Looked more carefully into article and found it really disappointing. The code which most likely beginners will be following in their applications contains a number of significant issues:
Using mutable POJO for storing users is a poor habit
Using non-synchronized collection for storing data which can be accessed concurrently will result to unpredictable behavior
Modifying collection while iterating over it is a poor habit
No input validation for REST methods
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Looked more carefully into article and found it really disappointing. The code which most likely beginners will be following in their applications contains a number of significant issues: