External Chaining Hash Map Solved

It is important that your code is not only functional, but written clearly and with good programming style. Your code will be checked against a style checker. The style checker is provided to you, and is located on Canvas. It can be found under Files, along with instructions on how to use it. A point is deducted for every style error that occurs. If there is a discrepancy between what you wrote in accordance with good style and the style checker, then address your concerns with the Head TA.

Javadocs

Javadoc any helper methods you create in a style similar to the existing javadocs. If a method is overridden or implemented from a superclass or an interface, you may use @Override instead of writing javadocs. Any javadocs you write must be useful and describe the contract, parameters, and return value of the method. Random or useless javadocs added only to appease checkstyle will lose points.

Vulgar/Obscene Language

Any submission that contains profanity, vulgar, or obscene language will receive an automatic zero on the assignment. This policy applies not only to comments/javadocs, but also things like variable names.

Exceptions

When throwing exceptions, you must include a message by passing in a String as a parameter. The message must be useful and tell the user what went wrong. “Error”, “BAD THING HAP-PENED”, and “fail” are not good messages. The name of the exception itself is not a good message. For example:

Bad: throw new IndexOutOfBoundsException(‘‘Index is out of bounds.’’);

Good: throw new IllegalArgumentException(‘‘Cannot insert null data into data structure.’’);

Generics

If available, use the generic type of the class; do not use the raw type of the class. For example, use new LinkedList<Integer>() instead of new LinkedList(). Using the raw type of the class will result in a penalty.

Forbidden Statements

You may not use these in your code at any time in CS 1332.

    • package

    • System.arraycopy()

    • clone()

    • assert()

    • Arrays class

    • Array class

    • Thread class

    • Collections class

    • Collection.toArray()


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Homework 6: ExternalChainingHashMap    Due: See Canvas



    • Reflection APIs

    • Inner or nested classes

    • Lambda Expressions

    • Method References (using the :: operator to obtain a reference to a method)


If you’re not sure on whether you can use something, and it’s not mentioned here or anywhere else in the homework files, just ask.

Debug print statements are fine, but nothing should be printed when we run your code. We expect clean runs - printing to the console when we’re grading will result in a penalty. If you submit these, we will take off points.

JUnits

We have provided a very basic set of tests for your code. These tests do not guarantee the correctness of your code (by any measure), nor do they guarantee you any grade. You may additionally post your own set of tests for others to use on the Georgia Tech GitHub as a gist. Do NOT post your tests on the public GitHub. There will be a link to the Georgia Tech GitHub as well as a list of JUnits other students have posted on the class Piazza.

If you need help on running JUnits, there is a guide, available on Canvas under Files, to help you run JUnits on the command line or in IntelliJ.





































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Homework 6: ExternalChainingHashMap    Due: See Canvas



HashMap

You are to code an ExternalChainingHashMap, a key-value hash map with an external chaining collision resolution strategy. A HashMap maps unique keys to values and allows O(1) average case lookup of a value when the key is known.

The table should not contain duplicate keys, but can contain duplicate values. In the event of try-ing to add a duplicate key, replace the value in the existing (key, value) pair with the new value and return the old value.

You should implement two constructors for this HashMap. As per the javadocs, you should use con-structor chaining to implement the no-arg constructor.

Capacity

The starting capacity of the ExternalChainingHashMap using the default constructor should be the constant INITIAL CAPACITY defined in ExternalChainingHashMap.java. Reference the constant as-is. Do not simply copy the value of the constant. Do not change the constant. Do not regrow the backing array when removing elements.


If adding to the table would cause the load factor (LF) to exceed (greater than, not greater than or equal to) the max load factor constant provided in the java file, the table should be resized to have a capacity of 2n + 1, where n is the current capacity before adding the parameterized element. See the javadocs for specific instructions on when to resize. There is a method called resize that you should use for resizing.

Hash and Compression Functions

You should not write your own hash functions for this assignment. Instead, use the hashCode() method that every Object has. For the compression function, mod by table length first, then take the absolute value (it must be done in this order to prevent overflow in certain cases). As a reminder, you should be using the hashCode() method on only the keys (and not the ExternalChainingMapEntry object itself) since that’s what is used to look up the values. After converting a key to an integer with a hash function, it must be compressed to fit in the array backing the HashMap.

External Chaining

Your hash map must implement an external chaining collision policy. That is, in the event of a collision, colliding entries are stored as a chain of ExternalChainingMapEntry objects at that index. As such, if you need to search for a key, you’ll need to traverse the entire chain at the hashed index to look for it. See ExternalChainingMapEntry.java to see what is stored and what methods are available for use; do not use Java’s LinkedList to handle the chaining functionality.

Adding

When adding a key/value pair to a hash map, add the pair to the front of the chain in the correct position. Also remember that keys are unique in a hash map, so you must ensure that duplicate keys are not added. Each index of the table should point to an ExternalChainingMapEntry that represents the head of a linked list. That linked list contains all elements that collide at that index.

Removing

When removing a key/value pair from a hash map using external chaining, you can safely remove the item unlike in open addressing techniques such as linear probing where you must use a DEL marker. Removing from a chain is very similar to removing from a Singly-Linked List, treating the first table


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Homework 6: ExternalChainingHashMap    Due: See Canvas



entry as the head, so refer to your notes and homework assignments from earlier in the course if you need a refresher. As usual, if the entry you are removing is the only entry at that index, you should make sure to null out that spot rather than leaving it there.































































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Homework 6: ExternalChainingHashMap    Due: See Canvas



Grading

Here is the grading breakdown for the assignment. There are various deductions not listed that are incurred when breaking the rules listed in this PDF and in other various circumstances.

Methods:



constructor
2pts


put
18pts


remove
10pts


get
10pts


containsKey
10pts


keySet
5pts


values
5pts


resizeBackingTable
10pts


clear
5pts


Other:



Checkstyle
10pts


Efficiency
15pts


Total:
100pts



Provided

The following file(s) have been provided to you. There are several, but we’ve noted the ones to edit.

    1. ExternalChainingHashMap.java

This is the class in which you will implement the ExternalChainingHashMap. Feel free to add private helper methods but do not add any new public methods, inner/nested classes, instance variables, or static variables.

    2. ExternalChainingMapEntry.java

This class stores a key-value pair and the next reference for your hash map. Do not alter this file.

    3. ExternalChainingHashMapStudentTest.java

This is the test class that contains a set of tests covering the basic operations on the ExternalChainingHashMap class. It is not intended to be exhaustive and does not guarantee any type of grade. Write your

own tests to ensure you cover all edge cases.


Deliverables

You must submit all of the following file(s) to the course Gradescope. Make sure all file(s) listed below are in each submission, as only the last submission will be graded. Make sure the filename(s) matches the filename(s) below, and thatonly the following file(s) are present. If you resubmit, be sure only one copy of each file is present in the submission. If there are multiple files, do not zip up the files before submitting; submit them all as separate files.

Once submitted, double check that it has uploaded properly on Gradescope. To do this, download your uploaded file(s) to a new folder, copy over the support file(s), recompile, and run. It is your sole responsibility to re-test your submission and discover editing oddities, upload issues, etc.



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Homework 6: ExternalChainingHashMap    Due: See Canvas



    1. ExternalChainingHashMap.java

































































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