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• HashMap is a class implementing the Map interface, which allows us to store collection of object as key value pairs.
• It does not maintain the insertion order.
• It is not synchronized. It allows only one null key with null values.

Creating a HashMap:

Syntax:

HashMap obj = new HashMap(); //Without Generics

Syntax:

HashMap<KeyType, ValueType> obj = new HashMap<KeyType, ValueType>(); //using Generics

• By default a hashmap with a capacity of 16 is created and when the size of the hashmap (number of elements) exceeds 3/4th of the capacity (12), the capacity of the hashmap is doubled (32).

Example:

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

Syntax:

HashMap obj = new HashMap(int initialCapacity);

• HashMap object ‘obj’ is created with the specified initial capacity. When the size of the hashmap exceeds 3/4th of the initial capacity, the capacity of the hashmap is doubled.

Example:

HashMap<Integer,String> Number = new HashMap<Integer,String>(12);

HashMap object Number is created with initial capacity as 12. When 10th element is added, the capacity is doubled to 24.

Syntax:

HashMap obj = new HashMap(int initialCapacity, float loadFactor);

• Creates an empty HashMap object with the specified initial capacity and load factor.
• Load factor is a value based on which the capacity of the hashmap is doubled.
• When the size of the hashmap reaches the value which is the product of initial capacity and the load factor, the capacity of hashmap is doubled

Example:

HashMap<Integer,String> Number = new HashMap<Integer,String>(12,0.5);

• Hashmap object is created with an initial capacity of 12 and with load factor as 0.5. When the number of elements exceeds the product value (12*0.5 = 6), then the capacity will be doubled.

Methods of HashMap:

Methods of HashMap	Description
void clear()	Removes all the key-value pairs from the Hashmap.
boolean containsKey(Object key)	Returns true, if the hashmap object contains a value for the specified key.
boolean containsValue(Object value)	Returns true, if the map contains a key for the specified value.
Object get(Object key)	Returns the value to which the specified key is mapped, or null if the map contains no mapping for the key.
Set entrySet()	Returns a Set which contains the key value pairs of the map.
boolean equals(Object o)	Compares the specified object with the map object for equality.
Value put(K key, V value)	Maps the specified key with the specified value in the map.
int size()	Returns the number of key-value pairs in the map.
Value remove(Object key)	Removes the key value pair for the specified key from the map if it is present.
Collection values()	It returns the collection of values present in the map.

Inserting the key-value pair:

Key value pairs can be mapped by using put() method.

Syntax:

put(K key, V value)  where ‘K’ refers to the type of key and ‘V’ to the type of value.

Example:

Output:

{100=Anjali, 101=Anu, 102=Banu} Key:100 value:Anjali Key:101 value:Anu Key:102 value:Banu

Removing the Mapping of a specified key value:

Association of the key to the value can be removed using the remove() method.

Syntax:

remove(Object key)

Example:

Output:

{100=Anjali, 101=Anu, 102=Banu} Mapping associated with the key 101 is removed.. {100=Anjali, 102=Banu}

Calculating length of the HashMap:

Length of the HashMap can be found using the size() method.

Example:

Output:

{100=Anjali, 101=Anu, 102=Banu} size of the student objects:3

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