Question

Possible Duplicate:
Concatenating null strings in Java

I’m just wondering if someone can explain why does the following code works the way it does:

String a = null;
String b = null;
String c = a + b;
System.out.println(c.toString());

This prints “nullnull” to the console.

I was kind of expecting that the operation + would thrown an exception or, to a lesser degree, that “c” would be null.

Answer 1

“String conversion,” required by the concatenation operator, mandates that null values are to be mapped to the four characters null whenever a string representation is needed.

Spec for string conversion: http://docs.oracle.com/javase/specs/jls/se7/html/jls-5.html#jls-5.1.11

Spec for string concatenation: http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.18.1