Synchronized solve the problem of bank withdraw money __java

when multiple threads simultaneously access a common resource and make modifications, an error occurs, such as the question of the bank taking money: Create a new Acount.java to encapsulate bank accounts and balances, as follows:

A new Firstthread.java is created to represent the current thread:
Run results, the account balance will appear negative, it is obvious, when one of the threads to withdraw money, another thread also take money, the results are as follows:

At this time should be the account acount lock, that is, the use of the Synchronized keyword, after the lock, when the cable access, will acquire a lock, and its resources to modify operations, other threads only when the thread has been modified and release the lock to access it, This locking--modification--the mode of releasing locks solves the error caused by multiple threads modifying resources at the same time. in the code on the Fristthread.java diagram, remove 11 lines and 27 lines of comments, which run as follows: