HashTable

A hash table is a data structure that stores data in key-value pairs. It provides a way of mapping a key to a value for fast retrieval. The hash table works by using a hash function. This hash function computes an index into an array of buckets, where each bucket is a linked list of key-value pairs. In Java, the HashTable class is used to implement a hash table.

Creating a HashTable

To create a HashTable in Java, you can use the following syntax.

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Hashtable<KeyType, ValueType> hashTable = new Hashtable<KeyType, ValueType>();

Here, KeyType is the type of the key, and ValueType is the type of the value.

Adding Elements to a HashTable

To add elements to a HashTable, you can use the put() method. The put() method takes two arguments - the key and the value.

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hashTable.put(key, value);

Retrieving Elements from a HashTable

To retrieve an element from a HashTable, you can use the get() method. The get() method takes one argument - the key.

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ValueType value = hashTable.get(key);

Removing Elements from a HashTable

To remove an element from a HashTable, you can use the remove() method. The remove() method takes one argument - the key.

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hashTable.remove(key);

Iterating Over a HashTable

To iterate over a HashTable, you can use a for-each loop to iterate over the key-value pairs.

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for (KeyType key : hashTable.keySet()) {
    ValueType value = hashTable.get(key);
    System.out.println(key + " = " + value);
}

When to use a HashTable

A HashTable is an efficient way to store and retrieve data in key-value pairs. It is useful when you need to look up a value quickly based on a key.

Some common use cases for a HashTable include:

Caching data to improve performance.
Storing and retrieving user preferences or settings.
Implementing a dictionary or a word frequency counter.

Differences between HashTable and HashMap

In Java, there are two main classes that implement a hash table - HashTable and HashMap. The main differences between the two are:

Thread-safety: HashTable is thread-safe, while HashMap is not.
Performance: HashTable is slower than HashMap due to its synchronization.
Null values: HashTable does not allow null values for keys or values, while HashMap does.

Advantages of HashTable	Disadvantages of HashTable
Fast Retrieval: `HashTable` provides fast retrieval of values using keys. The hash function ensures that the key is mapped to a specific index in the array, which makes retrieval of the value O(1) time complexity.	Performance: `HashTable`'s performance can be affected by the quality of the hash function. If the hash function is not well-designed, it can cause collisions, which can lead to slower performance.
Efficient Storage: `HashTable` uses an array to store the key-value pairs, which makes it an efficient data structure in terms of memory usage. It allocates memory dynamically and can grow or shrink as needed.	Overhead: `HashTable` has some overhead associated with it. Since it is a synchronized data structure, it can be slower than unsynchronized data structures such as `HashMap`.
Thread-Safe: `HashTable` is a synchronized data structure, which means that it is safe to use in a multi-threaded environment. It ensures that only one thread can modify the data at a time, preventing race conditions.	Limited Flexibility: `HashTable` is a fixed-size data structure, which means that once the size of the table is set, it cannot be changed. If the table is too small, it can cause collisions, and if it is too big, it can waste memory.

1. Which of the following classes in Java is used to implement a hash table?

2. What is the main difference between HashTable and HashMap in Java?

Everything was clear?

Thanks for your feedback!

Section 1. Chapter 4