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CS1332 Homework 3: Hashing Solution
CS 1332 Section C
1 Important
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.
5. Do not use anything that would trivialize the assignment. (e.g. don’t import/use java.util.ArrayListfor an ArrayList assignment. Ask if you are unsure.)
6. Always consider the efficiency of your code. Even if your method is O(n), traversing the structure multiple times is considered inefficient unless that is absolutely required (and that case is very rare).
7. You must submit your source code - the .java files. Do not submit compiled code - the .class files.
8. Only the last submission will be graded. Make sure your last submission has all required files. Resubmitting voids prior submissions.
2 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 it located on Canvas. A point is deducted for every style error that occurs. Please double check before you submit that your code is in the appropriate style so that you don’t lose any unnecessary points!
2.1 Javadocs
Javadocs should be written for any private helper methods that you create. They should follow a style similar to the existing javadocs on the assignment. 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.
2.2 Vulgar/Obscene Language
Any submission that contains profanity, vulgar, or obscene language will receive an automatic zero on the assignment. This policy applies to all aspects of your code, such as comments, variable names, and javadocs.
1
3 FORBIDDEN STATEMENTS
2.3 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”, ”Oof - Bad things are happening”, and ”FAIL” are not good messages. Additionally, the name of an exception itself is not a good message.
2.4 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.
3 Forbidden Statements
• package
• System.arraycopy()
• clone()
• assert()
• Arrays class
• Thread class
• Collections class
• Collection.toArray()
• Refleciton APIs
• Inner or nested classes
• Lambda Expressions
• Method References (using the :: operator to obtain a reference to a method)
4 JUnits
5 DELIVERABLES
5 Deliverables
You must submit all of the following file(s) to the corresponding assignment on Gradescope. Make sure all file(s) listed below are in each submission, as only the last submission is graded. Make sure the filename(s) matches the filename(s) below, and that only the following file(s) are present.
1. HashMap.java
5.1 Hash Map
You are to code an HashMap, 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.
5.1.1 Capacity
The starting capacity of the HashMap using the default constructor should be INITIAL CAPACITY defined in HashMap.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 resizeBackingTable that you should use for resizing.
5.1.2 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 MapEntry 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.
5.1.3 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 MapEntry 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 MapEntry.java to see what is stored and what methods are available for use; do not use Java’s LinkedList to handle the chaining functionality.
5.1.4 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 MapEntry that represents the head of a linked list. That linked list contains all elements that collide at that index.
6 GRADING
5.1.5 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.
6 Grading
Here is the grading breakdown for the assignment. There are various deductions not listed that are incurred when breaking the rules listed in the PDF and in other various circumstances. Note that for this assignment efficiency is explicitly mentioned as contributing to your point total. In contrast to past assignments, unimplemented methods will result in losing efficiency points.
• HashMap − 80pts
• Efficiency − 10pts
• Checkstyle − 10 pts
• Total: 100 pts
1 Important
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.
5. Do not use anything that would trivialize the assignment. (e.g. don’t import/use java.util.ArrayListfor an ArrayList assignment. Ask if you are unsure.)
6. Always consider the efficiency of your code. Even if your method is O(n), traversing the structure multiple times is considered inefficient unless that is absolutely required (and that case is very rare).
7. You must submit your source code - the .java files. Do not submit compiled code - the .class files.
8. Only the last submission will be graded. Make sure your last submission has all required files. Resubmitting voids prior submissions.
2 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 it located on Canvas. A point is deducted for every style error that occurs. Please double check before you submit that your code is in the appropriate style so that you don’t lose any unnecessary points!
2.1 Javadocs
Javadocs should be written for any private helper methods that you create. They should follow a style similar to the existing javadocs on the assignment. 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.
2.2 Vulgar/Obscene Language
Any submission that contains profanity, vulgar, or obscene language will receive an automatic zero on the assignment. This policy applies to all aspects of your code, such as comments, variable names, and javadocs.
1
3 FORBIDDEN STATEMENTS
2.3 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”, ”Oof - Bad things are happening”, and ”FAIL” are not good messages. Additionally, the name of an exception itself is not a good message.
2.4 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.
3 Forbidden Statements
• package
• System.arraycopy()
• clone()
• assert()
• Arrays class
• Thread class
• Collections class
• Collection.toArray()
• Refleciton APIs
• Inner or nested classes
• Lambda Expressions
• Method References (using the :: operator to obtain a reference to a method)
4 JUnits
5 DELIVERABLES
5 Deliverables
You must submit all of the following file(s) to the corresponding assignment on Gradescope. Make sure all file(s) listed below are in each submission, as only the last submission is graded. Make sure the filename(s) matches the filename(s) below, and that only the following file(s) are present.
1. HashMap.java
5.1 Hash Map
You are to code an HashMap, 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.
5.1.1 Capacity
The starting capacity of the HashMap using the default constructor should be INITIAL CAPACITY defined in HashMap.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 resizeBackingTable that you should use for resizing.
5.1.2 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 MapEntry 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.
5.1.3 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 MapEntry 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 MapEntry.java to see what is stored and what methods are available for use; do not use Java’s LinkedList to handle the chaining functionality.
5.1.4 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 MapEntry that represents the head of a linked list. That linked list contains all elements that collide at that index.
6 GRADING
5.1.5 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.
6 Grading
Here is the grading breakdown for the assignment. There are various deductions not listed that are incurred when breaking the rules listed in the PDF and in other various circumstances. Note that for this assignment efficiency is explicitly mentioned as contributing to your point total. In contrast to past assignments, unimplemented methods will result in losing efficiency points.
• HashMap − 80pts
• Efficiency − 10pts
• Checkstyle − 10 pts
• Total: 100 pts
1 file (133.3KB)
