Difference Between Hashtable and Synchronized Map in Java

In Java, thread safety is an important concern when multiple threads access shared data structures. Two commonly used thread-safe map implementations are Hashtable and Synchronized Map. Although both provide synchronization, they differ in design, performance, and usage.

Hashtable

Hashtable is a legacy class that implements the Map interface. It stores data in the form of key–value pairs and is synchronized by default, meaning all its public methods are thread-safe.

• Does not allow null keys or null values.
• Uses hashCode() and equals() methods to store and retrieve elements.
• Provides Enumeration (not fail-fast) and Iterator (fail-fast).
• Default initial capacity is 11 with a load factor of 0.75.

3 Joe Biden
2 Donald Trump
1 James Bond

Explanation:

• Creates a Hashtable that stores Integer keys and String values, and is thread-safe by default.
• Inserts key-value pairs using the put() method; null keys or values are not allowed.
• Uses entrySet() to iterate over all key–value pairs in the table.
• Prints each key and its corresponding value; the order of output is not guaranteed.

Synchronized Map

A Synchronized Map is created by wrapping a non-synchronized map (like HashMap) using Collections.synchronizedMap(). This provides thread safety by synchronizing access to the map.

• Synchronization is applied at the object level.
• Entire map is locked during read/write operations.
• Allows one null key and multiple null values (because it wraps HashMap).
• Iterators are fail-fast.
• Requires external synchronization while iterating.

1 Brad
2 Anil
3 Sachin

Explanation:

• Creates a regular HashMap and then wraps it using Collections.synchronizedMap() to make it thread-safe.
• Synchronization is applied at the object level, so all read/write operations acquire a lock on the map.
• While iterating, the map is explicitly synchronized using synchronized (syncMap) to prevent concurrent modification.
• Uses a fail-fast iterator, which throws ConcurrentModificationException if the map is modified during iteration.

Hashtable vs SynchronizedHashMap