HashMap vs HashSet vs HashTable in java
HashMap
- Implements Map interface
- uses put method to store key-value pairs
- Doesn't allow duplicate keys but values can be duplicated
- Maximum of one NULL key but any number of NULL values
- Internally uses an array
HashSet
- Implements Set interface
- uses add method to store only unique objects
- Doesn't allow duplicate objects
- Allows maximum of one NULL object
- Internally uses HashMap to store unique objects
HashTable
- Synchronized
- Considered legacy code
- Doesn't allow NULL keys or values
Whatever can be done in HashTable can be done with a HashMap, so stick to HashMaps.
When to use HashMap?
- Synchronization is not needed
- Key value pairs are wanted
- Search operation on multiple threads
When to use HashSet?
- Order is not required
- Mathematical sets (ex: {1, 2, 3, 4})
- Don't want duplicate keys