Java HashMap vs HashTables – Explained

Hash tables are a fundamental data structure used in computer science to implement dictionaries, associative arrays, and sets. In Java, the two most commonly used hash table implementations are the HashMap and HashTable classes. Both classes provide similar functionality, but they differ in their performance characteristics, thread-safety, and feature set

In this blog post, we will explore the functional and performance differences between Java’s HashMap and HashTable classes.

Functional Differences
The functional differences between HashMap and HashTable are relatively minor, but they can be significant depending on your use case.

1. Thread-Safety
   The most significant difference between HashMap and HashTable is that HashTable is thread-safe, while HashMap is not. HashTable provides synchronized methods to ensure that only one thread can modify the table at a time. On the other hand, HashMap is not synchronized, which means that multiple threads can modify the table simultaneously, leading to data corruption and unpredictable behavior.
2. Null Keys and Values
   HashTable does not allow null keys or values. Any attempt to store a null key or value will result in a NullPointerException. In contrast, HashMap allows null keys and values, making it more flexible in some cases.
3. Iteration Order
   The iteration order of HashMap is not guaranteed and can change over time as elements are added or removed. In contrast, HashTable guarantees a predictable iteration order based on the order in which elements were added.

Performance Differences
The performance differences between HashMap and HashTable are more substantial, with HashMap typically outperforming HashTable in most use cases.

1. Synchronization Overhead
   As mentioned earlier, HashTable is thread-safe, while HashMap is not. This means that HashTable incurs synchronization overhead in every operation, even when used in a single-threaded context. In contrast, HashMap does not incur this overhead, making it faster in single-threaded scenarios.
2. Hashing Algorithm
   Both HashMap and HashTable use a hash function to map keys to an index in the table. However, HashMap uses a more efficient hash function that reduces collisions and improves performance. Additionally, HashMap uses a technique called “resizing” to dynamically adjust the size of the table as the number of elements grows. This allows HashMap to maintain a low load factor, which further reduces collisions and improves performance.
3. Initial Capacity
   HashTable has a default initial capacity of 11, while HashMap’s default initial capacity is 16. This means that HashMap can store more elements without having to resize the table, which can lead to better performance in scenarios where many elements are stored.

Both HashMap and HashTable provide similar functionality but differ in their performance characteristics, thread-safety, and feature set. HashMap is faster and more flexible than HashTable but is not thread-safe. HashTable is thread-safe but incurs synchronization overhead in every operation, making it slower than HashMap in most scenarios. Ultimately, the choice between HashMap and HashTable depends on your specific use case, and you should carefully consider the functional and performance differences when making your decision.

More Information

1. Oracle HashMap (https://docs.oracle.com/javase/8/docs/api/java/util/HashMap.html)
2. Oracle HashTable (https://docs.oracle.com/javase/8/docs/api/java/util/Hashtable.html)