Hierarchy of Java HashMap

Characteristics of Java HashMap

A HashMap is a data structure that is used to store and retrieve values based on keys. Some of the key characteristics of a hashmap include:

• Fast access time: HashMaps provide constant time access to elements, which means that retrieval and insertion of elements are very fast, usually O(1) time complexity.
• Uses hashing function: HashMaps uses a hash function to map keys to indices in an array. This allows for a quick lookup of values based on keys.
• Stores key-value pairs: Each element in a HashMap consists of a key-value pair. The key is used to look up the associated value.
• Supports null keys and values: HashMaps allow for null values and keys. This means that a null key can be used to store a value, and a null value can be associated with a key.
• Not ordered: HashMaps are not ordered, which means that the order in which elements are added to the map is not preserved. However, LinkedHashMap is a variation of HashMap that preserves the insertion order.
• Allows duplicates: HashMaps allow for duplicate values, but not duplicate keys. If a duplicate key is added, the previous value associated with the key is overwritten.
• Thread-unsafe: HashMaps are not thread-safe, which means that if multiple threads access the same hashmap simultaneously, it can lead to data inconsistencies. If thread safety is required, ConcurrentHashMap can be used.
• Capacity and load factor: HashMaps have a capacity, which is the number of elements that it can hold, and a load factor, which is the measure of how full the hashmap can be before it is resized.

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). One object is used as a key (index) to another object (value). If you try to insert the duplicate key in HashMap, it will replace the element of the corresponding key.