchecked Exception
Throwable Exception
Thread Exception

33. Separate class but accesses private fields
Problem
Separate class but accesses private fields

34. Separate But Not Independent
Most often adopt clean solution using inner class
Inner class is independent class & follows Java rules
Parameters, instance variables, & locals can use as type
Must instantiate (new InnerClassName()) before use
Can be used as return value & usable beyond outer class

35. Separate But Not Independent
Most often adopt clean solution using inner class
Inner class is independent class & follows Java rules
Parameters, instance variables, & locals can use as type
Must instantiate (new InnerClassName()) before use
Can be used as return value & usable beyond outer class
BUT

36. Separate But Not Independent
Most often adopt clean solution using inner class
Inner class is independent class & follows Java rules
Parameters, instance variables, & locals can use as type
Must instantiate (new InnerClassName()) before use
Can be used as return value & usable beyond outer class
BUT
Inner class can use outer classes private fields

37. Declaring Inner Class Inner classes declared like normal Java classes
But class declaration must be INSIDE main (outer) class
Filename unchanged: still OuterClassName.java
Typical Java class rules still apply to inner classes
Inheritance (subclass and interface) is perfectly legal
Can have instance variables, constructors, & methods
Static & instance-based members can be declared
Inner class’s methods same as other Java methods

38. Inner Class Access

39. Inner Class Access Can directly accesses own instance variables
Just a normal Java class, so should not be surprise
But (& this is change) also accesses outer class’s fields
Protections do not matter, access is always allowed
Like gut bacteria, permission only in 1 direction
Inner class protections limit outer class actions
This is by design: inner classes support outer class

40. Inner Class Access
public class ArrayMultiSet<E> /* cut for space */ { private E[] _store; private int _size; private long _modCount; // Code exists here, but not shown due to space limits public class AMSIterator implements Iterator<E>{ private int cursor; // More code is here, but not shown to fit on screen public boolean hasNext() { return (cursor < _size); } } // More from ArrayMultiSet here; not shown due to screen space }

41. Instantiating Inner Classes
public class ArrayMultiSet<E> /* cut for space */ { private E[] _store; private int _size; private long _modCount; // Code exists here, but not shown due to space limits public Iterator<E> iterator() { return new AMSIterator(); } public class AMSIterator implements Iterator<E>{ // More code is here, but not shown to fit on screen } // More from ArrayMultiSet here; not shown due to screen space }

42. Fill In the Blank assertTrue Cannot know -- behavior is undefined
ArrayMultiSet<Integer> ams = new ArrayMultiSet<Integer>(); ams.add(2); Iterator<Integer> it=ams.iterator(); assertEquals(2, it.next()); ams.add(3); _____________(it.next()==3);
assertTrue
Cannot know -- behavior is undefined
None of the above – code will crash
Ungraded; only being asked once. 1

43. Could return 3; added 3 to _store at the index after where 2 is found
Fill In the Blank
ArrayMultiSet<Integer> ams = new ArrayMultiSet<Integer>(); ams.add(2); Iterator<Integer> it=ams.iterator(); assertEquals(2, it.next()); ams.add(3); _____________(it.next()==3);
assertTrue
Cannot know -- behavior is undefined
None of the above – code will crash
Could return 3; added 3 to _store at the index after where 2 is found

44. Fill In the Blank assertTrue Cannot know -- behavior is undefined
ArrayMultiSet<Integer> ams = new ArrayMultiSet<Integer>(); ams.add(2); Iterator<Integer> it=ams.iterator(); assertEquals(2, it.next()); ams.add(3); _____________(it.next()==3);
assertTrue
Cannot know -- behavior is undefined
None of the above – code will crash

45. Fill In the Blank assertTrue Cannot know -- behavior is undefined
ArrayMultiSet<Integer> ams = new ArrayMultiSet<Integer>(); ams.add(2); Iterator<Integer> it=ams.iterator(); assertEquals(2, it.next()); ams.add(3); _____________(it.next()==3);
assertTrue
Cannot know -- behavior is undefined
None of the above – code will crash

46. Behavior undefined; any of these could be correct
Fill In the Blank
ArrayMultiSet<Integer> ams = new ArrayMultiSet<Integer>(); ams.add(2); Iterator<Integer> it=ams.iterator(); assertEquals(2, it.next()); ams.add(3); _____________(it.next()==3);
assertTrue
Cannot know -- behavior is undefined
None of the above – code will crash
Behavior undefined; any of these could be correct

47. Fail-fast Iterator
Iterator results after data changed could be wrong

48. Fail-fast Iterator Iterator results after data changed could be wrong
May lead to bad outcomes & unpredictable results
Best avoid blame; must make certain Not Your Fault™
Idea behind fail-fast Iterators – avoid potential mistakes
Fail-fast Iterators track data’s modification count
Count increases each time data changed in program
When it is created, Iterator records modification count
Error in next() when different saved & current count

49. Fail-fast Iterator Iterator results after data changed could be wrong
May lead to bad outcomes & unpredictable results
Best avoid blame; must make certain Not Your Fault™
Idea behind fail-fast Iterators – avoid potential mistakes
Fail-fast Iterators track data’s modification count
Count increases each time data changed in program
When it is created, Iterator records modification count
Error in next() when different saved & current count

50. Fail-Fast Iterator Skeleton
public class ArrayMultiSet<E> /* cut for space */ { private E[] _store; private int _size; private long _modCount; // Code exists here, but not shown due to space limits public class AMSIterator implements Iterator<E>{ private int cursor; private long expectedModCount; // More code is here, but not shown to fit on screen public AMSIterator() { cursor = 0; expectedModCount = _modCount; } } }

51. Fail-Fast Iterator Skeleton
public class ArrayMultiSet<E> /* cut for space */ { private E[] _store; private int _size; private long _modCount; // Code exists here, but not shown due to space limits public class AMSIterator implements Iterator<E>{ private int cursor; private long expectedModCount; // More code is here, but not shown to fit on screen public AMSIterator() { cursor = 0; expectedModCount = _modCount; } } }

52. _modCount & expectedModCount?
Similar fields found in most fail-fast Collections
Counts changes in elements that comprise the ADT
Updated once per change, even if change to mult. data
Provide versioning for Iterator to use in its checks
expectedModCount is used by Iterator to fail
Assigned to _modCount in Iterator’s constructor
expectedModCount checked against _modCount
Values differ once change in elements occurs

53. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray() 1

54. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray()
Convince your neighbor your answer is correct

55. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray() 1

56. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray()

57. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray()

58. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray()

59. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray()

60. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray()

61. ArrayMultiSet Methods Changed?
add()
remove()
contains()
fromArray()
toArray()

62. How To Use _modCount Increment _modCount when elements changed
After element added to backing store in add()
Success in remove() deleting element from ADT
Having ADT elements replaced via fromArray()
Any changes that might create mistakes in Iterator

63. How To Use _modCount Increment _modCount when elements changed
After element added to backing store in add()
Success in remove() deleting element from ADT
Having ADT elements replaced via fromArray()
Any changes that might create mistakes in Iterator

64. Fail-Fast Iterators
Algorithm next() if hasNext() then E retVal = value at cursor’s location Advance cursor to refer to next location return retVal else throw new NoSuchElementException(); endif end

65. Fail-Fast Iterators
Algorithm next() if hasNext() then if expectedModCount == _modCount then E retVal = value at cursor’s location Advance cursor to refer to next location return retVal else throw new ConcurrentModificationException(); endif else throw new NoSuchElementException(); endif end

66. Iterable holds data processed with for-each loop
Iterable Key Concept #1
Iterable holds data processed with for-each loop

67. Iterator accesses data BUT separate class
Iterator Key Concept #1
Iterator accesses data BUT separate class

68. Iterator’s cursor used to access next value
Iterator Key Concept #2
Iterator’s cursor used to access next value

69. NO! For Next Lecture Section 3.1 & web page reading for Wednesday
Is all code really equal?
How can we figure out which is faster?
Week #6 weekly assignment due Monday
Today is new, rest of the week reviews for test #1
NO!