Homework 6: HashMap Solution

HashMap




You are to code a LinearProbingHashMap, a key-value hash map with a linear probing collision reso-lution 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.




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 le, 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 speci c instructions on when to resize.




Do not use magic numbers in your code. That is, use the provided INITIAL CAPACITY in your code rather than hardcoding its values.







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 rst, then take the absolute value (it must be done in this order to prevent over ow in certain cases). As a reminder, you should be




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Homework 6: HashMap Due: See Canvas










using the hashCode() method on only the keys (and not the LinearProbingMapEntry 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 t in the array backing the HashMap.




Linear Probing




Your hash map must implement a linear probing collision policy. If the index corresponding to the hash value of the key is occupied, probe in linear increments. For example, if the hash value of your key is 7 with a backing array of capacity 9, and index 7 mod 9 in the array is occupied, check index 7 + 1 mod 9, then 7 + 2 mod 9, then 7 + 3 mod 9, etc. until you hit a null spot in the array or after you have attempted table.length probes.




Adding Items




When adding a key/value pair to a HashMap, add the pair to the array in the correct position. Also remember that keys are unique in a hash map, so you must ensure that duplicate keys are not added. When searching for a spot to add, after ensuring no duplicates, you should add at the rst encountered removed spot (if there are any). If no removed spots were encountered, add at the null spot that terminated your search.




Removing Items




Since linear probing is an open addressing scheme, you should not set removed entries to null. Instead, you need to implement a \soft remove" using the removed ag in LinearProbingMapEntry.java. All the ag does is keeps track of what entries have been removed, but you need to implement the logic for what to do with the removed entries. Though the objects may still be in memory, as far as the user is concerned, the data has been removed, and your code’s behavior should re ect this expectation. Do not null out the key and value of removed entries.




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
3pts




put
17pts




remove
20pts




get
10pts




contains
10pts




keySet
5pts




values
5pts




resizeBackingTable
5pts




Other:






Checkstyle
10pts




E ciency
15pts




Total:
100pts







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







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Homework 6: HashMap Due: See Canvas










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.




Collaboration Policy




Every student is expected to read, understand and abide by the Georgia Tech Academic Honor Code.




When working on homework assignments, you may not directly copy code from any source (other than your own past submissions). You are welcome to collaborate with peers and consult external re-sources, but you must personally write all of the code you submit. You must list, at the top of each le in your submission, every student with whom you collaborated and every resource you consulted while completing the assignment.




You may not directly share any les containing assignment code with other students or post your code publicly online. If you wish to store your code online in a personal private repository, you can use Github Enterprise to do this for free.




The only code you may share is JUnit test code on a pinned post on the o cial course Piazza. Use JUnits from other students at your own risk; we do not endorse them. See each assignment’s PDF for more details. If you share JUnits, they must be shared on the site speci ed in the Piazza post, and not anywhere else (including a personal GitHub account).




Violators of the collaboration policy for this course will be turned into the O ce of Student Integrity.




Style and Formatting




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.’’);







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Homework 6: HashMap Due: See Canvas










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()




Re ection 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 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 we’ve noted the ones to edit.




LinearProbingHashMap.java



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




LinearProbingMapEntry.java



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










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Homework 6: HashMap Due: See Canvas










LinearProbingHashMapStudentTest.java



This is the test class that contains a set of tests covering the basic operations on the LinearProbingHashMap 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). Make sure all le(s) listed below are in each submission, as only the last submission will be graded. Make sure the lename(s) matches the lename(s) below, and that only the following le(s) are present. The only exception is that Canvas will automatically append a -n depending on the submission number to the le name(s). This is expected and will be handled by the TAs when grading as long as the le name(s) before this add-on matches what is shown below. If you resubmit, be sure only one copy of each le is present in the submission. If there are multiple les, do not zip up the les before submitting; submit them all as separate les.




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




LinearProbingHashMap.java
























































































































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