Null reference converted to a String

Question

I read somewhere that when a null reference is converted to a string, the result is the String "null", hence a null reference can be used freely within any string concatenation expression.
for example:
String str1 = null;
String str2 = null;
System.out.println(str1+=str2);//outputs "nullnull"
Before I read about this, I would have thought that a NullPointerException would have been thrown.
Can anyone explain the rationale behind this?

Answer 1

This is not a property of String, its the property of
the println method in the PrintStream class.No, that's not correct.
Whenever Strings are concatenated, non-Strings are converted to Strings and null Strings are converted to the String "null". This is a property of the Java language itself.
The expression a + b, where either a or b is a String, is converted into the expression:
new StringBuffer().append(a).append(b).toString();
It is the append method of StringBuffer that calls the toString method of non-String objects, calls String.valueOf on primitives, and converts null String references into the String "null". Even though it's an API call that technically produces this behavior, it is actually a feature of the Java language itself as is therefore explained in the JLS. See http://java.sun.com/docs/books/jls/second_edition/html/expressions.doc.html#39990 for details.