Question

OMG. I have a little project to do and the Strings are killing me!

Now, I have a String which is null (is taken the value from invoking getParameter() from a servlet).

The problem is that, I’m trying to see if it’s null, and, even if it’s null, in the program is telling me that is not null, but later in program, when I’m using the variable, I receive a exception saying the variable is null.

    System.out.println("In  " + ID); // in console: In null
    if ((ID == null) || (ID == "null") || ID.equals(null) || **ID.equals("null")**)
    {

       // after I put the 4th condition, the if is working right (WHAT IS THE PROBLEM?)            
        System.out.println("==null");
        this.ID = "";
    }
    else
    {
        System.out.println("!=null");
        this.ID = ID;
    }
    System.out.println("After " + ID);

What I’m doing wrong?

Only the forth condition is working! What about the rest(except second one, because that condition i put it because I was desperate)

I taught ID == null or ID.equals(null) will be ok, but no.

Edit:
The problem is that, I’m getting the value of the ID from a form(form 1 let’s say- usually). But in this case, I’m using form 2 which doesn’t have any ID inputs, so ID must be null and not "null"

Answer 1

Since you get the string from a servlet i can say that this is normal.

Java converts a null string to a “null” string on some conditions.

Obviously the string you retrieve is not a null value, but it is a 4 char string “null”

Why don’t you try debugging? Or just see what does this return:

System.out.println("Length of ID:  " + ID.Length);

Edit: If you don’t get exception here, this means that the string is not null and also output “Length of ID: 4” will mean that the string is really ID = “null”

EDIT2: Alright it seems that some guys do not understand what is going on here and they say how can a null string be “null” in some conditions in Java? They find it riddiculus. I prefer them to try this on java:

String abc = null;
String xyz = "hello"; 
System.out.println(xyz + abc); 

The output will be “hellonull” Nothing else…

Also here we have a servlet. There is a null data. Servlet sends the null data as “null” what should it do? An empty string? Come on!!! ”