Homework 6: Hash Maps Solution

Important




Due: See T-Square




There are general homework guidelines you must always follow. If you fail to follow any of the following guidelines you risk receiving a 0 for the entire assignment.




All submitted code must compile under JDK 8. This includes unused code, so don't submit extra les that don't compile. Any compile errors will result in a 0.



Do not include any package declarations in your classes.



Do not change any existing class headers, constructors, instance/global variables, or method sig-natures.



Do not add additional public methods.



Do not use anything that would trivialize the assignment. (e.g. don't import/use java.util.ArrayList for an Array List assignment. Ask if you are unsure.)



Always be very conscious of e ciency. Even if your method is to be O(n), traversing the structure multiple times is considered ine cient unless that is absolutely required (and that case is extremely rare).



You must submit your source code, the .java les, not the compiled .class les.



After you submit your les, redownload them and run them to make sure they are what you intended to submit. You are responsible if you submit the wrong les.



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 resized to have a capacity of 2n + 1 where n is the current capacity 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.




There are two constructors in HashMap.java. As per the javadocs, you should use constructor chaining to implement the no-arg constructor.




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, mod by table length rst, then take the absolute value (it must be done in this order to prevent over ow in certain cases).




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.






1
Homework 6: Hash Maps Due: See T-Square










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:






put
17pts




remove
15pts




get
10pts




containsKey
10pts




keySet
5pts




values
5pts




resizeBackingTable
8pts




clear
5pts




Other:






Checkstyle
10pts




E ciency
15pts




Total:
100pts







Keep in mind that some functions are dependent on others to work, such as remove methods requiring the add methods to work. Also, the size function is used many times throughout the tests, so if the size isn't updated correctly, many tests can fail.




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 o a point for every style error that occurs. If you feel like what you wrote is in accordance with good style but still sets o the style checker please email Raymond Ortiz (rortiz9@gatech.edu) with the subject header of \CheckStyle XML".




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.




Exceptions




When throwing exceptions, you must include a message by passing in a String as a parameter. The mes-sage 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.







2
Homework 6: Hash Maps Due: See T-Square










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.




break may only be used in switch-case statements




continue package




System.arraycopy() clone()




assert()




Arrays class Array class




Collections class




Collection.toArray()




Re ection APIs




Inner or nested classes Lambda Expressions Method References




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




Debug print statements are ne, 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 o points.




Provided




The following le(s) have been provided to you. There are several, but you will only edit one of them.
















3
Homework 6: Hash Maps Due: See T-Square










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 le.




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 vari-ables, or static variables.




MapEntry.java



This class stores a key-value pair and a removed attribute for your hash map. Do not alter this le.




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.







Deliverables




You must submit all of the following le(s). Please make sure the lename matches the lename(s) below, and that only the following le(s) are present. T-Square does not delete les from old uploads; you must do this manually. Failure to do so may result in a penalty.




After submitting, be sure you receive the con rmation email from T-Square, and then download your uploaded les 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.




HashMap.java


























































































4