Java AP Computer Science Design Patterns Owen Astrachan

Top 10 reasons continued 3. In grading AP exams, we can have a usage error for capitalization problems, e. g. , the real-estate error public class Fish { public Location location() { return my. Loc; } } Array. List list = new Array. List(); Iterator it = list. iterator(); // code from Array. List class follows // public Iterator iterator() {…} APCS Workshop 14

#2 Reason to switch AB to Java 1. Source code: java. util. Hash. Map and stl_hashtable. h public boolean contains. Key(Object key) { Entry tab[] = table; if (key != null) { int hash = key. hash. Code(); int index = (hash & 0 x 7 FFFFFFF) % tab. length; for (Entry e = tab[index]; e != null; e = e. next) if (e. hash==hash && key. equals(e. key)) return true; } return false; } size_type count(const key_type& __key) const { const size_type __n = _M_ bkt_num_key(__key); size_type __result = 0; for (const Node* __cur = _M_buckets[__n]; __cur = __cur->_M_next) if (_M_equals(_M_get_key(__cur->_M_ val), __key)) ++__result; } return __result; APCS Workshop 15

#1 Reason for using Java in AB 1. No friends, but there is free garbage collection APCS Workshop 16

java. util. Collection l Interface for collections of objects Ø See API for details, AP subset requires only a few of the methods for subinterfaces List and Set, and for separate interface Map l Which interface and class should be used? Tradeoffs. Ø Why choose Array. List vs Linked. List? Ø Why choose Hash. Set vs Tree. Set? l What about classes, interfaces, and methods not in the subset? Ø Collection interface compared to Collections class Collection. add. All(Collection); // object method Collections. sort(Collection); // static method APCS Workshop 17

Collection hierarchy, tradeoffs? Collection List list. Iterator Array. List get(int) set(int, o) add(int, o) remove(int) add size iterator Linked. List Hash. Set contains* remove** Tree. Set get. First get. Last add. First add. Last remove. First remove. Last *Collection method **Collection/optional APCS Workshop 18

Searching, Maps, Tables l Searching is a fundamentally important operation Ø We want to search quickly, very quickly Ø Consider searching using google. com or whitepages. com? Ø In general we want to search in a collection for a key l Finding unique words in a file (or webpage or disk or …) Ø Add words to Array. List, search before adding • Is it worth it keep the list sorted? • Add all words then remove duplicates, faster? Use java. util. Set implementations Tree. Set or Hash. Set What are the tradeoffs in these choices? Ø Algorithmic/theory, programming/engineering Ø l APCS Workshop 19

From Google to Maps l If we wanted to write a search engine we’d need to access lots of pages and keep lots of data Ø Given a word, on what pages does it appear? Ø This is a map of words->web pages l In general a map associates a key with a value Ø Look up the key in the map, get the value Ø Google: key is word/words, value is list of web pages Ø Anagram: key is string, value is words that are anagrams l Interface issues Ø Lookup a key, return boolean: in map or value: associated with the key (what if key not in map? ) Ø Insert a key/value pair into the map APCS Workshop 20

Interface at work: Map. Demo. java l Key is a string, Value is # occurrences, the Map interface at work: while (toks. has. More. Tokens()) { String s = toks. next. Token(). to. Lower. Case(); Counter c = (Counter) map. get(s); if (c == null) map. put(s, new Counter()); else c. increment(); } l Consider alternative of using map. contains. Key() if (map. contains. Key(s)) … l What about implementations and trade-offs? Ø If we use a search tree what are constraints on key values? Ø If we use a hash-table what are constraints on key values? APCS Workshop 21

Accessing values in a map (e. g. , print) l l What values will iterator. next() return? Choices? Ø The AP Java subset specifies key. Set() method; API provides entry. Set() and values() [the latter is a Collection, why? ] Iterator it = map. key. Set(). iterator(); int max = 0; String smax = null; while (it. has. Next()) { String s = (String) it. next(); int count = ((Counter)map. get(s)). get. Value(); if (count > max) { max = count; smax = s; } } Iterator has access to “guts” of a map, iterates over it Ø Tightly coupled: container and iterator, implementation? APCS Workshop 22

Iterators and the Map. iterator() method l We ask a map/container to provide us with an iterator Ø We don't know how the map is implemented, just want an iterator Ø Map object is an iterator factory: makes/creates iterator l Typically the iterator is a nested or inner class Ø Not part of the AP Java subset, useful to know about Ø Inner class is an object that has access to enclosing object Ø Static inner class has no associated object, like nested Node class in C++ APCS Workshop 23

Map example: finding anagrams l Anagram. Finder: alternative to finding anagrams Ø Maps string (normalized): key to Array. List of Strings value Ø Look up normalized string, associate all "equal" strings with normalized form Ø To print, loop over all keys, grab Array. List, print if ? ? ? l Each value in the map is list/collection of anagrams Ø How do we look up this value? Ø How do we create initial list to store (first time) l See also mapanagram. cpp for standard C++ using STL Ø The STL code is very similar to Java! (if you can read it) APCS Workshop 24

Eric Raymond Open source evangelist Ø The Cathedral and the Bazaar http: //tuxedo. org/~esr/writings/cathedralbazaar/ Ø How to construct software “Good programmers know what to write. Great ones know what to rewrite (and reuse). ” l How to convince someone that guns are a good idea? Put this sign up: l l THIS HOME IS A GUN-FREE ZONE APCS Workshop 25