How does null work in this code? [duplicate]

Answer 1

As String is a subclass of Object, Java will try and use the class furthest down in the hierarchy that is applicable. null is able to be interpreted as either here, therefore the String method is used.

Answer 2

Java will always try to use the most specific version of a method.

Since the call

t.doStuff(null);  

is applicable to both methods

t.doStuff(Object o)
t.doStuff(String o)

Java will choose the most specific method description, which is

t.doStuff(String o)