//instance variables //methods. Foo x = new Foo(); Interface: also a type of objects public interface Bar {

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1 BASIC CONCEPTS Class: a type of objects public class Foo { //instance variables //methods Foo x = new Foo(); Interface: also a type of objects public interface Bar { //method headers without implementation Bar y = new Bar (); [WRONG!] Abstract data type (ADT): interface + set of allowable behaviors e.g. deposit() and getbalance()

2 WHEN A CLASS IMPLEMENTS AN INTERFACE it must provide implementations for all the interface methods its objects can be used anywhere the interface type is required public class Thing implements Bar { // all of the Bar s methods are implemented new Thing () => object of type Thing, but also of type Bar Thing x = new Thing(); Bar y = new Thing(); Bar z = x; Bar w = new Bar (); [WRONG]

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5 HOW TO MANAGE COLLECTIONS OF OBJECTS Grocery shopping 1. declare an instance variable for each item public class ShoppingList{ String item1; String item2;. Problem: a) Don t how many b) refer to each by name 2. Better: a data type that can manage a collection of items a) add items; b) order items; c) find d) remove

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7 IMPLEMENTATION OF SET, LIST, AND MAP Set HashSet faster, but unordered TreeSet keeps items ordered List ArrayList fast access to any item by index LinkedList fast adding and removing Map HashMap -- faster, but unordered TreeMap -- keeps items ordered

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12 EXAMPLE: USING A LIST AND A MAP List<String> grocerylist = new LinkedList<String>(); grocerylist.add( bananas ); grocerylist.add( pop tarts );. Suppose grocery store has a Map that relates grocery items (keys) to their locations (values) Map<String, String> locations = new HashMap(String, String>() ; locations.get( pop tarts ) Aisle 8 Print a list all items and their locations for(string item: grocerylist) { String where = locations.get(item); System.out.println(item + at +where);

13 ITERATORS Key ideas of OO programming: Encapsulation, Abstraction Exposing the rep. is a big NO NO! Don t let users of object see how you are storing the data Why? User might modify the internal data unpredictably Ties the hands of the developers Loss of abstraction

14 ADTS FOR MANAGING COLLECTIONS OF OBJECTS Set, List, Map interfaces (ADTs) Sometimes design objectives compete abstraction vs. efficiency exposing the rep? Problem: iterating over collections Want : A loop that will visit each item Requirements: a) maintain abstraction b) efficiency

15 ADTS FOR MANAGING COLLECTIONS OF OBJECTS Problem: iterating over collections Want : A loop that will visit each item Options: a) maintain abstraction b) efficiency 1. Let user keep a reference to the current position in the collection (efficient, but exposes the internal rep) 2. Hid the rep., but provide a method getnext(object x) finds x in the collection and returns the next one (encapsulates, but not efficient since need to search every time)

16 ADTS FOR MANAGING COLLECTIONS OF OBJECTS 3. More encapsulation Create an iterator in between user and the object. Iterator provides getnext(), but users don t see details. public interface Iterator { public boolean hasnext() true if there are more public Object next() return the next object & advance the iterator public void remove() remove the last returned item

17 EXAMPLE OF ITERATORS List<Integer> ls task: calculate the sum of all items in ls public int sum(list<integer> ls) { int total = 0; Iterator<Integer> it = ls.iterator(); while(it.hasnext()){ int x = it.next(); total = total + x; return total;

18 EXAMPLE OF ITERATORS Map<String, Definition> dict Task: remove all 4-letter words from dictionary public void censor(map<string, Definition> dict) { Iterator<String> it = dict.ketset.iterator(); while(it.hasnext()) { if(it.next().length == 4) it.remove();

19 EXAMPLE OF ITERATORS List<Integer> ls1 and List<Integer> ls 2 Task: calculate their dot product Example (2 1 5) * (4 3 0) = 2*4 + 3 * * 0 = 11 public int dotproduct(list<integer> ls1, List<Integer> ls2) { int sum = 0; Iterator<Integer> it1, it2; it1 = ls1.iterator(); it2 = ls2.iterator(); while(it1.hasnext()) { sum = sum + it1.next()*it2.next(); return sum;

20 EXAMPLE OF NOT USING ITERATORS List<Integer> ls task: calculate the sum of all items in ls public int sum(list<integer> ls) { int total = 0; for(int x: ls) //syntactic sugar for iterator total = total + x; return total;