StringBuilder append() and null values

Question

I have a list of Strings, and I want to concatenate them with spaces in between. So I'm using StringBuilder. Now if any of the Strings are null, they get stored in the StringBuilder literally as 'null'. Here is a small program to illustrate the issue:

public static void main(String ss[]) {
    StringBuilder sb = new StringBuilder();

    String s;
    s = null;

    System.out.println(sb.append("Value: ").append(s));
}

I'd expect the output to be "Value: " but it comes out as "Value: null"

Is there a way around this problem?

Answer 1

You can do a check on the object before appending it:

sb.append("Value: ");
if (s != null) sb.append(s);
System.out.println(sb);

A key point to make is that null is not the same an an empty String. An empty String is still a String object with associated methods and fields associated with it, where a null pointer is not an object at all.

From the documentation for StringBuilder's append method:

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.

Answer 2

Null check as @Shynthriir has mentioned is the simplest most efficient way (AFAIK) to get around the problem.

I would however strongly recommend initiating strings to an empty string String str = ""; instead, which would save you a lot of headache in a more complex programming effort.

Answer 3

I'm not sure why you'd expect it to come out empty, given that the documentation is pretty clear:

If str is null, then the four characters "null" are appended.

Basically you need to either not call append at all if you have a null reference, or switch the value for "".

You could write a method to do this substitution if you find yourself doing it a lot:

public static String nullToEmpty(String text) {
    return input == null ? "" : text;
}

Indeed, I've just looked at the Guava documentation and the Strings class has exactly that method (but with a parameter called string instead of text).