Question

Here is a simple code snippet and I cannot figure out why does it throw a NullPointerException.

  String lastGroup = "";
  menuTevekenysegekGrouped = new ArrayList<MenuElem>();
  for(MenuElem me : menuA) {
    // double checked that me objects are never null
    // double checked that menuA is never null
    if(me.getGroup() != null && !me.getGroup().equals(lastGroup)) { /* NPE!!! */
        lastGroup = me.getGroup();
        MenuElem separ = new MenuElem();
        separ.setCaption(lastGroup);
        separ.setGroupHead(true);
        menuTevekenysegekGrouped.add(separ);
        menuTevekenysegekGrouped.add(me);
    } else {
        menuTevekenysegekGrouped.add(me);
    }
  }

In the first iteration the me.getGroup() returns null. So the first operand of the && is false and second operand should not evaluate according to the JLS, as far as I know. However when I debug the code I get NPE from the marked line. I’d like to know why. (Using JRockit 1.6.0_05 if it matters..)

Answer 1

Are you sure that me itself is not, in fact, null?