How to make updating BigDecimal within ConcurrentHashMap thread safe

Question

I am making an application that takes a bunch of journal entries and calculate sum.

Is below way of doing it is thread/concurrency safe when there are multip

Answer 1

Yes, you need to synchronize since otherwise you can have two threads each getting the same value (for the same key), say A and thread 1 add B to it and thread 2 adds C to it and store it back. The result now will not be A+B+C, but A+B or A+C.

What you need to do is lock on something that is common to the additions. Synchronizing on get/put will not help, unless you do

synchronize {
    get
    add
    put
}

but if you do that then you will prevent threads from updating values even if it is for different keys. You want to synchronize on the account. However, synchronizing on the string seems unsafe as it could lead to deadlocks (you don't know what else locks the string). Can you create an account object instead and use that for locking?