I think the example mentioned by you will throw a NullpointerException ( Runtime) , since in the system.out.println you are trying to use a reference variable not referring to any object. ( Its having null value).
Actually it doesn't throw NullpointerException. Indeed the null is first concatinated with "" string. After that when null has been concatinated to "" String then it convert the null+"" into String with toString() method. So if we convert null+"" into String, no reason for NullpointerException.
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