Java String.valueOf(null) throws NPE, but Object a = null; String.valueOf(a) returns 'null' [duplicate]

Question

Is there a logical language-design-type explanation for the following behaviour (Java 7 and I suspect earlier editions as well):

    Object  a = null;     String as = String.valueOf(a);              // as is assigned "null"     System.out.println(as+":"+as.length());     // prints: "null:4"     System.out.println ( String.valueOf(null)); // NPE 

Answer 1

In statement System.out.println(String.valueOf(null)); there is a call of method public static String valueOf(char data[]), which source code is as follows:

public static String valueOf(char data[]) {   return new String(data); } 

That is why you get NPE

On the other hand, in statement Object a = null; String as = String.valueOf(a); there is a calls of method public static String valueOf(Object obj), which source code is as follows:

public static String valueOf(Object obj) {     return (obj == null) ? "null" : obj.toString(); } 

That is why you get "null" instead of NPE

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A bit of theory from Java Language Specification: 15.12.2.5 Choosing the Most Specific Method

If more than one member method is both accessible and applicable to a method invocation, it is necessary to choose one to provide the descriptor for the run-time method dispatch. The Java programming language uses the rule that the most specific method is chosen.

A char[] is of type Object, but not all Object are of type char[]. Type char[] is more specific than Object and as described in the Java Language Specification, the String.valueOf(char[]) overload is chosen in this case.

EDIT

It is also worth mentioning what Ian Roberts mentioned (in his comment below):

It's important to note that it's a compile error if there is no single overloading that is more specific than all the others - if there were a valueOf(String) method as well as valueOf(Object) and valueOf(char[]) then a call to the untyped String.valueOf(null) would be ambiguous