In Java SE 17, both `HashMap` and `Hashtable` are part of the Collections Framework and are used to store key-value pairs. Despite their similar purpose, there are significant differences in their design, functionality, and usage. These differences are crucial for developers to understand in order to choose the appropriate implementation for their specific needs.
Synchronization
- `Hashtable`: It is a legacy class from the original version of Java and is fully synchronized, meaning it is thread-safe out of the box. Each method call on a `Hashtable` is wrapped in a synchronized block, which ensures that only one thread can access the table at a time. This makes `Hashtable` suitable for use in multithreaded environments where multiple threads might access and modify the table concurrently. However, this synchronization comes at the cost of performance, making `Hashtable` less efficient in scenarios where thread safety is not a concern.
- `HashMap`: Introduced in Java 2, version 1.2, `HashMap` is not synchronized, which means it does not provide thread safety by default. This lack of synchronization improves its performance in single-threaded or read-heavy environments. For multithreaded scenarios, external synchronization must be applied if atomic operations are required, or alternatively, one can use `Collections.synchronizedMap(new HashMap<...>())` to synchronize a `HashMap` or use `ConcurrentHashMap` for better concurrency.
Null Keys and Values
- `Hashtable`: It does not allow null keys or values. Attempting to insert a null key or value into a `Hashtable` will result in a `NullPointerException`.
- `HashMap`: It allows one null key and multiple null values. This flexibility can be useful in applications where null has semantic meaning, such as representing the absence of a value.
- Iteration Order: Both `Hashtable` and `HashMap` do not guarantee any specific order of the elements during iteration. The iteration order can change with the addition or removal of elements. However, for ordered or sorted iteration, one might consider using `LinkedHashMap` or `TreeMap`, respectively.
- Performance: Due to the lack of synchronization in `HashMap`, it generally offers better performance in scenarios where thread safety is not a concern. The cost of synchronization in `Hashtable` can be significant in terms of performance, especially in highly concurrent scenarios where contention for the Hashtable might be high.
- Legacy Status: `Hashtable` is considered a legacy class. Although it is still supported, it has been superseded by `ConcurrentHashMap` in Java 1.5 for highly concurrent scenarios and `HashMap` for non-threaded applications. The Java Collections Framework (JCF) does not include `Hashtable`, and it is recommended to use `HashMap` or `ConcurrentHashMap` in new development.
In summary, the choice between `HashMap` and `Hashtable` in Java SE 17 should be guided by the specific requirements regarding thread safety, performance, and the need to support null keys and values. `HashMap` is generally preferred in most modern Java applications due to its performance advantages and flexibility, while `Hashtable` may still be used in legacy systems or specific scenarios where its thread-safe nature is required without the need to move to `ConcurrentHashMap`.
Hashtable is the
synchronized counterpart to HashMap.
When we say Vector and Hashtable
are synchronized, we mean that the key methods of the class
are synchronized.
A
HashMap lets you have null values as well as one null key, a Hashtable does not allow anything that is null.