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 create a bank interface in java. Q ahd 4 parts, each of the next part needed some refactoring in previous parts. We had to support operations like:

Deposit in a account P1
Withdraw from account P1
Check balance P1
Find top K users with maximum transaction activity P2
2-3 more functions, cannot remember for P2
Begin Money Transfer to a given account - P3 . It should deduct from the account but not deposit unless second account accepts the transaction. Incorrect inputs must be taken care of.
Accept Transfer into account from a given account. P3. Note that this should update the financial activity only on accepting transfer for both accounts.
Merge two accounts but history for individual should be available P4
Give me balance at some timestamp T.