Strange Java compiler warning: incorrect "potential null access warning"

Question

(JDK 1.6.0_23, Eclipse 3.7.0 with "Potential null pointer access" warning level at "Warning") Consider the following example code:

Object obj = null;
for (;;) {
    obj = getObject();
    if (obj != null) break;
    Thread.sleep(25);
}
obj.toString();

I'm getting the following warning on the last line: Potential null pointer access: The variable obj may be null at this location. Is there any real way for obj to actually be null or why the compiler thinks so?

Answer 1

I'd say the compiler sees this as:

Object obj = null;
[Too dumb to interpret this loop]
obj.toString();

So obj.toString() could be null if you cannot interpret what the loop does.

For example you can trick the compiler by replacing:

void x(int x){
    return;  
    x++;  //error unreachable code
}

with

void x(int x){
    if(true) return;
    x++;  //compiles
}