Mini-Project 4 : Associative arrays

Introduction

An associative array is a common data structure, similar to the association lists that you may have learned about in CSC-151. Since it’s a data structure, we’ll start by considering the layout of the associative array in memory.

In essence, an associative array is an (expandable) array of key/value pairs, intended to support looking up values by key. I think of them something like the following:

        +---+
  size: | 3 |
        +---+
   
  pairs:
  +---+                                                +---+---+
  | *-------------------------------------------------> | * | * |
  +---+                                +---+---+        +-|-+-|-+
  | *--------------------------------> | * | * |          |   v 
  +---+              +---+---+         +-|-+-|-+          | +----------+
  | *--------------> | * | * |           |   v            | | V value0 |
  +---+              +-|-+-|-+           | +----------+   | +----------+
  | / |                |   v             | | V value1 |   v            
  +---+                | +----------+    | +----------+ +--------+  
  | / |                | | V value2 |    |              | K key0 |   
  +---+                | +----------+    v              +--------+      
  | / |                |               +--------+                   
  +---+                |               | K key1 |                   
  |   |                v               +--------+                   
    .                +--------+
    .                | K key2 |
    .                +--------+

That is, we have an array of Key/Value pairs along with an accompanying size field to keep track of how many pairs are in the array. The array may have some empty space, which we fill with null values. In the diagram, we’ve put all the nulls at the end. However, you might decide that it’s more natural to leave some nulls in the middle.

Associative arrays are most frequently used to implement the Dictionary or Map abstract data types. Both are names for structures that allow you to store associate values with corresponding keys. (Each key has only one value; multiple valeus may have the same key.)

The central for such types include the following:

void set(K key, V value)

Set the value associated with a given key. If there is already another value associated with the given key, this new value replaces that value.

V get(K key)

Get the value associated with a given key. If there is no such key, throws an exception.

boolean hasKey(K key)

Determines if the given key appears in the associative array.

void remove(K key)

Remove the key/value pair associated with the given key. If the key does not appear in the associative array, does nothing.

int size()

Determine how many key/value pairs are currently stored in the associative array.

String toString()

Return a string of the form "{ key0: value0, key1: value1, ... keyn: valuen }" If the array is empty, you should return "{}".

As you might expect, the first four procedures will need to iterate the array of key/value pairs, stopping when they find a matching key or run out of pairs. We normally use the equals method to determine matching keys.

The assignment

1. Add at least three tests, including one edge case, to the shared AssociativeArrayTests.java shared repository. Please name them with a form like yourNameTest1(), yourNameTest2(), and yourNameEdge1(). For example, mine would be samuelRebelskyTest1(), samuelRebelskyTest2(0, and samuelRebelskyEdge1().

2. Implement an AssociativeArray<K,V> class in Java. You may not use any other Java classes that provide similar features; you must rely on an underlying plain Java array.

Starter code

• KeyNotFoundException.java
• AssociativeArray.java
• KVPair.java
• AssociativeArrayExperiments.java`
  • ReportingAssociativeArray.java

Rubric

Redo or above

Submissions that fail to meet any of these requirements will get an I.

[ ] Includes the specified `.java` files, correctly named.  (They should
    be in the appropriate package.)
[ ] Each class has an introductory Javadoc comment that indicates
    the author and purpose. 
[ ] Includes a `README.md` file that contains the appropriate information 
    (authors, purpose, acknowledgements if appropriate)
[ ] All files compile correctly.

Meets expectations or above

Submissions that fail to meet any of these requirements but meet all previous requirements will receive an R.

[ ] Appears to follow Google Java style guidelines for indentation and such.
[ ] Passes all of the tests.
[ ] Added three tests to the `AssociativeArrayTests.java` file, including
    at least one edge case (preferably named as such).
[ ] The `toString()` method appears to behave correctly.

Exemplary / Exceeds expectations

Submissions that fail to meet any of these requirements but meet all previous requirements will receive an M.

[ ] All (or most) repeated code has been factored out into individual 
    methods.  
[ ] All or most variable names are appropriate.
[ ] Handles `null` keys in `set`, `get`, and `hasKey`.  Since `set` 
    and `hasKey` don't throw exceptions, you can choose whether to
    ignore or permit `null` keys.
[ ] Permits `null` values in `set` and `get`.

Q&A