1. Ch 16: Data Structures - Set and Map
Yonglei Tao

2. A set is a collection of elements
Sets
A set is a collection of elements
Every element is unique
Ordering is irrelevant
Elements can be added, located, and removed
Adding a duplicate of an existing element is silently ignored
No way to visit elements in the order in which they are added

3. Set Implementation in Standard Java Library
Class HashSet
Uses a hash table as its data structure
It arranges its elements so they can be located quickly
Class TreeSet
Uses a balanced binary tree as its data structure
It keeps its elements in a way that makes it easy to fetch in ascending order
Implementation with Linked lists - relatively slow to add, remove, and locate elements
As a rule of thumb, use a hash set unless you want to visit the set elements in sorted order

4. Add and remove elements:
Using a Set
Construct the Set:
Set<String> names = new HashSet<String>(); or
Set<String> names = new TreeSet<String>();
Add and remove elements:
names.add("Romeo");
names.remove("Juliet");
Test whether an element is contained in the set:
if (names.contains("Juliet")) . . .

5. Visiting All Elements with an Iterator
Using the “for each” loop
for (String name : names){ // do something with name here }
Or using an iterator
Iterator<String> iter = names.iterator(); while (iter.hasNext()){ String name = iter.next(); // do something with name here }
A set iterator does not visit the elements in the order in which they were added
An element cannot be added to a set at an iterator position
A set element can be removed at an iterator position

6. Sample Program - SpellCheck
public class SpellCheck {
public static void main(String[] args) {
// Read the dictionary and the document
try {
Set<String> dictionaryWords = readWords(“dictionary.txt");
Set<String> documentWords = readWords(“report.txt");
} catch (IOException e) {
System.out.println(“Invalid input”); }
// Print all words that are in the document but not the dictionary
for (String word : documentWords) {
if (!dictionaryWords.contains(word))
System.out.println(word);

7. Sample Program – SpellCheck (Cont.)
/**
Reads all words from a file.
@param filename the name of the file
@return a set with all lowercased words in the file. Here, a
word is a sequence of upper- and lowercase letters. */
public static Set<String> readWords(String filename) {
Set<String> words = new HashSet<String>();
Scanner in = new Scanner(new File(filename));
// Use any characters other than a-z or A-Z as delimiters
in.useDelimiter("[^a-zA-Z]+");
while (in.hasNext()) {
words.add(in.next().toLowerCase()); }
return words;
If you changed the code to use a TreeSet instead of a HashSet. How would the output change?
Answer: The words would be listed in sorted order

8. Sample Program – SpellCheck (Cont.)
Program Run:
neighbouring
croqueted
pennyworth
dutchess
comfits
xii
dinn
clamour
...
When would you choose a tree set over a hash set? Answer: When it is desirable to visit the set elements in sorted order.

9. Maps
A map is a set of key / value pairs
It is a function from one set, the key set, to another set, the value set
Every key in a map has a unique value, but a value may be associated with several keys
Used to find the value that is associated with a key

10. Map Classes and Interfaces
Class HashMap
Uses a hash table to minimize the search time
Class TreeMap
Needs extra time to insert and retrieve keys
Can produce keys in sorted order
To find all keys and values in a map, iterate through the key set and find the values that correspond to the keys

11. Example: Associate names with colors Construct the Map:
Using a Map
Example: Associate names with colors
Construct the Map:
Map<String, Color> favoriteColors =
new HashMap<String, Color>(); or
new TreeMap<String, Color>();
Add an association:
favoriteColors.put("Juliet", Color.RED);
Change an existing association:
favoriteColors.put("Juliet",Color.BLUE);

12. Get the value associated with a key:
Using a Map
Get the value associated with a key:
Color julietsFavoriteColor =
favoriteColors.get("Juliet”);
Remove a key and its associated value:
favoriteColors.remove("Juliet");
Find if this map contains a mapping for a key:
favoriteColors.containsKey(“Jackson");
Print key/value pairs
Set<String> keySet = m.keySet(); for (String key : keySet) { Color value = m.get(key); System.out.println(key + ” : " + value); }

13. MapDemo.java public class MapDemo {
public static void main(String[] args) {
Map<String, Color> favoriteColors = new HashMap<String, Color>();
favoriteColors.put("Juliet", Color.BLUE);
favoriteColors.put("Romeo", Color.GREEN);
favoriteColors.put("Adam", Color.RED);
favoriteColors.put("Eve", Color.BLUE);
// Print all keys and values in the map
Set<String> keySet = favoriteColors.keySet();
for (String key : keySet) {
Color value = favoriteColors.get(key);
System.out.println(key + " : " + value); }

14. MapDemo.java (cont.) Program Run:
Romeo : java.awt.Color[r=0,g=255,b=0]
Eve : java.awt.Color[r=0,g=0,b=255]
Adam : java.awt.Color[r=255,g=0,b=0]
Juliet : java.awt.Color[r=0,g=0,b=255]
What is the difference between a set and a map? Answer: A set stores elements. A map stores associations between keys and values.
Why is the collection of the keys of a map a set? Answer: The ordering does not matter, and you cannot have duplicates.