Lec #6: Review, Generics and Recursion

Transcription

1 Lec #6: Review, Generics and Recursion 1 Introductions and sounding out? (5 min) 2 Review (25 min) 3 Generics (15 min) 4 Recursion (40 min) 5 Forward guidance (5 min) Timings approximate: Interruptions welcome and appreciated!

2 How goes??

3 Clay Pigeons A person loads a gun with one bullet, spins the barrel, points the gun at a clay pigeon and pulls the trigger. What classes should we have?

4 The first Gun class A mess! public class Gun public int capacity=6; public int current=0; public int bullet=-1; pubblic boolean loaded=false; void public load() if (!loaded) System.out.println("Already loaded"); else bullet = current loaded = true; ; public void spin(int positions) current += positions % chambers; public boolean fire() if (current = bullet) current = (current + 1) % capacity; return true; else return false; A real mess! Let s focus on syntax errors first.

5 The first Gun class at least it compiles! public class Gun public int capacity=6; public int current=0; public int bullet=-1; public boolean loaded=false; public void load() if (!loaded) System.out.println("Already loaded"); else bullet = current; loaded = true; ; public void spin(int positions) current += positions % capacity; public boolean fire() if (current = bullet) current = (current + 1) % capacity; return true; else return false; Great! But does it work at run time? What is wrong with it logically? What is wrong with it stylistically?

6 The first Gun class fixed public class Gun private int capacity=6; private int current=0; private int bullet=-1; private boolean loaded=false; Gun() Gun(int capacity) this.capacity = capacity; public void load() if (loaded) System.out.println("Already loaded"); else bullet = current; loaded = true; public void spin(int positions) current = (current + positions) % capacity; public boolean fire() if (current == bullet) current = (current + 1) % capacity; bullet = -1; loaded = false; return true; else current = (current + 1) % capacity; return false; OK, but could we do a little better? How is it limited?

7 A better Gun class import java.util.random; public class BetterGun private boolean[] chambers; private int current=0; BetterGun() chambers = new boolean[6]; empty(); BetterGun(int capacity) chambers = new boolean[capacity]; empty(); public void empty() for(int i=0; i<chambers.length; i++) chambers[i]=false; public void load() if (chambers[current]) System.out.println("Already loaded"); chambers[current] = true; Not perfect, but let s go with this. public void spin(int positions) current = (current + positions) % chambers.length; private void advance() spin(1); public boolean fire() if (chambers[current]) System.out.println("\n BANG!!! \n"); chamber[current] = false; advance(); return true; else System.out.println("\n click \n"); advance(); return false; public void spin() Random random = new Random(); current = (current + random.nextint(100)) % chambers.length;

8 Give me a Person and give me a Pigeon! public class Person String name; public class ClayPigeon private boolean broken = false; public Person(String name) this.name= name; public void play(claypigeon pigeon, BetterGun gun) gun.spin(); gun.load(); gun.spin(); if (gun.fire()) pigeon.takebullet(); public void takebullet() broken = true; System.out.println("Clay pigeon breaks" + " into \n a thosand pieces"); Person george = new Person("George"); BetterGun gun = new BetterGun(8); ClayPigeon pigeon = new ClayPigeon(); george.play(pigeon,gun);

9 We clear on this??

10 Generics In the beginning (Gosling,1995), Java did not have generics Added in Java 5 (Wadler,2004). Remember linked-list implementation of a collection in lecture 5? Linked list of String. Some code. What if we also wanted to have a linked lists collections of int, float, Person, Gun, ClayPigeon, BowlingBall, etc? A ton of duplicated code! Instead let s use generics!: ArrayList<int> int_collection = new ArrayList<>(); ArrayList<float> float_collection = new ArrayList<>(); ArrayList<Person> person_collection = new ArrayList<>(); ArrayList<BetterGun> gun_collection = new ArrayList<>();... in general: ArrayList<E> E_collection = new ArrayList<>(); ArrayList<E> E_collection = new ArrayList<E>(); if you are not into brevity.

11 Use of ArrayList in Person import java.util.arraylist; import java.util.iterator; public class Person... same as above... ArrayList<Person> friends = new ArrayList<>(); ArrayList<ClayPigeon> pigeoncollection = new ArrayList<>(); public void addfriend(person friend) friends.add(friend); public void addpigeon(claypigeon pigeon) pigeoncollection.add(pigeon); public boolean ownspigeon(claypigeon pigeon) Iterator<ClayPigeon> it = pigeoncollection.iterator(); boolean found=false; while (it.hasnext() &&!found) if (it.next() == pigeon) found=true; return found; public boolean isfriend(person person) int i = 0; boolean found=false; while (i < friends.size() &&!found) if (friends.get(i) == person) found=true; i++; return found; What s going on here? How is it limited? Could we do better?

12 Generics Take away message In the previous, we get better behavior with: HashSet<Person> friends = new HashSet<>(); Lots of other collections in Java use generics. We will meet these later. Really shortens code. More advantages that we will talk about later.

13 Recursion Can a method call itself? Add this method to Person: public void goforbroke(claypigeon pigeon, BetterGun gun) play(pigeon,gun); if (!pigeon.broken) goforbroke(pigeon,gun); What is going on here? Let s work it out on the board!

14 Recursion a simple pure case Let s sum up the first n natural numbers. For example the sum of the first 4 is = 10 public class RecursionDemo public long whilesumup (int n) long total=0; int i= 0; while (i <= n) total += i; i++; public long closedformsumup (int n) return n * (n+1) / 2; return total; public long recsumup (int n) if (n == 0) return 0; return n + recsumup(n - 1); What s going on here? Let s show on the board!

15 Recursion Fibonacci Numbers! Recall the Fibonacci sequence: 1,1,2,3,5,8,13,... One of those magic sequences that appears everywhere in nature. Let us calculate the n-th Fibonacci Add this method to RecursionDemo: public long fib (int nth) if (nth <=2) return 1; return fib(nth-2) + fib(nth-1); What s going on here? Let s show on the board!

16 Why is Recursion useful? can give us very compact/elegant code helps us get to our root understanding of an algorithm can be inefficient (see fib above). Let s discuss this, time permitting...

17 Forward Guidance on rest of lectures in LP2 Current plans... Lec #7 The JAVA library + Thinking about Algorithms Lec #8 Testing + Asymptotic notation Lec #9 Assembly language Lec #10 Program Design + Review Lec #11 Project + Misc.