Chapter 10 Collections

Transcription

1 Chapter 10 Collections I. Scott MacKenzie 1 Outline 2 1

2 What is a Collection? A collection is an aggregation with a variable number of components, or elements Examples Portfolio - a collection of Investment objects GlobalCredit a collection of CreditCard objects 3 Types of Collections List Elements are in a sequence Order is significant (i.e., [a,b] is not the same as [b,a]) Duplicates allowed E.g., a string of characters ( hello ) Set Elements viewed as a mathematical set Order is not significant (i.e., [a,b] is the same as [b,a]) Duplicates not allowed E.g., IDs for students enrolled in a course Map Elements are a mapping between keys and values A key maps to only one value E.g., student IDs (keys) and names (values) 4 2

3 Collections Framework Java defines a collections framework (e.g., a link is provided in the ArrayList API) 5 Implementing Collections Java s collections framework defines interfaces and provides classes that implement the interfaces Interface List Set Map Implementing Classes AbstractList, ArrayList a, LinkedList, Vector AbstractSet, HashSet a, LinkedHashSet, TreeSet AbstractMap, Attributes, HashMap a, HashTable IdentityHashMap, RenderingHints, TreeMap, WeakHashMap a recommended (see next slides for demos) 6 3

4 Collections Class Java includes a static class called Collections in the java.util package Contains only static methods odds and ends to facilitate working with collections 7 import type.lang.*; import java.util.*; ArrayList Last slide on Mar 15 public class DemoArrayList public static void main(string[] args) if (args.length!= 1) IO.println("usage: java ArrayList file\n"); IO.println("where 'file' is any text file"); IO.println("(NOTE: list elements are tokens in file)"); System.exit(0); UniReader ur = new UniReader(args[0]); ArrayList al = new ArrayList(); String s; while ((s = ur.readline())!= null) StringTokenizer st = new StringTokenizer(s); while (st.hasmoretokens()) al.add(st.nexttoken()); IO.println("Size = " + al.size()); Collections.sort(al); IO.println("Elements: " + al); DemoArrayList.java Demo 8 4

5 import type.lang.*; import java.util.*; HashSet public class DemoHashSet public static void main(string[] args) if (args.length!= 1) IO.println("usage: java HashSet file\n"); IO.println("where 'file' is any text file"); IO.println("(NOTE: list elements are tokens in file)"); System.exit(0); UniReader ur = new UniReader(args[0]); HashSet hs = new HashSet(); String s; while ((s = ur.readline())!= null) StringTokenizer st = new StringTokenizer(s); while (st.hasmoretokens()) hs.add(st.nexttoken()); IO.println("Size = " + hs.size()); IO.println("Elements: " + hs); DemoHashSet.java Demo 9 import type.lang.*; import java.util.*; HashMap Last slide on Mar 17 public class DemoHashMap public static void main(string[] args) if (args.length!= 1) IO.println("usage: java HashMap file\n"); IO.println("where 'file' is file of map elements"); IO.println("(NOTE: elements are key-value pairs, one"); IO.println("per line, with comma or space separator"); System.exit(0); UniReader ur = new UniReader(args[0]); HashMap hm = new HashMap(); String s; while ((s = ur.readline())!= null && s.length() > 0) // use StringTokenizer to separate keys and values // (assume comma or space as separator) StringTokenizer st = new StringTokenizer(s, ", "); DemoHashMap.java Demo 10 5

6 Using the Framework The sorts of things we want to do with collections include Adding elements Removing elements Accessing elements Searching the elements Traversing the elements Etc. 11 Adding Elements 12 6

7 Removing Elements 13 Accessing Elements 14 7

8 Searching the Elements 15 Traversing the Elements 16 8

9 Etc. 17 Word Frequency Application import type.lang.*; import java.util.*; public class DemoWordFreq public static void main(string[] args) if (args.length!= 1) IO.println("usage: java WordFreq file\n"); IO.println("where 'file' is any text file"); IO.println("(NOTE: list elements are tokens in file)"); System.exit(0); final String DELIMITER = " \t\f\n\r~!@#$%^&*()_+= []\\:\";<>?,./"; UniReader ur = new UniReader(args[0]); TreeMap tm = new TreeMap(); String s; while ((s = ur.readline())!= null) StringTokenizer st = new StringTokenizer(s, DELIMITER); Try with while (st.hasmoretokens()) DemoWordFreq.java Demo sample.txt GulliversTravels.txt 18 9

10 Programming Challenge Last slide on Mar 19 Make a copy of DemoWordFreq.java and name it DemoCharFreq.java. Modify the application to build a character-frequency map, instead of a word-frequency map. Provide some command-line options to make the program flexible in its operation. 19 Thank You 20 10