Java 9 Concurrency Cookbook - Second Edition

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The synchronized keyword penalizes the performance of the application, so you must only use it on methods that modify shared data in a concurrent environment. If you have multiple threads calling a synchronized method, only one will execute them at a time while the others will remain waiting. If the operation doesn't use the synchronized keyword, all the threads can execute the operation at the same time, reducing the total execution time. If you know that a method will not be called by more than one thread, don't use the synchronized keyword. Anyway, if the class is designed for multithreading access, it should always be correct. You must promote correctness over performance. Also, you should include documentation in methods ...