Lab 9: Hash Maps

In this lab, you'll create MyHashMap, an implementation of the Map61B interface, which represents a hash map. This will be very similar to Lab 8, except this time we're building a Hash Map rather than a Tree Map.

After you've completed your implementation, you'll compare the performance of your implementation to a list-based Map implementation ULLMap as well as the built-in Java HashMap class (which also uses a hash table).

1: MyHashMap

Create a class MyHashMap that implements the Map61B interface. You must do this in a file named Your implementation is required to implement all of the methods given in Map61B except for remove. For this methods you should throw an UnsupportedOperationException. Note that you should implement keySet and iterator this time. For these methods, we recommend you simply create a HashSet instance variable that holds all your keys.

Additionally, you should implement the following constructors:

public MyHashMap();
public MyHashMap(int initialSize);
public MyHashMap(int initialSize, double loadFactor);

You should increase the size of your HashMap when the loadFactor exceeds some number of your choice, unless the HashMap was instantiated with the loadFactor parameter, in which case you should use that number. Your Hashmap should initially have a number of buckets equal to initialSize. You are not required to resize down. When resizing, make sure to multiplicatively increase the size, not additively (e.g. multiply by 2, don't add 100 or something). Your HashMap operation should all be constant amortized time, assuming that the hashCode of any objects inserted spread things out nicely.

You should handle collisions by chaining. You may not import any libraries other than ArrayList, LinkedList, HashSet, iterator and Set.

You can test your implementation using the TestMyHashMap class in the lab9tester package.

You may find the following resources useful:

2: So... How Fast Is It (Redux)?

There are two interactive speed tests provided in and Do not attempt to run these tests before you've completed MyHashMap. Once you're ready, you can run the tests in IntelliJ.

The InsertRandomSpeedTest class performs tests on element-insertion speed of your MyHashMap, ULLMap (provided), and Java's built-in HashMap. It works by asking the user for an input size N, then generates N Strings of length 10 and inserts them into the maps as pairs.

Try it out and see how your data structure scales with N compared to the naive and industrial-strength implementations. Record your results in a file named speedTestResults.txt. There is no standard format required for your results, and there is no required number of data points.

Now try running InsertInOrderSpeedTest, which behaves similarly to InsertRandomSpeedTest, except this time the Strings in <String, Integer> key-value pairs are inserted in lexicographically-increasing order. Note that unlike lab8, your code should be in the rough ballpark of Java's built in solution -- say within a factor of 10 or so. What this tells us is that state-of-the-art HashMaps are relatively easy to implement compared to state-of-the-art TreeMaps.