1. JCF Hashmap & Hashset
Winter 2005
CS-2851 Dr. Mark L. Hornick

2. JCF HashMap stores <Key, Value> pairs in a hash table
Example: HashMap<String, Student> students; //String is the key //Student is the value
Key/Value pairs are stored in a Map.Entry object
public class HashMap<K, V>
implements Map<K, V>
extends AbstractMap<K, V>
Winter 2005
CS-2851 Dr. Mark L. Hornick

3. JCF HashMap Implements methods such as
put(k,v) – insert value v with key k into the table
get(k) – return the value associated with key k
remove(k) – remove entry associated with key k
containsKey(k) – search for entry with key k
containsValue(v) – search for entry with value v
size()
clear()
Winter 2005
CS-2851 Dr. Mark L. Hornick

4. HashMap’s advantage is overall performance
Specifically, performance for the following operations is O(k); that is, constant time:
put(k,v) – insert value v with key k into the table
get(k) – return the value associated with key k
remove(k) – remove entry associated with key k
containsKey(k) – search for entry with key k
Winter 2005
CS-2851 Dr. Mark L. Hornick

5. Performance is gained in exchange for memory utilization efficiency
Hash tables map keys to table indices
Some keys map to the same table index
Some indices remain unmapped
Winter 2005
CS-2851 Dr. Mark L. Hornick

6. The JCF HashMap collision handling mechanism
At index 873 in the table, store the linked list of all elements whose keys hash to 873
This is called chaining
It implements a simple singly-linked list
Winter 2005
CS-2851 Dr. Mark L. Hornick

7. As chains get long, performance degrades to O(M)
M is the number of entries chained together
To maintain good performance, once a JCF HashMap becomes 75% full, it is resized
All indices are recalculated
Chains are removed or reduced
Winter 2005
CS-2851 Dr. Mark L. Hornick

8. Winter 2005
CS-2851 Dr. Mark L. Hornick

9. Winter 2005
CS-2851 Dr. Mark L. Hornick

10. Another collision handler
In Open Address hashing, when a collision occurs, the next available index is used to store the key/value
This leads to some interesting practical implementation problems (see the text)
Winter 2005
CS-2851 Dr. Mark L. Hornick

11. HashSets
Winter 2005
CS-2851 Dr. Mark L. Hornick

12. A HashSet is an unordered Collection in which the Value is also the Key
All Values in a HashSet must be unique
Since Values are the Keys
The HashSet class has all of the methods in the Collection interface (rather than Map)
add, remove, size, contains, …
plus toString (inherited from AbstractCollection)
Winter 2005
CS-2851 Dr. Mark L. Hornick