Hash-table

Hashtable in Java

Overview

The Hashtable class in Java is a part of the java.util package and implements the Map interface. It is similar to HashMap, but with two key differences: it is synchronized (making it thread-safe) and does not allow null keys or values. It stores data in key-value pairs and provides constant-time performance for basic operations, such as get() and put().

---

Characteristics of Hashtable

Feature	Details
Package	java.util
Allows Duplicates	No (Allows duplicate values, but not keys)
Thread-Safe	Yes
Allows null Values	No (Does not allow null keys or values)
Initial Capacity	11 (default)
Load Factor	0.75 (default)

---

Key Features of Hashtable

• Key-Value Pairs: Stores data in key-value pairs.
• Thread-Safe: Ensures thread safety with synchronization, though it may be slower compared to other Map implementations.
• Does Not Allow null Keys or Values: Unlike HashMap, Hashtable does not allow null keys or values.
• Fast Lookup: Provides constant time complexity (O(1)) for get() and put() operations under ideal conditions.

---

How to Use Hashtable

Importing Hashtable

import java.util.Hashtable;

Creating a Hashtable

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

• Example:

  Hashtable<String, String> hashtable = new Hashtable<>();
  Hashtable<Integer, String> hashtableWithIntegers = new Hashtable<>();

---

Hashtable Specific Methods

The Hashtable class provides several methods beyond those in the Map interface. Here are some of the most commonly used:

Method	Description	Return Type	Example
put(K key, V value)	Associates the specified value with the specified key.	V	hashtable.put("Name", "Alice");
get(Object key)	Returns the value associated with the specified key.	V	hashtable.get("Name");
remove(Object key)	Removes the mapping for the specified key from the map.	V	hashtable.remove("Name");
containsKey(Object key)	Checks if the map contains a mapping for the specified key.	boolean	hashtable.containsKey("Name");
containsValue(Object value)	Checks if the map contains the specified value.	boolean	hashtable.containsValue("Alice");
size()	Returns the number of key-value mappings in the map.	int	hashtable.size();
clear()	Removes all mappings from the map.	void	hashtable.clear();
isEmpty()	Checks if the map is empty.	boolean	hashtable.isEmpty();
keySet()	Returns a set view of the keys contained in the map.	Set	hashtable.keySet();
values()	Returns a collection view of the values contained in the map.	Collection	hashtable.values();
entrySet()	Returns a set view of the mappings contained in the map.	Set<Map.Entry<K, V>>	hashtable.entrySet();
putAll(Map<? extends K, ? extends V> m)	Copies all of the mappings from the specified map to the current map.	void	hashtable.putAll(anotherMap);
equals(Object o)	Compares the specified object with this map for equality.	boolean	hashtable.equals(otherMap);

---

Example Code

import java.util.Hashtable;

public class HashtableDataStructure {
    public static void main(String[] args) {
        // Create a Hashtable
        Hashtable<String, String> hashtable = new Hashtable<>();

        // Add key-value pairs
        hashtable.put("Name", "Alice");
        hashtable.put("Age", "25");
        hashtable.put("City", "New York");

        // Access value by key
        System.out.println("Value for 'Name': " + hashtable.get("Name"));

        // Check size
        System.out.println("Size of the hashtable: " + hashtable.size());

        // Check if a key exists
        System.out.println("Contains key 'Age': " + hashtable.containsKey("Age"));

        // Remove a key-value pair
        hashtable.remove("City");
        System.out.println("Hashtable after removal: " + hashtable);

        // Iterate over key-value pairs
        System.out.println("Iterating over hashtable:");
        for (Map.Entry<String, String> entry : hashtable.entrySet()) {
            System.out.println(entry.getKey() + ": " + entry.getValue());
        }

        // Clear the hashtable
        hashtable.clear();
        System.out.println("Is hashtable empty? " + hashtable.isEmpty());
    }
}

For more: HashtableDataStructure.java