Description

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.
1. All submitted code must compile under JDK 11. This includes unused code, so don’t submit extra files that don’t compile. Any compile errors will result in a 0.
2. Do not include any package declarations in your classes.
3. Do not change any existing class headers, constructors, instance/global variables, or method signatures. For example, do not add throws to the method headers since they are not necessary.
4. Do not add additional public methods.
5. Do not use anything that would trivialize the assignment. (e.g. Don’t import/use java.util.ArrayList for an ArrayList assignment. Ask if you are unsure.)
6. Always be very conscious of efficiency. Even if your method is to be O(n), traversing the structure multiple times is considered inefficient unless that is absolutely required (and that case is extremely rare).
7. You are expected to implement all of the methods in this homework. Each unimplemented method will result in a deduction.
8. You must submit your source code, the .java files, not the compiled .class files.
9. Only the last submission will be graded. Make sure your last submission has all required files. Resubmitting will void all previous submissions.
10. After you submit your files, redownload them and run them to make sure they are what you intended to submit. You are responsible if you submit the wrong files.
Style and Formatting
Javadocs
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 HAPPENED”, 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
• package
• System.arraycopy()
• clone()
• assert()
• Arrays class
• Array class
• Thread class
• Collections class
• Collection.toArray()
• 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
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.
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 trying 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 constructor 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 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.
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 that only 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.
1. ExternalChainingHashMap.java