7011326: Add informative example to @SafeVarargs type or language discussion

Reviewed-by: mcimadamore, mduigou

src/share/classes/java/lang/SafeVarargs.java

 /*
- * Copyright (c) 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2010, 2011, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -32,7 +32,7 @@ import java.lang.annotation.*;
  * constructor does not perform potentially unsafe operations on its
  * varargs parameter.  Applying this annotation to a method or
  * constructor suppresses unchecked warnings about a
- * <i>non-reifiable</i> variable-arity (vararg) type and suppresses
+ * <i>non-reifiable</i> variable arity (vararg) type and suppresses
  * unchecked warnings about parameterized array creation at call
  * sites.
  *
@@ -41,11 +41,10 @@ import java.lang.annotation.*;
  * additional usage restrictions on this annotation type; it is a
  * compile-time error if a method or constructor declaration is
  * annotated with a {@code @SafeVarargs} annotation, and either:
-
  * <ul>
- * <li>  the declaration is a fixed-arity method or constructor
+ * <li>  the declaration is a fixed arity method or constructor
  *
- * <li> the declaration is a variable-arity method that is neither
+ * <li> the declaration is a variable arity method that is neither
  * {@code static} nor {@code final}.
  *
  * </ul>
@@ -55,15 +54,28 @@ import java.lang.annotation.*;
  *
  * <ul>
  *
- * <li> The variable-arity parameter has a reifiable element type,
+ * <li> The variable arity parameter has a reifiable element type,
  * which includes primitive types, {@code Object}, and {@code String}.
  * (The unchecked warnings this annotation type suppresses already do
  * not occur for a reifiable element type.)
  *
  * <li> The body of the method or constructor declaration performs
  * potentially unsafe operations, such as an assignment to an element
- * of the variable-arity parameter's array that generates an unchecked
- * warning.
+ * of the variable arity parameter's array that generates an unchecked
+ * warning.  Some unsafe operations do not trigger an unchecked
+ * warning.  For example, the aliasing in
+ *
+ * <blockquote><pre>
+ * &#64;SafeVarargs // Not actually safe!
+ * static void m(List&lt;String&gt;... stringLists) {
+ *   Object[] array = stringLists;
+ *   List&lt;Integer&gt; tmpList = Arrays.asList(42);
+ *   array[0] = tmpList; // Semantically invalid, but compiles without warnings
+ *   String s = stringLists[0].get(0); // Oh no, ClassCastException at runtime!
+ * }
+ * </pre></blockquote>
+ *
+ * leads to a {@code ClassCastException} at runtime.
  *
  * <p>Future versions of the platform may mandate compiler errors for
  * such unsafe operations.