This is the expected behaviour. Println converts any of its non-String arguments to their string representation using wrapper classes and their toString methods when necessary.
This is what the API documentation says about the toString method -
Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.
Since we are dealing with null references and/or null literals displaying "null" as a string seems certainly is more friendlier than raising an exception.
You might want to read JLS section
15.18.1.1 String Conversion which explains how null references and null literals ought to be treated -
Any type may be converted to type String by string conversion.
If the reference is null, it is converted to the string "null" (four ASCII characters n, u, l, l). Otherwise, the conversion is performed as if by an invocation of the toString method of the referenced object with no arguments; but if the result of invoking the toString method is null, then the string "null" is used instead.
Hope that helps!
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Ajith Kallambella M. Sun Certified Programmer for the Java�2 Platform.
IBM Certified Developer - XML and Related Technologies, V1.