1. CS2110: Software Development Methods
Maps and Sets in Java
These slides are to help with the lab, Finding Your Way with Maps
More on these topics in lecture.
Today’s lab uses Maps (and Sets but just a little).

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 HashSet

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. 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?

7. 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?

8. Java Type: Maps
Maps
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

9. 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.

10. Concrete Classes
Map is an interface. We need concrete classes that implement it.
HashMap
Most commonly used. Allows nulls as values.
Need a hashCode() method
String and other classes Java gives us have this
More later on doing this for our own classes
TreeMap
Uses a search tree data structure
Values stored in a sorted order
More on this later.

11. 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!

12. Demo Download the MapLab.zip file from the lab page
In Eclipse, do the following:
Select Import from the File menu
Select “General” and then “Existing Projects into Workspace”
Choose “Select Archive File” and the find the MapLab.zip file
Two files in there:
MapDemo0.java – the demo the TAs will do now
MapDemo.java – the lab exercise you and your partner will do afterwards

13. 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 22900
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