Hash Tables in Java

Data Structures and Algorithms with Java is a course that covers the fundamentals of computer science, including the use of hash tables. Hash tables are a data structure that can be used to store and retrieve data quickly. They are a powerful tool for solving many problems, and are a cornerstone of modern computer science. In this article, we will discuss the basics of hash tables, how to implement them using arrays, the concept of hash functions, collision handling, and some applications of hash tables.

Overview of Hash Tables

Hash tables are a type of data structure that store and retrieve data quickly. They are also known as hash maps or dictionaries. Hash tables are composed of a set of key-value pairs, and the keys are used to find the associated values. The keys and values can be of any type, and the structure of a hash table is dynamic, meaning that it can be changed as needed.

Hash tables are usually implemented using an array as the underlying data structure, and the keys are used to calculate the index of the associated value in the array. This makes them very efficient for searching, as the time to find an element is proportional to the size of the array. Hash tables also use a hash function to map the keys to indexes. This makes them very efficient for inserting and deleting elements, as the time required is constant regardless of the size of the array.

Implementing Hash Tables Using Arrays

To implement a hash table using an array, we must first create a data structure that stores the key-value pairs. This can be done using a class such as the following:

public class HashTableEntry {
    private int key;
    private String value;
 
    public HashTableEntry(int key, String value) {
        this.key = key;
        this.value = value;
    }
 
    public int getKey() {
        return key;
    }
 
    public String getValue() {
        return value;
    }
}

Next, we need to create an array that will store the key-value pairs. This can be done using the following code:

public class HashTable {
    private int size;
    private HashTableEntry[] table;
 
    public HashTable(int size) {
        this.size = size;
        this.table = new HashTableEntry[size];
    }
}

We can now use the array to store key-value pairs in the hash table. To do this, we need to use a hash function to map the keys to indexes in the array.

Hash Functions

A hash function is a function that maps a key to an index in the array. The goal of a hash function is to minimize collisions, which occur when two or more keys are mapped to the same index in the array. There are many different hash functions that can be used, but the most commonly used hash function is the modulus function. This function takes the key and divides it by the size of the array, and the remainder is used as the index in the array. The following code shows an example of a modulus hash function:

public int hashFunction(int key) {
    return key % size;
}

Collision Handling

Collisions can occur when two or more keys are mapped to the same index in the array. To handle collisions, we need to use a collision resolution strategy. There are several different strategies, but the most commonly used are linear probing and chaining.

Linear Probing

Linear probing is a collision resolution strategy that uses an array to store the key-value pairs. When a collision occurs, the key-value pair is stored in the next available position in the array. The following code shows an example of linear probing:

public void insert(int key, String value) {
    int index = hashFunction(key);
 
    while (table[index] != null) {
        index = (index + 1) % size;
    }
 
    table[index] = new HashTableEntry(key, value);
}

Chaining

Chaining is a collision resolution strategy that uses a linked list to store the key-value pairs. When a collision occurs, the key-value pair is stored in the linked list. The following code shows an example of chaining:

public void insert(int key, String value) {
    int index = hashFunction(key);
 
    if (table[index] == null) {
        table[index] = new LinkedList<HashTableEntry>();
    }
 
    table[index].add(new HashTableEntry(key, value));
}

Applications of Hash Tables

Hash tables are a powerful tool for solving many problems. They are used in databases for indexing and searching, in caches for storing frequently used data, in programming languages for creating dictionaries and maps, and in networking protocols for routing data. Hash tables are also used in cryptography for storing and retrieving encrypted data.

Conclusion

Hash tables are a powerful data structure that can be used to store and retrieve data quickly. They are usually implemented using an array, and use a hash function to map the keys to indexes in the array. Collisions are handled using either linear probing or chaining. Hash tables are used in many applications, such as databases, caches, programming languages, networking protocols, and cryptography.

Exercises

public String search(int key) {
    int index = hashFunction(key);
 
    if (table[index] == null) {
        return null;
    }
 
    for (HashTableEntry entry : table[index]) {
        if (entry.getKey() == key) {
            return entry.getValue();
        }
    }
 
    return null;
}

public void insert(int key, String value) {
    int index = hashFunction(key);
 
    if (table[index] == null) {
        table[index] = new LinkedList<HashTableEntry>();
    }
 
    table[index].add(new HashTableEntry(key, value));
}

public void delete(int key) {
    int index = hashFunction(key);
 
    if (table[index] == null) {
        return;
    }
 
    Iterator<HashTableEntry> iter = table[index].iterator();
    while (iter.hasNext()) {
        HashTableEntry entry = iter.next();
        if (entry.getKey() == key) {
            iter.remove();
            return;
        }
    }
}

public int hashFunction(int key) {
    return key % size;
}

public void handleCollision(int key, String value) {
    int index = hashFunction(key);
 
    while (table[index] != null) {
        index = (index + 1) % size;
    }
 
    table[index] = new HashTableEntry(key, value);
}