ceving ceving - 3 months ago 5
Java Question

What is the difference between "(Object)null" and "null" in Java?

Take a look at the following example:

class nul
{
public static void main (String[] args)
{
System.out.println (String.valueOf((Object)null));
System.out.println (String.valueOf(null));
}
}


The first
println
writes
null
but the second throws a
NullPointerException
.

Why is only the second line worth an exception? And what is the difference between the two
null
s? Is there a real
null
and a fake
null
in Java?

Answer

The first invocation will call the String.valueOf(Object) method, as you have explicitly typecasted null to Object reference. Conversely, the second one will invoke the overloaded String.valueOf(char[]) method, as char[] is more specific than Object for a null argument.

There are other overloaded versions of this method that accept primitive parameters, but those are not a valid match for a null argument.

From JLS §15.12.2:

There may be more than one such method, in which case the most specific one is chosen. The descriptor (signature plus return type) of the most specific method is one used at run time to perform the method dispatch.

A method is applicable if it is either applicable by subtyping (§15.12.2.2), applicable by method invocation conversion (§15.12.2.3), or it is an applicable variable arity method (§15.12.2.4).

[...]

If several applicable methods have been identified during one of the three phases of applicability testing, then the most specific one is chosen, as specified in section §15.12.2.5.

Now check the source code of both the methods:

// This won't throw NPE for `obj == null`
public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

// This will throw `NPE` for `data == null`
public static String valueOf(char data[]) {
    return new String(data);
}