C6: Working with Collections
CPSC 210
Learning Goals
- Gain an overview over Java collections
- Use the Map<K, V> type in the Java library
- Select the appropriate collection type from the Java library
Java Collections
Collection
Map
Iterable
List
Queue
Set
HashMap
HashSet
ArrayList
LinkedList
...
closely related
Maps
-
Map<K, V> interface with two type parameters:
- K - represents the type for the key
- V - represents the type for the value
- In this course we will focus on HashMap<K, V>
- You have seen a collection of key-value pairs using a binary search tree in CPSC 110
HashMap Example
Map<String, Instructor>Map<K, V>
values: instructor objects
keys: names
Map<String, Instructor> instructors;
instructors = new HashMap<String, Instructor>();
instructors.put("Felix", new Instructor());
instructors.put("Steve", new Instructor());
instructors.put("Meghan", new Instructor());Instructor felix = instructors.get("Felix");
Instructor steve = instructors.get("Steve");
Instructor meghan = instructors.get("Meghan");
"Felix"
"Steve"
"Meghan"
Which Collection to Use?
Also preserves order.
Which one to use? (2)
- Lists: for maintaining the order in which items were added
- Sets: if there are no duplicate items in the collection
- Maps: to represent collections of key-value pairs
HashSet
ArrayList
LinkedList
HashMap
Lecture Ticket C6
public class Section {
private Collection<Topic> topics;
public Section(Collection<Topic> topics) {
this.topics = topics;
}
}public class Topic {
private Text text;
private Video video;
public Topic(Text text, Video video) {
this.text = text;
this.video = video;
}
}
Lecture Lab
We're not going to run the code today. Instead:
- Write down some course recommendations of the form: If you've taken Course A, you should consider taking Course B.
- Write code for a
Map<Course, Set<Course>>representing those recommendations. (Create all the objects and their associations.)
Do NOT use the existing CourseRecommender class!
- Give good reasons why we chose a
Mapabove and why we might choose each of aSet(as above) or aListinstead.
Sample call to Course constructor:
new Course("CPSC", 210)
Sample Solution
Recommendations:
- CPSC 210: CPSC 213, CPSC 221, CPSC 310
- CPSC 103: CPSC 107, CPSC 203
Map<Course, Set<Course>> recs = new HashMap<>();
Course c103 = new Course("CPSC",103), c107 = new Course("CPSC",107),
c203 = new Course("CPSC",203), c210 = new Course("CPSC",210), c213 = new Course("CPSC",213),
c221 = new Course("CPSC",221), c310 = new Course("CPSC",310);
// We show two ways to establish a recommendation set for learning..
// but consistency would be better in reality :)
Set<Course> courses = new HashSet<>();
courses.add(c213);
courses.add(c221);
courses.add(c310);
recs.put(c210, courses);
recs.put(c103, new HashSet<>());
recs.get(c103).add(c107);
recs.get(c103).add(c203);- A
Mapgets us easily from a taken to its recommended follow-ups - A
Setensures the follow-ups for a given course are unique - A
Listwould let us preserve order (like for ranked recommendations).
C6: Working with Collections
The End - Thank You!
C6 Working with Collections
By Steven Wolfman
