HashMap Methods and Usage

• A HashMap can be defined as an 'associate array that stores and retrieves values using hash values for keys, and whose size dynamically increases based on the number of key-value pairs.'
• This associate array is also called a Map, Dictionary, or Symbol Table.

Simply put, it's a fundamentally unordered data structure where keys and values form 1:1 mappings as pairs, and duplicate keys are not allowed.

By default, equals() is used to determine duplicates, so primitive data types are filtered out. However, objects are not filtered out because equals() considers them different even if their values are the same. Therefore, to prevent duplicate objects, you must override equals().

HashMap Methods #

HashMap Constructor #

Basically, primitive data types are not allowed for the type parameters.

HashMap<String , Integer> map8 = new HashMap<>();

• When data is added, HashMap approximately doubles its storage capacity.
• Therefore, if you know the initial number of data items to store, it's good practice to specify the initial capacity.

void clear(); #

• Clears all existing elements within the HashMap. It has no return value.

boolean isEmpty() #

• Checks if the HashMap contains any elements. Returns true if empty, false otherwise.

boolean containsKey(Object Key) #

• Determines if the given Key exists in the current HashMap and returns a boolean value. (true if present, false if not).

boolean containsValue(Object value) #

• Determines if a Key associated with the given Value exists in the current HashMap and returns a boolean value.

Set<Map.Entry<K,V>> entrySet(); #

• Returns all elements of the HashMap as a Set, bundled in "key=value" form.

Set KeySet() #

• Returns all keys of the HashMap as a Set, bundled in key form.

Collection values() #

• Returns all values of the HashMap, bundled together.

V get #

• Returns the value mapped to the given key.
• If the key is not present in the HashMap, it returns null.

V put(K key, V value) #

• Adds the given key=value pair to the HashMap.
• If the Key already exists in the HashMap, the value from the later put operation will be stored.

V remove(Object key) #

• If the given key exists in the HashMap, it removes that key=value pair and returns the value.
• If the given key is not in the HashMap, it returns null.

V replace(key, value) #

• Replaces an existing key=old_value in the HashMap with a new key=value.
• If the replace operation is successful, it returns the old_value that existed previously; if the key does not exist and replace fails, it returns null.

void forEach #

• You can access each key=value pair of the HashMap using forEach.
• Lambda expressions can also be used, allowing for simpler code compared to iteration via an Iterator.
• Sets created through keySet(), entrySet(), or values() can also be accessed with a forEach loop.