HashMap

HashMap is a Map based collection class that is used for storing Key & value pairs

I would recommend you to go through my post 'How HashMap works in Java

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HashMap permits null values and null key.

The HashMap performs the get and put operations in constant time.

HashMap has two parameters that effects the Performance

a. Capacity

b. Load Factor

Capacity

capacity is the number of buckets in the hash table.

Load Factor

The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. By default Load Factor for HashMap is 0.75.

For Example let us assume

initial capacity is 10 (I.e, Number Of Buckets)

Load Factor is 0.75

Each Bucket has the capacity to store 100 data items.

So when all the items reached 0.75 * 100 * 10 = 750, then the internal data structures rehashed and the Capacity(Number Of Buckets) increases depends on implementation. The same logic applies in future also.

HashMap is not synchronized, So user has to taken care while performing put kind of operations.

Collections.synchronizedMap returns a synchronized map backed by the specified map.

Note

HashMap implements Serializable and Cloneable interface.

public class HashMap<K,V> extends AbstractMap<K,V>

implements Map<K,V>, Cloneable, Serializable

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