... checks¹

Commercial software tools, such as PREfixco Corp.'s PREfix, that are based on software component simulation technology, may provide means to detect some errors prior to software deployment that would otherwise be caught only at run-time.

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... instance²

By an instance of an interface we mean an object that is an instance of a class that implements the interface.

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...Stack³

Note that this example does not reflect the current design of the Java APIs, where Stack is a class with one fixed implementation as a subclass of Vector. This includes all Vector operations in the interface of Stack, enabling to bring a Stack into an inconsistent state, for example, by directly calling the method add(atIndex, elem).

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