CS201 - Spring 2014 - Class 31
- look at OpenAddressedHashtable class in
- what is the "put" method doing?
- write the "contains" method
- notice that the class is abstract since we haven't defined what the probe sequence will be
- there are some cases where we want to make sure that a class has a particular method (e.g. probSequence), however, we know that there are many different ways that could be implemented
- we can declare a method to be "abstract", which means:
- we can declare the method header (parameters, name and return type)
- we can use the method in the code
- HOWEVER: we must also declare the class as abstract, which means it cannot be instantiated
- how does this help us?
- classes that inherit the class can define the method
- look at LinearAddressedHashtable class in
- extends OpenAddressedHashtable
- has a non-abstract (normal) probeSequence method
- since it doesn't have any abstract methods anymore, we can instantiate this class
deleting in open addressing
- what is the challenge with deleting in open addressing?
- let's us linear probing (but it happens regardless of probing scheme)
- let h(x) = h(y) = h(z)
- insert x, y and z
- delete y
- now search for z. What happens?
- we won't find z because we will stop our search when we find an empty entry
- besides just being occupied or not occupied, keep track if it was deleted
- for inserting, if we find a deleted item, fill it in
- in searching if we find a deleted item, keep searching
- any problems with this approach?
- if we delete a lot of items, our search times can remain large even though our table isn't very full
- in general, if you plan on doing a lot of deleting, use a chained hashtable
- so far, we've just talked about storing sets of things
- often, we want to store store things based on a key, but we want to store some data/value associated with that key
- social security number
- name, address, etc.
- bank account number
- counting the number of times a word occurs in a document
- key is the word
- data/value is the frequency
- look at Map interface in
- similar to Set
- the put method has a value as well
- the get method instead of containsKey, which returns a value
- how would this change the code?
- need to store both the key and the value
- all the hashing is still based on the key; the value is just a tagalong item
hashtables in java
- Set interface (
- HashSet (
- what do you think TreeSet and HashSet look like?
- Map interface (
- HashMap (
- Hashtable (sort of like ArrayList and Vector)