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Java HashMap. In the ArrayList chapter, you learned that Arrays store items as an ordered collection, and you have to access them with an index number (int type). A HashMap however, store items in " key / value " pairs, and you can access them by an index of another type (e.g. a String).
In Java, HashMap is a part of Java’s collection since Java 1.2. This class is found in java.util package. It provides the basic implementation of the Map interface of Java. HashMap in Java stores the data in (Key, Value) pairs, and you can access them by an index of another type (e.g. an Integer).
A hash map is a concrete implementation of the abstract data type known as an associative array. In a hash map, keys are hashed to determine the index where the corresponding values will be stored, allowing for efficient retrieval and storage of key-value pairs.
A map implemented by a hash table is called a hash map. Most hash table designs employ an imperfect hash function. Hash collisions, where the hash function generates the same index for more than one key, therefore typically must be accommodated in some way.
Class HashMap<K,V>. Hash table based implementation of the Map interface. This implementation provides all of the optional map operations, and permits null values and the null key. (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.)
Hashmaps and hashtables are datastructures (like arrays and lists), that use hashing to store data. In a hashtable, a hash is produced (either from a provided key, or from the object itself) that determines where in the table the object is stored.
Basic Usage. Let’s first look at what it means that HashMap is a map. A map is a key-value mapping, which means that every key is mapped to exactly one value and that we can use the key to retrieve the corresponding value from a map. One might ask why not simply add the value to a list.