Java provides two main classes for implementing a hash table data structure – HashMap and Hashtable. Although they seem similar, there are several key differences between the two that make them appropriate for different use cases.
Thread Safety
The most significant difference between HashMap and Hashtable is that HashMap is not thread-safe while Hashtable is thread-safe. This means that if multiple threads try to access a HashMap at the same time, there is a possibility of data being corrupted. On the other hand, Hashtable is synchronized, which means that only one thread can access it at a time, making it safe to use in a multi-threaded environment.
Null Values
Another difference between HashMap and Hashtable is the handling of null values. HashMap allows for one null key and multiple null values, while Hashtable does not allow for any null keys or values.
Performance
Hashtable is synchronized, which means that only one thread can access it at a time. This makes it slower than HashMap, which is not synchronized and therefore faster.
Iteration Order
HashMap provides an iterable collection view of the values contained in the map, which can be accessed in the order in which they were inserted into the map. However, Hashtable does not guarantee the order in which the elements will be retrieved.
Legacy Code
Hashtable was part of the original java.util package and has been part of the Java language since Java 1.0. On the other hand, HashMap was added in Java 1.2 as a more efficient alternative to Hashtable.
If you are working with legacy code that uses Hashtable, it may not be possible to switch to HashMap without making significant changes to the code. In such cases, it may be better to stick with Hashtable.
Example Code
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Map;
public class Main {
public static void main(String[] args) {
// HashMap Example
Map<String, Integer> hashMap = new HashMap<>();
hashMap.put("A", 1);
hashMap.put("B", 2);
hashMap.put("C", 3);
System.out.println("HashMap: " + hashMap);
// Hashtable Example
Map<String, Integer> hashtable = new Hashtable<>();
hashtable.put("A", 1);
hashtable.put("B", 2);
hashtable.put("C", 3);
System.out.println("Hashtable: " + hashtable);
}
}
Concurrent Modification Exception
Another difference between HashMap and Hashtable is the handling of concurrent modification exceptions. If a thread tries to modify a HashMap while another thread is iterating over it, a concurrent modification exception will be thrown. On the other hand, if a thread tries to modify a Hashtable while another thread is iterating over it, the modification will be blocked until the iteration is complete.
Enumeration vs. Iterator
Hashtable provides an enumeration of its elements, while HashMap provides an iterator. Enumeration is a legacy class that was introduced in Java 1.0, while Iterator is a more modern class that was introduced in Java 1.2.
Enumeration has several limitations compared to Iterator, including the fact that it does not support the remove() method and it can only be used to traverse the elements of a collection in a forward direction. On the other hand, Iterator supports the remove() method and can be used to traverse a collection in both forward and backward directions.
Key-Value Pairs
Both HashMap and Hashtable store data in the form of key-value pairs. The key is used to access the value, and the value is the data that is stored. In both classes, the key must be unique, but the value can be duplicated.
Conclusion
In conclusion, HashMap and Hashtable are both useful classes for implementing a hash table data structure in Java. The choice between the two will depend on the specific requirements of the project, including the need for thread safety, the handling of null values, and the need to support concurrent modification exceptions. When choosing between the two, it is important to consider the trade-offs between performance and functionality, and to select the class that best meets the needs of the project.