CS-151 Labs > Lab 8. Hash Map

Part 1. Hash Map

First you’ll implement your own hash map with separate chaining in a class called MyHashMap<K,V>. You will build your hash table on top of an array; this array should consist of an array of LinkedLists, one linked list per “bucket”. A very small amount of starter code has been provided to prevent the MarkovModel class from showing errors. You’ll need to implement those methods as well as some others.

Data Members

You’re going to use an array to store the buckets of your hash map. Because you’re using separate chaining, each of these buckets will be a linked list of elements, in fact, they’ll be a linked list of (key, value) pairs (since each element is really one such pair).

Java has an interface java.util.Map.Entry for (key, value) pairs as well as a simple class that implements the interface called java.util.AbstractMap.SimpleEntry. You’re going to be using SimpleEntry rather than making your own class. Check out the JavaDoc for SimpleEntry, but here’s an example using the relevant methods.

SimpleEntry<K, V> entry = new SimpleEntry<K, V>(key, value);
entry.getKey(); // Returns the key.
entry.getValue(); // Returns the value.
entry.setValue(newValue); // Returns the old value.

Notice that SimpleEntry is parameterized by K and V where K is the type of the key and V is the type of the value. Conveniently, MyHashMap is also parameterized by K and V!

Since our MyHashMap will use separate chaining, we’ll need the following instance members in the class.

private LinkedList<SimpleEntry<K, V>>[] table;
The array of buckets used in the hash map.
private int size;
Current number of items in the table (not the number of buckets, use table.length for that).
private float loadFactor;
Maximum permitted load factor for the table
private static final int DEFAULT_CAPACITY = 11;
private static final float DEFAULT_LOAD_FACTOR = 0.75;
Constants for the default hash map capacity and a default load factor.

Constructors

public MyHashMap(int capacity, float loadFactor)
Create a hash map with capacity buckets and a maximum load factor of loadFactor. First you need to initialize the array of linked lists using a method from the starter code: 
this.table = createTable(capacity);

This will create an array of size at least capacity that is prime. (Not every prime is included in the list of allowable sizes. In fact, each prime in the list is at least twice the size of the previous prime.)

MyHashMap()
Create a hash map with the default capacity and load factor using the constants you created above. You can do this in one line by calling the previous constructor.

Public Methods

public int size()
Return the number of items in the hash map, not the number of buckets.
public boolean isEmpty()
Return true if size is 0, false otherwise (note you do not need an if statement for this).
public void clear()
Empties out the hash map
public V put(K key, V value)
Associate the specified value with the given key.

To compute the index of the bucket (the linked list of SimpleEntry<K, V>s), use

int index = (key.hashCode() & Integer.MAX_VALUE) % this.table.length;

This convoluted line computes the hash code of the key, transforms it to a nonnegative integer (that’s what the & Integer.MAX_VALUE is doing), and then takes the remainder of dividing that result by the length of the table.

Associating the value with the key does not change the size of the hash map if the key is already in the map. If the key is not in the map, then the size increases by 1.

If the load factor threshold has been reached, call the private resize() method.

Attempts to insert null values or keys should throw a NullPointerException.

Return the previous value associated with the key, or null if there was no mapping.

public V get(K key)
Return the value associated with the key. If no value exists, return null.

This operation should not examine every index in the table, but rather, should only examine the one linked list indicated by the hash code. Use the same algorithm used in put() to find the index of the linked list.

Before continuing, you should test your MyHashMap class if you haven’t already. That is, you should create a JUnit test class called MyHashMapTest and check the methods you have so far before continuing.

public V remove(K key)
Delete the mapping (key ,value) from the hash map. Return the previous value or null if there was no such value.
public boolean containsKey(K key)
Return true if key is already in the hash map.

You should only have to examine a single bucket in order to get your answer.

public boolean containsValue(V value)
Return true if value is already in the hash map.

This may require inspection of all buckets.

public List<SimpleEntry<K, V>> entryList()
Returns a list of some sort containing all of the entries in the hash map, in any order.

Java collections like ArrayList have an addAll(otherCollection) method that adds all of the elements of otherCollection.

Private Methods

private void resize()
Dynamically resize the array.

Make a new array of at least double the capacity, and then rehash the items into the new array (not every item in a bucket-chain will have the same key and therefore will hash differently in your new array!)

This should be called whenever the maximum load factor is exceeded.

The new array size should be an prime number that’s at least twice the size of the existing array.

createTable(this.table.length * 2)

will return an appropriately sized array.

Testing

Be sure to test your hash map methods with JUnit tests before continuing. You can test your implementation by creating MyHashMap<String, Integer> and calling methods. Things to test: