Java – synchronize on values, not objects

I want to do this in Java

public void giveMoney(String userId,int money) {
    synchronized (userId) {

        Profile p = fetchProfileFromDB(userId);
        p.setMoney(p.getMoney() + userId);
        saveProfileToDB(p);

    }
   }

But of course, string synchronization is incorrect What is the right way to do such a thing?

Solution

If the user ID collection is limited, you can synchronize on the build string

Use string Intern () (it has some drawbacks) or something like guava interners if you need more control internships