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collision, and quality of hash function

load factor

When the hash function keep on targeting the same position in
HashTable In case of h(k) is always same, example we take %2 and all numbers are even so it always go into the even slot of hashTable.

Quality of Hash function Depends on your data so you can never blame such thing, rather quality can differ from Data to Data even with highly efficient hash technique .

locking .. Hashtable is synchronized.

its hashcode and size of the hashatble. Efficient hashcode() & proper size will reduce the collission and rehashing