CS2110: Software Development Methods. Maps and Sets in Java

Transcription

1 CS2110: Software Development Methods Maps and Sets in Java These slides are to help with the lab, Finding Your Way with Maps This lab uses Maps, and Sets too (but just a little). Readings from textbook: (a) Map introduction: Section (pp ) (b) Map interface: Section (pp ) (c) Set interface: Section (pp ) -- Maps use Sets so this is needed, but it's simple

2 Lists, Sets and Maps in Java Java has what s called a Collections Framework Includes very useful classes like ArrayList Also, other useful classes for other data types Sets Data model: a collection of items Operations: membership, insert, delete Maps: like a lookup table Data model: each data item stored with a lookup key Operations: retrieve with the key, etc.

3 Collection Interface All collections implement the Collection interface. Helpful! all the collection classes have common operations! boolean add(object obj); Iterator iterator(); int size(); boolean isempty(); boolean contains(object obj); boolean containsall (Collection other); See Java API documentation for all methods

4 Set and HashSet in Java Collections A Set is a collection of items that are unordered are unique (no duplicates) What kind of operations do we think of? element membership (add, remove, ismember, etc.) manipulation (union, intersection, ) Java gives us: Set an interface Several implementation classes, including TreeSet And HashSet, but we ll use TreeSet for now

5 Java s Set Interface (see MSD, p. 625) The Set interface looks just like the Collection interface, but with more specific behaviors boolean add(elem) returns false if already there boolean remove(elem) returns false if not there What s sweet here: Try to add something that s already there? Remove something that s not there? No problem! It basically ignores that attempt! This allows some powerful methods to be very short See examples on pages 625 and 626

6 Examples using Set How many unique items in any Collection object? int numuniq = new TreeSet(someList).size(); Creates a new set on the fly using the constructor that adds all items from an existing collection Remove duplicates from any Collection object? Set nodups = new TreeSet( somelist ); somelist.clear(); somelist.addall( nodups ); Note we don't have to iterate We rely on useful constructors and bulk operation

7 Using Sets in Java: TreeSet class TreeSet is a concrete class that implements Set How's it work inside? How is it implemented? We won't focus on that in CS2110 (not now anyway) Because it's a well-implemented abstraction, you don't need to understand what's in the black-box The tree refers to type of data structure used But you do need one thing: All items must be Comparable All items must correctly implement compareto() Bonus! Iterating over a TreeSet uses compareto()'s ordering. So prints in correct order

8 Common Situation: data lookup We have some some information, and we want to retrieve it or look it up We find it quickly by using some special data value. Let s call that a key Examples of keys and data values used in lookup? Often use term lookup table for these. Databases work this way, don t they?

9 What if we had a Type for a Table? Called Map in Java: find data given some key value Map s model of information Some set of values, each associated with a key Operations Lookup: given a key, retrieve the values Insert, remove, updates, etc. How could we implement this? You ve been using lists (arrays, ArrayLists) and the contains() and indexof() methods Disadvantages?

10 Java Type: Maps Map Interface Like sets, but have <key, value> instead of <value> Examples in real life? Dictionary: word to its definition (or list of definitions) to Facebook Profile Host Names to IP addresses How to declare: Map<KeyType,ValueType> Examples: Map<String,Integer> // maps string to a number Map<Student,String> // maps Student to a String Map<String,ArrayList<Course>> // e.g. a schedule

11 Important Map Methods Keys and values put (key, value), get(key), remove(key) containskey(key), containsvalue(value) Can think of a Map as a set of keys and a set of values keyset() // returns a Set of key values values() // returns a Collection of values Others methods too! See doc or a Java reference.

12 Concrete Classes Map is an interface. We need concrete classes that implement it. HashMap Often used. Allows nulls as values. Need a hashcode() method String and other classes Java gives us have this TreeMap More later on doing this for our own classes We ll use this one in CS2110! Uses a search tree data structure Values stored in a sorted order Keys must implement Comparable!

13 Why Are Maps Useful and Important? Lookup values for a key -- an important task We often want to do this! Need flexibility for what values are stored for a given key Need to use objects other than Strings or ints as key values Efficiency matters for large sets of data These are fast!

14 In Our Lab on Maps Demo now in class! Pay attention please and learn In lab, you ll do more on this kind of demo In lab, you ll have access to a Zip file with: MapDemo0.java the demo we will do now MapDemo.java the lab exercise you and your partner will do afterwards

15 MapDemo0.java file We define a set and a map We get a scanner for a file that looks like this: Cavman 101 JPA, Apt. 3. C'ville, VA Tom Monticello, C'ville, VA Task 1: store the names in the set Task 2: store (name,address) pairs in the map Task 3 (if time): split the address-line into words and store each of them in the set Task 4: read key and look it up in Map