Synchronized and Unsynchronized Types in Java

Types	Synchronized	Unsynchronized
Hashtable	Y	-
HashMap	-	Y
Vector	Y	-
ArrayList	-	Y
StringBuffer	Y	-
StringBuilder	-	Y

Hashtable & HashMap & HashSet

Hashtable

1. A table of records
2. Stores key-value pairs
3. Implements Map interface
4. Does not allow null key
5. Does not allow null values
6. Synchronized

HashMap

1. Stores key-value pairs
2. Implements Map interface
3. Allows null key (at most one)
4. Allows null values (arbitrary number)
5. Unsynchronized

HashSet

1. A collection of elements
2. Implements Set interface
3. Allows null element (at most one)
4. Unsynchronized