Well, that picture is incorrect, because it says the yellow classes and interfaces are in java.util, but Buffer and ArrayStack are definitely not in the package java.util.
ArrayStack from Apache Commons Collections and java.util.Vector indeed have the same functionality, but as the API documentation explains, ArrayStack is based upon java.util.ArrayList, whereas Stack is based upon Vector. The main difference between ArrayList and Vector is that in Vector, all methods are synchronized, but not in ArrayList. The differences between Vector and ArrayList have been discussed before; if you
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