CS 1332 Homework 6: Hash Maps




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Hash Maps
In this homework, you will implement a key-value hash map with an external chaining collision policy.
A hash map maps keys to values and allows O(1) average case lookup of a value when the key is known.
This hash map must be backed by an array of initial size 11, and must be resized to have a size of 2n+ 1
when the table exceeds (greater than, not greater than or equal to) a load factor of 0.67. These values
are provided as constants in the interface, and the constants should be used within your code.
The table should not add duplicate keys, but duplicate values are acceptable. In the event of a
duplicate key, replace the existing value with the new value.
Neither keys nor values may be null.
Hash Functions
You should not write your own hash functions for this assignment; use the hashCode() method (every
Object has one). If this is a negative value, use the absolute value of the hash code (and then mod by
the table length).
External Chaining
Your hash map must implement an external chaining collision policy. If the hash value of the key is
occupied and the key is not already in the map, add the key to the front of the chain.
Adding Items
When adding a key/value pair to a hash map, add the pair to 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.
Homework 6: Hash Maps Due: See T-Square
A note on JUnits
We have provided a very basic set of tests for your code, in HashMapStudentTests.java. These tests
do not guarantee the correctness of your code (by any measure), nor does it 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 T-Square under Resources, to help
you run JUnits on the command line or in IntelliJ.
Style and Formatting
It is important that your code is not only functional but is also written clearly and with good style. We
will be checking your code against a style checker that we are providing. It is located in T-Square, under
Resources, along with instructions on how to use it. We will take off a point for every style error that
occurs. If you feel like what you wrote is in accordance with good style but still sets off the style checker
please email Jonathan Jemson (careyjmac@gatech.edu) with the subject header of “CheckStyle XML”.
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
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 HAPPENED”,
and “fail” are not good messages. The name of the exception itself is not a good message.
For example:
throw new PDFReadException(“Did not read PDF, will lose points.”);
throw new IllegalArgumentException(“Cannot insert null data into data structure.”);
If available, use the generic type of the class; do not use the raw type of the class. For example, use new
LinkedList() instead of new LinkedList(). Using the raw type of the class will result in a
Forbidden Statements
You may not use these in your code at any time in CS 1332.
• break may only be used in switch-case statements
• continue
• package
• System.arraycopy()
• clone()
Homework 6: Hash Maps Due: See T-Square
• assert()
• Arrays class
• Array class
• Collections class
• Collection.toArray()
• Reflection APIs
• Inner, nested, or private classes
Debug print statements are fine, but nothing should be printed when we run them. We expect clean
runs – printing to the console when we’re grading will result in a penalty. If you use these, we will take
off points.
The following file(s) have been provided to you. There are several, but you will only edit one of them.
1. HashMapInterface.java
This is the interface you will implement in HashMap. All instructions for what the methods should
do are in the javadocs. Do not alter this file.
2. HashMap.java
This is the class in which you will implement HashMapInterface. Feel free to add private helper
methods but do not add any new public methods, inner/nested classes, instance variables, or static variables.
3. MapEntry.java
This class stores a key-value pair and a next attribute for your hash map. Do not alter this file.
4. HashMapStudentTests.java
This is the test class that contains a set of tests covering the basic operations on the HashMap 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.
You must submit all of the following file(s). Please make sure the filename matches the filename(s)
below, and that only the following file(s) are present. T-Square does not delete files from old uploads;
you must do this manually. Failure to do so may result in a penalty.
After submitting, be sure you receive the confirmation email from T-Square, and then download your
uploaded files to a new folder, copy over the interfaces, recompile, and run. It is your responsibility to
re-test your submission and discover editing oddities, upload issues, etc.
1. HashMap.java