In this article, we will discuss Hashtable class – one of the Map implemented classes in detail

1. Key points about Hashtable:

Hashtable is legacy class and based on hashcode of keys where keys are unique and it is exactly same as HashMap with few differences with respect to following points

Synchronized methods
Performance
Null key insertion
Null value insertion

2. Hashtable:

Java Hashtable is the implementation class of Map interface (i.e.; Hashtable implements Map)
Java Hashtable uses hash table data structure to store Key-Value pairs, which is also known as Map entry
Java Hashtable is legacy class introduced in Java 1.0 version
Java Hashtable allows only unique keys but there is no restriction on values which can be duplicated
At any time, Java Hashtable contains only unique keys
Insertion-order is NOT maintained
While iterating through Java Hashtable, we will get map entries in random-order, as against insertion-order
Doesn’t allows NULL insertion for keys and values
Without generics, Java Hashtable allows to insert any type of Key/Values;
With generics, it is type-bounded except, if we take both Key-Value as Objects within angle brackets
Java Hashtable is synchronized, all methods of Java Hashtable is thread-safe (i.e.; only one thread is allowed to access, at a any given point of time)
Search operation is faster i.e.; searching any element from Java Hashtable is faster, as it uses hashing to store key-value pairs
Present in java.util package and extends java.util.Dictionary abstract class implements java.util.Map interface
Also, implements java.lang.Cloneable, java.io.Serializable marker interfaces which provides special ability to Hashtable (provided by JVM at run time)
java.lang.Cloneable: to create a duplicate object or to clone an object
java.io.Serializable: to transfer objects across network

Source: Team BenchResources.Net

3. Hashtable constructors:

3.1 Hashtable ht = new Hashtable();

creates an empty Hashtable object of size 11
with default fill ratio of 0.75

3.2 Hashtable ht = new Hashtable(int initialCapacity);

creates an empty Hashtable object of specified size (or initial capacity)
with default fill ratio of 0.75

3.3 Hashtable ht = new Hashtable(int initialCapacity, float loadFactor);

creates an empty Hashtable object of specified size (or initial capacity)
and specified fill ratio (for example 0.85)

3.4 Hashtable ht = new Hashtable(Map m);

creates an equivalent Hashtable object for the specified map
it is basically used for inter-conversion between map objects

4. Fill ratio (or Load factor)

Fill ratio is also known as Load factor
This factor determines when to increase the size of Java Hashtable automatically
For example, for the 1^st two constructors the default load factor is 0.75 –> which means after filling 75% of original Java Hashtable, new Hashtable of bigger size will be created
For 3^rd constructor, programmer can define load factor while creating Hashtable object. If programmer defines it to be 0.95, then after filling 95% of Hashtable, size of Hashtable will be increased automatically
The value of Load factor should be in between 0 to 1.0

5. Hashtable examples:

HashtableAddAndRemove.java

package in.bench.resources.java.collection;

import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class HashtableAddAndRemove {

	public static void main(String[] args) {

		// creating Hashtable object of type <Integer, String>
		Hashtable<Integer, String> hm = new Hashtable<Integer, String>();

		// adding key-value pairs to Hashtable object
		hm.put(1, "Google");
		hm.put(2, "Facebook");
		hm.put(3, "Yahoo");
		hm.put(4, "Amazon");
		hm.put(5, "Reddit");

		System.out.println("Printing all key-value pairs inside {}\n"
				+ hm + "\n");

		System.out.println("\nIterating using keySet\n");

		// Iterating key-pairs using keySet
		Set<Integer> keys = hm.keySet();
		for(Integer key : keys) {
			System.out.println(key + "  " + hm.get(key));
		}

		System.out.println("\n\nIterating using Map Entry interface\n");

		// Iterating key-pairs using Map entry
		Set set = hm.entrySet();
		Iterator iterator = set.iterator();

		while(iterator.hasNext()) {

			Map.Entry mapEntry = (Map.Entry)iterator.next();
			System.out.println(mapEntry.getKey() + "  "
					+ mapEntry.getValue());
		}

		// removing map entry at 4th position
		System.out.println("\n\nEntry removed at 4th position : "
				+ hm.remove(4));
	}
}

Output:

Printing all key-value pairs inside {}
{5=Reddit, 4=Amazon, 1=Google, 3=Yahoo, 2=Facebook}

Iterating using keySet

5  Reddit
4  Amazon
1  Google
3  Yahoo
2  Facebook

Iterating using Map Entry interface

5  Reddit
4  Amazon
1  Google
3  Yahoo
2  Facebook

Entry removed at 4th position : Amazon

6. Difference between HashMap and Hashtable ?

The main difference between HashMap and Hashtable is that, all methods of Hashtable class is synchronized whereas HashMap methods are non-synchronized
Hashtable is thread-safe, while working in multi-threaded environment
To make HashMap methods synchronized, we need to execute below code

Map map = Collections.synchronizedMap(hashMap);

References:

Happy Coding !!
Happy Learning !!

BenchResources.Net

Java tutorial for beginners

Java – Hashtable class with examples

1. Key points about Hashtable:

2. Hashtable:

3. Hashtable constructors:

3.1 Hashtable ht = new Hashtable();

3.2 Hashtable ht = new Hashtable(int initialCapacity);

3.3 Hashtable ht = new Hashtable(int initialCapacity, float loadFactor);

3.4 Hashtable ht = new Hashtable(Map m);

4. Fill ratio (or Load factor)

5. Hashtable examples:

HashtableAddAndRemove.java

6. Difference between HashMap and Hashtable ?

Related Articles:

References:

Related

1. Key points about Hashtable:

2. Hashtable:

3. Hashtable constructors:

3.1 Hashtable ht = new Hashtable();

3.2 Hashtable ht = new Hashtable(int initialCapacity);

3.3 Hashtable ht = new Hashtable(int initialCapacity, float loadFactor);

3.4 Hashtable ht = new Hashtable(Map m);

4. Fill ratio (or Load factor)

5. Hashtable examples:

HashtableAddAndRemove.java

6. Difference between HashMap and Hashtable ?

Related Articles:

References:

Related

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