According to "Requirements for Writing Java API Specifications"[1]:
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  Method Specification

    This section applies to Java methods and constructors. Each method and
    constructor specification must include:
       ...
       4. Null Argument Values - For each reference type argument, specify the
          behavior when null is passed in. See two examples. NOTE: If possible,
          document the general null argument behavior at the package or class
          level, such as causing a java.lang.NullPointerException to be thrown.
          Deviations from this behavior can then be documented at the method
          level.
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such cases should be described in the appropriate javadoc(s).

[1] http://java.sun.com/j2se/javadoc/writingapispecs/index.html#method