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.
because that code compiles to
String c = new StringBuilder().append(a).append(b).toString();
and StringBuilder.append(a) appends the result of String.valueof(a) which is "null" if a is null
String c = new StringBuilder(String.valueOf(a)).append(b).toString();. The real reason is the String.valueOf(Object) implementation.
append(null) also needs to append "null""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
When java find a null reference during the string concatenation convert it to the string "null". So "nullnull" is the right result.
Before concatenation for every object Java call
java.lang.String.valueOf(Object)
and if you look at the source code of that method you find this
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}