CSE 373 Homework #6: Hashing It Out

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In this assignment, you will practice different hashing concepts including different collision resolution techniques, implementing hashCode for different classes, and implementing the Map ADT backed by a hash table
that uses separate chaining as its collision resolution strategy.
Part A should be submitted electronically in a file named README.txt. Part B should be submitted electronically in a file named HashMap.java. For Part B, you will also need the additional supporting files:
Map.java (interface to implement), HashMap.java (a partially implemented class to which you should add the
get and remove methods), HashMapEntry.java (a class that models a key  value pair that can be inserted
into the HashMap), and HashMapClient.java (a client program containing a couple of testing methods).
Part A: Hashing Concepts
1. Given input {66, 53, 287, 96, 81, 941, 3}, a hash table of size 10, no rehashing, and a hash function h(x)
= x (mod 10), show the resulting:
a) Separate chaining hash table
b) Hash table using linear probing
c) Hash table using quadratic probing
d) Hash table with second hash function h2(x) = 7 – (x mod 7)
For each result, list 10 items—use a dash (-) to represent an empty element, e.g.
– 66 53 287 96 – 81 941 – 3
Here, element 0, 5, and 8 are empty.
For separate chaining, write the chain out as an array, e.g.
– [66, 53, 287] – 96 – – [81, 941] – 3 –
2. What are the advantages and disadvantages of the various collision resolution techniques (i.e. separate
chaining, linear probing, quadratic probing, and double hashing)?
3. Override the hashCode method for the two classes on the next page. Your hashCode methods should
use the principles discussed in class and found in the Bloch reading which can be found off of the
homework page of our course website. For both classes, if two objects are equal their hashCodes must
be equal. Hint: For fields that have a Java object type, take advantage of the hashCode methods implemented for those objects by Java when you are constructing your own hashCode method. Read the API
to discover how hashCode is implemented for different Java objects.
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public class BigNum {
private List digits;
private boolean isNegated;

public boolean equals(Object other) {
if (!(other instanceof BigNum)) {
return false;
}

BigNum o = (BigNum)other;

if (digits.isEmpty() && o.digits.isEmpty()) {
return true;
}
else {
return digits.equals(o.digits) && isNegated == o.isNegated;
}
}
}
public class ProjectPartners {
private int year;
private int quarter; // 0..3 -> AU,WI,SP,SU
private int prjNum;
private String student1, student2;


public boolean equals(Object other) {
if (!(other instanceof ProjectPartners)) {
return false;
}

ProjectPartners o = (ProjectPartners)other;

boolean sameStudents =
(student1.equals(o.student1) && student2.equals(o.student2))
|| (student1.equals(o.student2) && student2.equals(o.student1));

return sameStudents && prjNum == o.prjNum
&& quarter == o.quarter && year == o.year;
}
}
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Part B: Hash Map Implementation
For this portion of the assignment you will finish implementing the map data structure that we began in lecture.
The goal is to complete the implementation of the instructor-provided Map interface, which represents a map of
keys into values.
Here are its methods:
public interface Map {
public boolean containsKey(K key);
public V get(K key);
public void print();
public void put(K key, V value);
public V remove(K key);
public int size();
}
In lecture, we implemented containsKey and put; the print and size methods were already provided.
Methods to Implement:
Your job is to implement the get and the remove methods. These methods are specified below. Both of these
methods should have constant (O(1)) expected runtime assuming rehashing was properly implemented to keep
the load factor at an acceptable ratio; you do not need to implement rehashing to ensure this is the case.
public V get(K key)
Returns the value mapped to the given key, if any. If the given key is not contained in this map, returns
null. Your implementation should correctly fetch the value mapped to a null key.
public V remove(K key)
Removes the mapping for the given key from this map. If key is found in your HashMap, you should remove the mapping and return the value that was associated with the key. If the key is not found in your
HashMap, your HashMap should not be altered and null should be returned. null is a valid key value so
your method should behave the same way with a null key as it would with any other value. In other
words, if null is passed as the key and it is in the HashMap you should remove the mapping with the null
key and return the value that was mapped to null; otherwise, your HashMap should not be altered and
null should be returned.