Interview #172: Java - How does HashMap differ from Hashtable?

and are both classes in Java that implement the interface and are used to store key-value pairs. Although they have similar functionality, they differ significantly in several aspects, such as synchronization, performance, null key/value handling, legacy status, and internal implementation. Below is a detailed comparison:

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1. Synchronization and Thread Safety

• is synchronized. All the methods of are synchronized, which means only one thread can access the at any given time. This makes thread-safe and suitable for multi-threaded applications, but it also results in lower performance in single-threaded or low-contention environments.

• is not synchronized. It is not thread-safe by default, which means multiple threads can access it concurrently, potentially causing race conditions if proper synchronization is not applied manually. However, for better performance in single-threaded environments, is preferred.

To make a thread-safe, you can use or switch to in concurrent applications.

2. Null Keys and Values

• does not allow any null key or null value. Attempting to insert a null key or value will result in a .

• allows one null key and multiple null values. This provides more flexibility when handling mappings where null values or keys are acceptable.

3. Performance

• Because is synchronized, it performs slower compared to in non-concurrent or lightly concurrent environments.

• provides better performance in such situations due to the absence of synchronization overhead.

4. Legacy Status

• is a legacy class from the early versions of Java (introduced in JDK 1.0). Although it is still part of the Java API, it is generally discouraged in favor of newer alternatives.

• was introduced later (in JDK 1.2) as part of the Java Collections Framework and is considered a modern alternative with more flexibility and better performance.

5. Iterator vs Enumerator

• uses Enumerator to iterate over the keys and values, which is considered outdated and does not support fail-fast behavior.

• uses Iterator, which is fail-fast — it throws a if the map is modified structurally after the iterator is created (except through the iterator's own method).

6. Inheritance Hierarchy

• extends the Dictionary class, which is an abstract class from the early days of Java. The class itself is considered obsolete.

• extends AbstractMap, which is part of the Java Collections Framework and provides a more modern design.

7. Usage in Concurrent Applications

• In multi-threaded applications, instead of using , it is recommended to use , which offers better scalability and fine-grained locking.

• , when required to be thread-safe, should be wrapped with or replaced with .

Summary Table

Conclusion

While both and are used to store key-value mappings, is generally preferred due to its better performance, flexibility, and modern design. is retained mainly for backward compatibility and is not recommended for use in new applications. In concurrent scenarios, it's advisable to use instead of either or .