Question on String Operation.

Question

Hi all, My understanding of java String got wrong when i see this code. I am not sure how this is happening. Can anyone explain why is so?.

public class NewClass {
    public static void main(String[] args) {
        String str=null;

        System.out.println(str+"Added");
    }
}

output:nullAdded

Answer 1

String variable str is given the null value.

When you print it, the String#toString() converts that to the string 'null', and appends the string "Added". The result is

nullAdded

Answer 2

String concatenation treats a null reference as if it were "null" in Java. From the docs for StringBuilder.Append(String):

The characters of the String argument are appended, in order, increasing the length of this sequence by the length of the argument. If str is null, then the four characters "null" are appended.

Basically your code is similar to:

public class NewClass {
    public static void main(String[] args) {
        String str=null;

        System.out.println(new StringBuilder().append(str)
                                              .append("Added")
                                              .toString());
    }
}

Does that make it clearer?

Answer 3

What happens is that when you use the + operator, both operands are converted to a string (if required) via String.valueOf method. This returns the four-character string "null" for a null reference, rather than an actual null reference.

Thus null + "Added" is equal to "null" + "Added" and hence "nullAdded".

Answer 4

What you might want to do is initialise the string with the empty string:

String str = "";

Cheers

Tom

Answer 5

Java will/had always when concatanating a string converted null objects into the string null. via String.valueOf.

Java knows it is concatenating into a string, so even if the null is at the start (not at the end) of the expression it will still convert it to null.

you don't get a nullpointerException since no operation is called on the null object. A Little strange I know. but it is what it is.

link: concat null and String

Answer 6

With other words, you'd expect a NullPointerException here?

Well, the key is: the + is an operator, it is not a field or method of the String class. So there's no need to access the str reference.

You would only get NPE when you do something like

obj.someMethod();
// or
obj.someField;

while the obj is null.