Map is Interface and Hashmap is class that implements that
In Java, Map
is an interface that represents a collection of key-value pairs, where each key is associated with exactly one value. HashMap
is a specific implementation of the Map
interface.
Here are some key differences between Map
and HashMap
:
- Interface vs. Implementation:
Map
is the interface that defines the basic contract for implementing a map data structure.HashMap
is a class that implements theMap
interface and provides a specific way to store and retrieve key-value pairs.
- Synchronization:
Map
is not synchronized, meaning it is not thread-safe. Multiple threads can access it concurrently without any restriction.HashMap
is also not synchronized. If you need a synchronized version, you can useCollections.synchronizedMap(Map)
to obtain a synchronized map.
- Null Keys and Values:
Map
allows a singlenull
key and multiplenull
values.HashMap
allows a singlenull
key and multiplenull
values. In fact, it allows one null key and any number of null values.
- Ordering:
- The
Map
interface does not guarantee any specific order of elements. HashMap
does not guarantee any specific order of elements. If you need ordering, you can useLinkedHashMap
, which maintains the order of the elements based on their insertion order.
- The
- Performance:
HashMap
generally has better performance compared to some other map implementations for most operations (such as adding, removing, and retrieving elements). However, performance characteristics may vary depending on the specific use case.
- Iterating:
- When you iterate over a
Map
, the order of the elements is not guaranteed. - If you need predictable iteration order, you can use a
LinkedHashMap
.
- When you iterate over a
In summary, Map
is an interface, and HashMap
is a specific implementation of that interface with certain characteristics. Depending on your requirements, you may choose different implementations of the Map
interface, such as HashMap
, TreeMap
, or LinkedHashMap
.