How to check null on StringBuilder?

Question

I want to check for null or empty specifically in my code. Does empty and null are same for StringBuilder in Java?

For example:

StringBuilder state = new StringBuilder();
StringBuilder err= new StringBuilder(); 
success = executeCommand(cmd, state, err); 

/* here executeCommand() returns empty or null in state, I cant  make changes in  <br/> executeCommand() so can I check it in my code somehow for state, if its null or empty? */<br/>

if (state == null) { //do blabla1 }
if (state.tostring().equals("")) { //do blabla2 }

Does above code make sense or how should I change it?

Answer 1

No, null and empty are different for StringBuilder.

StringBuilder nullBuilder = null;
if(nullBuilder == null) {
    System.out.println("Builder is null");
}

&

StringBuilder emptyBuilder = new StringBuilder("");
if(emptyBuilder == null || emptyBuilder.toString().equals("")) {
    System.out.println("Builder is empty");
}

Answer 2

In Java, null is a reference literal. If a variable is null then is not referring to anything.

So, if you have StringBuilder s = null, that means that s is of type StringBuilder but it is not referring to a StringBuilder instance.

If you have a non-null reference then you are free to call methods on the referred object. In the StringBuilder class, one such method is length(). In fact if you were to call length() using a null reference then the Java runtime will throw a NullPointerException.

Hence, this code is quite common:

If (s == null || s.length() == 0/*empty if the length is zero*/){
    // do something

It relies on the fact that evaluation of || is from left to right and stops once it reaches the first true condition.