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Reports variables that are assigned to <code>null</code> outside a declaration.
<p>The main purpose of <code>null</code> in Java is to denote uninitialized
  reference variables. In rare cases, assigning a variable explicitly to <code>null</code>
  is useful to aid garbage collection. However, using <code>null</code> to denote a missing, not specified, or invalid value or a not
  found element is considered bad practice and may make your code more prone to <code>NullPointerExceptions</code>.
 