what does Dead Code mean under Eclipse IDE Problems Section

Dead code is code that will never be executed, e.g.

 boolean b = true
 if (!b) {
    .... 
    // dead code here
 }

Dead code means, that there is no way that this code will be executed.

Sometimes you even can't compile it (like this case:)

private Boolean dead_code()
    {
    return true;
    //Dead code below:
    dosomething();
    }

But in other cases this is not too obvious, eg this statement:

   b=true;
   [...]
   if (b==false)
    {
    //Dead code
    }

If you have this message, there is some major flaw in your code. You have to find it, otherwise your app won't work as intended.

In Eclipse, "dead code" is code that will never be executed. Usually it's in a conditional branch that logically will never be entered.

A trivial example would be the following:

boolean x = true;
if (x) {
   // do something
} else {
   // this is dead code!
}

It's not an error, because it's still valid java, but it's a useful warning, especially if the logical conditions are complex, and where it may not be intuitively obvious that the code will never be executed.

In your specific example, Eclipse has calculated that ar will always be non-null, and so the else length = 0 branch will never be executed.

And yes, it's possible that Eclipse is wrong, but it's much more likely that it's not.