Programming

Java Review¶

Map Interface¶

Although our map implementations will include essentially all the functionality of Java’s Map interface, you will only need to implement a few of its methods—all the others will have default implementations that use the methods you implement.

Iterator¶

An iterator is a type of object that lets a client efficiently iterate over a data structure using a forEach loop. Whenever we write code like:

for (String item : something) {
    // ...etc...
}

Java will internally convert that code into the following:

Iterator<String> iter = something.iterator();
while (iter.hasNext()) {
    String item = iter.next();
    // ...etc...
}

When you call iter.next for the first time, the iterator will return the first item. If you call it again, it’ll return the second item.

If the user calls iter.next after the iterator has gone through all items, the method will throw a NoSuchElementException. iter.hasNext helps avoid this by returning true if calling iter.next will safely return a value, and false otherwise.

Notes and Requirements¶

• You may NOT create any new temporary data structures inside of your iterators.
• Your iterator methods must run in constant time with respect to the size of the data structure.
• Your iterators may return entries in any order although the easiest approach is to return them in the same order as your map’s internal representation.

• You may assume that the user will not modify your maps while your iterators are in use.

  For example, the following will never happen:

  Iterator<Entry<String, Integer| itr = map.iterator();
  itr.next();
  // the following line should never happen if the same
  // iterator instance is used later
  map.put("hi", "373");
  itr.next();
  

ArrayMap¶

Overall Design¶

Your ArrayMap class will internally keep track of its key-value pairs by using an array of Map.Entry<K, V> objects. For example, after running the following code:

Map<String, Integer> map = new ArrayMap<>();
map.put("a", 11);
map.put("b", 22);
map.put("c", 33);

Your map’s internal array should look like this:

And if we add a few more entries:

map.put("d", 44);
map.remove("b");
map.put("a", 55);

Your internal array should now look like the image below.

There are a few important things to notice:

1. We’ve updated the old entry for “a” to store the new value.
2. We’ve replaced the entry for “b” with the one for “d”. Maps are inherently unordered so when removing an entry, we can simply replace it with the last entry in the array. For example, this is what the call to map.remove("b") would do in the example above:

Your implementation should include the following constructors:

Implementation Details¶

Requirements¶

• All methods must run in $\mathcal{O}(n)$ time, where $n$ is the size of the map.
• size() and iterator() must run in $\mathcal{O}(1)$ time.
• The iterator() function must return an iterator that adheres to all the requirements listed in the Iterator section.

Notes¶

• You are not required to downsize your array if the user removes many elements.
• Your internal array should store elements of type SimpleEntry<K, V> (take a look at SimpleEntry.java for more details). SimpleEntry<K, V> implements Entry<K, V>, so you can simply return your map’s SimpleEntry objects when implementing Iterator.next.

• The test files use AssertJ’s Map assertions. Here’s a summary of how each assertion method calls the methods you implement:

  Methods called by AssertJ assertions

  AssertJ Map assertion	Map method
  hasSize/hasSizeGreaterThan	size
  containsKey/doesNotContainKey	containsKey
  containsEntry	get (and containsKey if value is null)
  containsAllEntriesOf	get (and containsKey if value is null)
  containsExactly	iterator
  containsExactlyInAnyOrderEntriesOf	iterator

  AssertJ Iterator assertion	Iterator method
  hasNext/isExhausted	hasNext

Tips¶

• If the array is full and a new key is inserted, create a new array and copy over the old elements.
• If you need to check object equality but can’t guarantee that either object is non-null, use Objects.equals(a, b) under java.util—it does all the necessary null checks for you.
• By default, IntelliJ’s debugger may attempt to show the entries of your map instead of its fields. To disable this feature, open the project settings and navigate to Build, Execution, Deployment | Debugger | Data Views | Java, then deselect Enable alternative view for Collection classes.

ChainedHashMap¶

Overall Design¶

When we covered separate chaining in lecture, we used a list as the chaining data structure. In this assignment, instead of a list, you will reuse your own ArrayMap!

When you first create your array of chains, it will contain only null references. As entries are inserted into the table, you need to create the chains (ArrayMaps) as required. Let’s say you created a chain array of size 5 (you can choose any initial size), and you inserted the key “a” with the value 11.

Map<String, Integer> map = new ChainedHashMap<>();
map.put("a", 11);

In this example, the key “a” lands in index 2, but the index could change depending on your table size. Also, in this example, the ArrayMap (chain) has size 3, but you can choose a different initial size for your ArrayMap.

Now suppose you insert a few more keys:

map.put("f", 13);
map.put("c", 12);

Your internal hash table should now look like the figure below. In this example, keys “a” and “f” both hash to the same index (2).

Your implementation should include the following constructors:

ChainedHashMapIterator¶

As part of ChainedHashMap, you will have to implement your iterator class (we already wrote a placeholder class for you called ChainedHashMapIterator):

Remember that the actual elements of your ChainedHashMap are stored in the internal ArrayMap chains. This mean that you have to use the individual ArrayMap iterators to obtain elements. You also have to keep track of their states and advance to the next chain’s iterator when the current one runs out.

Before you write any iterator code:

• Try designing an algorithm using pencil and paper, and run through a few examples by hand i.e. draw the chains array that has some varying number of ArrayMap objects scattered throughout, and simulate what your algorithm does.
• Think about the invariants of your ChainedHashMap.

Implementation Details¶

Requirements¶

• All methods should run in $\mathcal{O}(1)$ time in practice (except when resizing, in which case put may run in $\Theta(n)$ time, where $n$ is the size of the data structure).
• The iterator must adhere to all the requirements listed in the Iterator section.

Notes¶

• The 0-argument constructor requires you to determine and define some reasonable defaults in the final fields at the top of the class.
• The second constructor has some parameters to configure the behavior of the map:
  • resizingLoadFactorThreshold: when the load factor exceeds this value, the array of chains resizes.
  • initialChainCount: the initial number of chains for your hash table
  • chainInitialCapacity: the initial capacity of each ArrayMap chain created by the map
• If your ChainedHashMap receives a null key, use a hashcode of 0 for that key.
• Do not try to implement your own hash function. Use Java’s Object.hashCode() method instead—e.g., int hashCode = key.hashCode().
• Use the provided createChain method to instantiate your ArrayMap, rather than calling the ArrayMap constructor.

Tips¶

• We recommend doubling the number of chains when resizing.
• You are NOT required to downsize your hash table if the user removes many elements.
• You should call the iterator method on each ArrayMap inside your chains array.
• You may add as many extra fields as you need to track your iterator’s state.

Good use of invariants can greatly reduce your code complexity by reducing the number of cases you need to consider. For example, as you (hopefully) found in the previous assignment, ensuring that a certain field is never null allows you to omit null checks.

Much of the complexity in your code execution will arise from handling edge cases—the empties and nulls and such. Think about what invariants you should uphold in both your main ChainedHashMap code and in your iterator. Obviously, this might involve tweaking the way your ChainedHashMap stores data, but it’s certainly worthwhile if you can reduce the overall complexity of your code.

For example, here are a couple invariants that you might include:

• Each index in the array of chains is null if and only if that chain has no entries.
• The currentChain field of the iterator always references the current chain being iterated through (the chain which contains the next entry that next will return).
• The currentChain field is null after the iterator has been exhausted of all entries.

Submission¶

If you’re working in a group, the partner who submits must add the other partner as a group member on Gradescope. Here’s a video demonstrating that process. You’ll also need to re-add group members whenever you resubmit to the same assignment, even if you already did so on a previous submission.