![\"HashMap](\"https:\/\/msgprogramator.sk\/wp-content\/uploads\/2025\/05\/HashMap-kolekcia-%E2%80%93-hierarchia-dedicnosti-min.webp\")
HashMap in Java – data structure introduction<\/h2>\n
The Java HashMap <\/strong>class is an implementation of the Map interface in the java.util<\/em> package. It allows the storage of key-value pairs, where each key is unique and maps to a specific value. This implementation provides all optional map operations and allows null<\/em> values and null<\/em> keys.<\/p>\n The HashMap class is roughly equivalent to the Hashtable class, except that HashMap is unsynchronized and allows null values. HashMap uses a hash table to store elements, providing fast lookup, insertion, and removal of elements with an average time complexity of O(1). It is important to note that HashMap does not maintain the order of the inserted elements.<\/p>\n We have several constructors available to create a HashMap instance:<\/p>\n 1. Empty HashMap with default capacity and load factor<\/strong><\/p>\n Creates an empty HashMap with default initial capacity (16) and load factor (0.75).<\/p>\n 2. HashMap with a specified initial capacity<\/strong><\/p>\n Creates an empty HashMap with the specified initial capacity and default load factor (0.75).<\/p>\n 3. HashMap with specified initial capacity and load factor<\/strong><\/p>\n Creates an empty HashMap with the specified initial capacity and load factor.<\/p>\n 4. HashMap constructed from Map<\/strong><\/p>\n Creates a new HashMap with the same mappings as the given Map.<\/p>\n These technical concepts related to creating a hash map have already been covered in the HashTable<\/a> class, so I won’t repeat them here.<\/p>\n Some of the basic operations we can perform with HashMap include:<\/p>\n![\"HashTable](\"https:\/\/msgprogramator.sk\/wp-content\/uploads\/2025\/05\/hashtable-java-datove-struktury-954x600-1.webp\")
<\/div>HashTable – Java data structures<\/span><\/h3>
HashMap – constructors<\/h2>\n
HashMap<K, V> map = new HashMap<>();<\/code><\/pre>\nHashMap<K, V> map = new HashMap<>(int initialCapacity);<\/code><\/pre>\nHashMap<K, V> map = new HashMap<>(int initialCapacity, float loadFactor);<\/code><\/pre>\nHashMap(Map<? extends K,? extends V> m)<\/code><\/pre>\nInitial capacity, Load Factor, Hash Function<\/h3>\n
HashMap – basic operations<\/h2>\n
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\nAdd a key-value pair using the put(K key, V value)<\/em> method. If a new pair with the same key is inserted, the old value is overwritten by the new one.<\/li>\n
\nGetting a value based on a key using the get(Object key)<\/em> method.<\/li>\n
\nRemove a key-value pair using the remove(Object key)<\/em> method.<\/li>\n
\nThe isEmpty()<\/em> method tells us if the HashMap contains any pair at all.<\/li>\n
\nChecking for the presence of a value is done using the containsValue(Object value)<\/em> method.
\nChecking for the presence of a key is done using the containsKey(Object key)<\/em> method.<\/li>\n
\nThe size()<\/em> method returns the number of pairs in the table.<\/li>\n
\nThe clear()<\/em> method removes all pairs from the table.<\/li>\n
\nUsing the keySet()<\/em>, values()<\/em> or entrySet()<\/em> methods we can iterate over keys, values or entire pair.<\/li>\n<\/ul>\nJava HashMap documentation<\/h2>\n
![\"HashMap](\"https:\/\/msgprogramator.sk\/wp-content\/uploads\/2025\/05\/hashmap-vystup-z-prikladu-min.jpg\")
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