Part 1: Excep-ons, part 2 Part 2: Iterator. Lecture 11 COMP 401, Spring /3/2015

Transcription

1 Part 1: Excep-ons, part 2 Part 2: Iterator Lecture 11 COMP 401, Spring /3/2015

2 The finally block Some-me we need some code to run no maker what happens. OMen this is the case in order to free up some system resource such as closing a file or closing a network connec-on. finally block will execute no maker what happens (excep-on or no). lec11.ex7

3 The Throwable Class Hierarchy Throwable Superclass for all objects that can be thrown Error Superclass for errors generally caused by external condi-ons. Excep-on Superclass for excep-ons generally caused by internal condi-ons.

4 Checked vs. Unchecked Error, Run-meExcep-on, and their subclasses are unchecked excep-ons. All other subclasses of Excep-on are checked.

5 Checked Excep-ons Checked excep-ons are subject to the catch or specify rule. A method that could poten-ally throw a checked excep-on must declare the possibility as part of its method signature. A method that calls another method that throws a checked excep-on must either: Catch: Include code that catches the excep-on. Specify: Declare the possibility of the excep-on as part of its own method signature. Why? Forces user of method to explicitly handle the excep-on or pass the buck to whomever calls it. Enforces good error handling. lec11.ex8

6 General Principle: Be Specific Use an exis-ng excep-on type. There are lots. If seman-cs of the excep-on match well, then go ahead and use it. Create your own excep-on type. Subclass either Run-meExcep-on or Excep-on lec11.ex9.v1 No-ce how less specific general excep-on raised by Scanner transformed into context- specific excep-on for Playlist. Also note how error message can be retrieved from excep-on object. See handling of PlaylistFormatError in main() See reference page for Excep-on for more. 6

7 General Principle: Catch Late Excep-ons should rise to level where applica-on has enough context to deal with them effec-vely. Catching excep-on just because you can is not always the right thing to do. Pass the buck unless response to this excep-on under all or nearly all circumstances is well- understood at this point. Look again at lec11.ex9.v1 In par-cular, note handling of FileNotFoundExcep-on lec11.ex9.v2 Note printstacktrace() method of Excep-on in Main1 Note differences in how FileNotFoundExcep-on handled in Main1 vs. Main2 Main1 not even given the chance because handled by playlist. Main2 has ability to re- prompt for new filename and does so. 7

8 General Principle: Throw Early Validate values as early as possible. Rather than wai-ng for excep-on generated by invalid values sent to other code. Par-cularly apropos for null values that may later cause a NullPointerExcep-on Excep-on generated by null pointer is not very specific Almost always have to look higher in the stack trace to see what the real problem is. lec11.ex9.v3 Changed how Main2 reads filename in order to demonstrate this point. 8

9 Odds and Ends Remember that the scope of a variable is limited to the surrounding block. Some-mes an issue if you declare a variable inside a try block but then need its value outside of the try block later. Sibling catch blocks can reuse the same variable name for the declared error object. A catch block associated with an excep-on class cannot precede a catch block associated with a subclass. Results in unreachable code. try- catch- finally blocks can be nested. Interfaces must declare any checked excep-ons thrown by any of its implementa-ons. Finally always runs Even if you return from within a try or catch block lec11.ex10

10 Part 2: Iterator

11 Design Situa-on Suppose we have an object that encapsulates some sort of collec-on. SongLibrary A collec-on of songs in an itunes- like system PolygonModel A collec-on of polygons in a 3D modeling system

12 Design Situa-on Now suppose we have code outside of this collec-on object that needs to examine each element of the underlying collec-on in turn. SongFilter A object that represents a search criteria to be applied to a collec-on of songs An intersec-on test in which each polygon of a PolygonModel needs to be evaluated

13 Strategy 1: Provide access to underlying collec-on as an array. SongLibrary public Song[] getsongs() PolygonModel public Polygon[] getpolygons() Drawbacks? May have to do a lot of work to create the array Collec-on may be result of genera-ve process There may be no end to the collec-on. Or the collec-on may be large so we don t want to provide the whole thing at once.

14 Strategy 2: Provide index access to each underlying item in collec-on SongLibrary public int getnumsongs(); public Song getsong(int song_idx); PolygonModel public int getnumpolygons(); public Polygon getpolygon(int polygon_idx); Drawbacks? Doesn t help with genera-ve collec-ons Imposes restric-ons on how collec-on is represented and linearized Deteriorates encapsula-on

15 Strategy 3: Internalize a cursor SongLibrary public void resetsongcursor(); public Song getnextsong(); public boolean iscursoratend(); Drawbacks? Can t have two traversals going at the same -me. But, this does come close.

16 Iterator Design PaKern Provide a way to access the elements of an aggregate object sequen-ally without exposing its underlying representa-on Gang of Four, Design Pa.erns Consider: for(int i=0; i<slist.size(); i++) {! }! Song next_song = slist.get(i);! // Do something with next_song.!

17 Iterator Design PaKern Iterator object encapsulates details of item traversal. Understands details of the underlying collec-on. Manages order of items May want a traversal that is not just first to last. Underlying collec-on may not have a natural linear order. Manages state of traversal Allows traversal to be picked up again later. Assump-on: underlying collec-on is not changed or modified while the traversal is occurring. Iterator should be able to detect this and signal an error Variant of pakern will have iterator provide methods that modify underlying collec-on safely

18 Components of Iterator PaKern Collec-on object is iterable Provides a method that returns an object that acts as an iterator. Iterator object provides access to the elements in turn. At the very least: A method to test whether more items exist. A method to retrieve the next item. Other possible features: Methods that remove an item safely. Method to peek at the next item. Method to reset the traversal.

19 Java Iterator PaKern Interfaces The Java Collec-ons Framework defines two generic interfaces for suppor-ng the iterable design pakern Implemented by the various collec-on types such as List<E>, Map<E>, Set<E>, etc. Iterable<E> Interator<E> iterator() Iterator<E>

20 Iterator<E> boolean hasnext() Are we at the end of the traversal? E next() Get the next item of the traversal. Throws a run-me excep-on if no next item. void remove() Not supported by all implementa-ons. Safely removes last item retrieved by next() from the underlying collec-on.

21 Iterable examples lec11pt2.ex1 Main1 Simple use Main2 Parallel iterators Main3 Simultaneous iterators Main4 for each syntac-c sugar

22 Main1 Visualized (1) ArrayList<Song> slist 0 Words and Guitar Dig Me Out Jenny LiKle Babies Buy Her Candy

23 Main1 Visualized (2) Iterator<Song> iter ArrayList<Song> slist list next_idx 0 0 Words and Guitar 1 Dig Me Out Jenny LiKle Babies Buy Her Candy

24 Main1 Visualized (3) Iterator<Song> iter ArrayList<Song> slist list next_idx 0 0 Words and Guitar 1 Dig Me Out public boolean hasnext() { if (next_idx < list.size()) { return true; } return false; } Jenny LiKle Babies Buy Her Candy NOTE: This may or may not be how it is actually implemented, but it is effec2vely what is going on.

25 Main1 Visualized (4) Iterator<Song> iter ArrayList<Song> slist list next_idx 1 0 Words and Guitar 1 Dig Me Out public Song next() { Song s = list.get(next_idx); next_idx++; return s; } Jenny LiKle Babies Buy Her Candy NOTE: Real implementa-on would first check to see if hasnext() is s-ll true and throw an excep-on otherwise.

26 lec11p2.ex1.main2 Parallel itera-on Processes two different lists Iterator associated with each. Iterators advance unevenly

27 lec11p2.ex1.main3 Simultaneous itera-on 2 Iterators, 1 List Insert your own joke here.

28 for - each Java provides syntac-c sugar for a par-cularly common use of iterators. for- each loop Supposing e_coll is Iterable<E>, then these are equivalent: Iterator<E> iter = e_coll.iterator();! while (iter.hasnext()) {!!E elem = iter.next();!!// Do something with element! }!! lec11p2.ex1.main4 for (E elem : e_coll) {!!// Do something with elem! }!

29 for- each with Array The for- each construct also works with Arrays Useful if you need to process the elements of an array but do not need the index. String[] names = new String[] { Amy, Mike,! for (String n : names) {! }! System.out.println(n);! Cameron, Claire };!

30 lec11p2.ex2 A more complicated iterator Can build iterators that do things other than just go through every item. Prior examples made use of Iterator<E> built into List<E>, here we are going to implement our own specialized version of Iterator<E>