The compiler issues unchecked warnings when it detects potential type safety issues arising from mixing raw types with generic code. This includes unchecked cast warnings, unchecked method invocation warnings, unchecked generic array creation warnings and unchecked conversion warnings [Bloch 08]. It is permissible to use the @SuppressWarnings("unchecked") annotation to suppress unchecked warnings if and only if the warning emitting code is guaranteed to be typesafe. A common usecase is mixing legacy code with new client code. The perils of relaxing ignoring unchecked warnings are discussed extensively in MSC05-J. Do not mix generic with non-generic raw types in new code.
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