A hash map (or hash table) is a data structure which contains "key-value" pairs and allows retrieving values by key.
The most attractive feature is fast lookup of elements, particularly for large numbers of elements. Hash maps work by using a hash function to transform keys into a hash number that is then used as an index into an array of "buckets" containing one or more elements. This allows constant time access to the relevant bucket, followed by a linear search for the desired element within the bucket. When the number of elements in each bucket is kept low (possibly by dynamically resizing the array of buckets as elements are inserted) this offers constant time lookup on average even when the number of elements in the hash map increases. This can be a significant advantage compared to lookup in a tree-based structures which needs to perform more steps as the number of elements increases.
A drawback of hash tables is that elements are not stored in an obvious or meaningful order, as a good hash function will not map neighbouring keys to neighbouring buckets.
If many mappings are to be stored in a
HashMap instance, creating it with a sufficiently large capacity will allow the mappings to be stored more efficiently than letting it perform automatic rehashing as needed to grow the table.
Note that this implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more mappings; merely changing the value associated with a key that an instance already contains is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedMap method. This is best done at creation time, to prevent accidental unsynchronized access to the map:
Map m = Collections.synchronizedMap(new HashMap(...));
The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a
ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw
ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
This class is a member of the Java Collections Framework.
Stackoverflow Link: Differences between HashMap and Hashtable?