What is the problem with HashMap in Java?

HashMap is a collection to store (key,value) pairs and According to the documentation of HashMap the keys are always unique . If you add a key which already exists(collision) in the hashmap, the old value will be replaced.

HashMap is not limited , provided to have a load factor is increased. … And the largest number of elements you can have before the HashMap will try to double its size to 2^31 (which it cannot do) is ( 2^30 * loadFactor ) or about 700 million for the default load factor.

HashMap’s methods are not synchronized . HashTable is more or less obsolete and people writing new code should avoid using a HashTable. In a HashMap, keys are always unique. i.e., there cannot be 2 entries with the same key.