Today’s Agenda  Generic  Iterators CS2336: Computer Science II

Parameterized Stack  We implemented stack of Strings  How about the stack of other type of objects!  Do we need to implement a separate stack for each type?  Rewriting code is error-prone  Maintaining cut-and-paste code is error-prone  How about having a stackOfObjects and use casting  Run-time error if the types mismatch CS2336: Computer Science II StackOfObj s = new StackOfObj(); Apple a = new Apple(); Orange o = new Orange(); s.push(a); s.push(o); a = (Apple) (s.pop()); Run-time error

Parameterized Stack : Java Generic  No casting needed  Discover mismatches at compile-time instead of run-time CS2336: Computer Science II Stack s = new Stack (); Apple a = new Apple(); Orange o = new Orange(); s.push(a); s.push(o); compile-time error

Generic stack: linked list implementation CS2336: Computer Science II Pubic class Stack { private Node first = null; Public void push(Obj item){ //implementation } Public Obj pop() { //implementation }

Generic stack: array implementation  Generic array creation is not allowed in java  We need to use casting ( we call it ugly cast) CS2336: Computer Science II Pubic class Stack { private Obj[] stackArray; Public Stack(int size){ stackArray = (Obj[]) new Object[size]; } Public void push(Obj item){ //implementation } Public Obj pop() { //implementation }

Iterators  Allow the user to iterate through any collection  Java provides Iterable interface  Make the data structure like stack implement the Iterable interface CS2336: Computer Science II

Iterable interface  It is an interface.  Has a method that returns an Iterator.  Iterator has methods:  hasNext()  next()  Java support elegant client code if we make data structures Iterable.  “foreach” statement that is so compact CS2336: Computer Science II for (String s: Stack) System.out.println(s); Iterator i = stack.iterator(); While(i.hasNext()){ String s = i.next(); System.out.println(s); } Equivalent code

LinkedList Stack CS2336: Computer Science II import java.util.Iterator; pubic class Stack implements Iterable { …. public Iterator iterator(){ return new ListIterator(); } //inner class private class ListIterator implements Iterable { private Node current = first; public boolean hasNext(){return current!=null;} public Obj next(){ Obj item = current.item; current = current.next; return item; }

Array Stack CS2336: Computer Science II import java.util.Iterator; pubic class Stack implements Iterable { …. public Iterator iterator(){ return new ReverseArrayItr(); } //inner class private class ReverseArrayItr implements Iterable { private int i = topOfArray; public boolean hasNext(){return i > 0; } public Obj next() {return stackArray[--i]; } }

Java Collection Library  List interface  java.util.List is API for sequence of items  List interface also extends Iterable  java.util.ArrayList uses resizable array  java.util.LinkedList uses Linked list  Why not just use the java library ??  The problem is that a lot of operations get added, and API become bloated. It is not a good idea to have lots of operations to the same API  We don’t know much about the performance! CS2336: Computer Science II

Example Generate random open sites in an N-by-N percolation System 1. Use an array: pick (i,j) at random ; if already open, repeat. Takes linear time o(n) 2. use java.util.Linkedlist pick an index at random and delete takes quadratic time (n 2 ) Reason: Java LinedList implementation take linear time to find an item within given index. Not constant time like an array. Don’t use a library until you understand its API. CS2336: Computer Science II