Concatenating null strings in Java

Question

Why does the following work? I would expect a NullPointerException to be thrown.

String s = null;

s = s + "hello";

System.out.println(s); // prints "nullhello"

Answer 1

The second line is transformed to the following code:

s = (new StringBuilder()).append((String)null).append("hello").toString();

The append methods can handle null arguments.