HashMap vs HashTable : Important Interview Question

Both `HashMap` and `HashTable` are classes in Java that provide key-value mappings. However, there are some important differences between them, especially when it comes to thread-safety.

1. Thread-safety:
— `HashTable`: It is considered thread-safe because it provides synchronized methods to ensure that multiple threads can access and modify it concurrently without causing data inconsistencies. Every method in `HashTable` is synchronized, meaning that only one thread can execute a method at a time, preventing concurrent modifications.
— `HashMap`: It is not inherently thread-safe. By default, `HashMap` does not provide synchronization, which means that concurrent modifications by multiple threads can lead to data corruption or unexpected behavior. If you need thread-safety with `HashMap`, you can use the `Collections.synchronizedMap()` method to create a synchronized version of the `HashMap`.

2. Performance:
— `HashTable`: Due to its synchronized nature, `HashTable` has additional overhead to enforce thread-safety. This can lead to lower performance in highly concurrent scenarios.
— `HashMap`: As `HashMap` is not synchronized by default, it avoids the overhead associated with synchronization. This makes it generally faster than `HashTable` in single-threaded or low-concurrency scenarios. However, in highly concurrent environments, proper synchronization needs to be applied externally to maintain thread-safety.

3. Null values and keys:
— `HashTable`: Neither `HashTable` allows null keys nor null values. Any attempt to store null keys or values will result in a `NullPointerException`.
— `HashMap`: `HashMap` allows null values and a single null key. You can store a null value and retrieve it later using the null key.

In summary, `HashTable` is thread-safe by providing synchronized methods but may have lower performance compared to `HashMap`. `HashMap` is not thread-safe by default but offers better performance. If you need thread-safety with `HashMap`, you can wrap it with `Collections.synchronizedMap()`.