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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 082008]. 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 ignoring unchecked warnings are discussed extensively in MSC00-J. Do not mix generic with non-generic raw types in new code.

According to the Java API [API 062006] Annotation Type SuppressWarnings documentation:

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This noncompliant code example is from the implementation of java.util.ArrayList. When the class is compiled, it emits an unchecked cast warning, as shown. As the return statement is not a declaration, the Java Language Specification [JLS 052005] makes it impossible to suppress the warning trivially. Consequently, the @SuppressWarnings is used over method scope. This can cause issues when some functionality that performs type-unsafe operations is added to the method at a later date. [Bloch 082008]

@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
  if (a.length < size)
    return (T[]) Arrays.copyOf(elements, size, a.getClass()); // Produces unchecked warning
 // ...
}

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Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[Bloch 082008] Item 24: "Eliminate unchecked warnings"

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