HashMap and concurrency- different keys

Question

Say I have a hash map and multiple threads. If I have a synchronized method that adds to the hash map, how would I make it possible that two different threads can put differ

Answer 1

However, if I am trying to add another String into an already contained key's corresponding LinkedList, I can just synchronize the hash map's get method. I think this will allow multiple threads to simultaneously (concurrently) add to the LinkedLists of different, already contained keys.

Read-only access to a HashMap is safe: you can have multiple threads call the get method with no synchronization at all and nothing breaks. If the linked lists are not shared between threads you don't need synchronization either. To be sure the threads never share a list the map key should be something specific to the thread, like an object created locally or the thread ID.

What's not safe is to let another thread modify a map or a list concurrently with a read or write. This is the use case for a read-write lock: it allows multiple concurrent reads but writes have to be exclusive.