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According to the Java API, Annotation Type SuppressWarnings documentation [API 2011],

As a matter of style, programmers should always use this annotation on the most deeply nested element where it is effective. If you want to suppress a warning in a particular method, you should annotate that method rather than its class.

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Limit the scope of the @SuppressWarnings annotation to the nearest code that generates a warning. In this case, it may be used in the declaration for the Set.:

class Legacy {
  @SuppressWarnings("unchecked")
  Set s = new HashSet();
  public final void doLogic(int a,char c) {
    s.add(a); // Produces unchecked warning
    s.add(c); // Produces unchecked warning
  }
}

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Because the return statement is not a declaration, the Java Language Specification [JLS 2011] makes it impossible to suppress the warning trivially by using @SuppressWarnings at that statement. Consequently, the @SuppressWarnings is used over method scope where it is allowed. This can cause issues when some functionality that performs type-unsafe operations is added to the method at a later date [Bloch 2008].

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This rule cannot be statically enforced in full generality; static analysis could be possible for some special cases.

Bibliography

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