If nothing is selected in Jlist

Question

I´m writing a program where different symbols are drawn on an image based on what the user selects in a JList. This works fine, but my problem is that I want to place a symbol on the image if nothing is selected in the JList as well. Is there some way to check if the selection is empty? This is my code right now, and it throws me a NullPointerException if I don´t select anything from the JList.

if(categoriesList.getSelectedValue().equals("Bus")) {
            BusSymbol bs = new BusSymbol(x, y);
            mp.add(bs);
        } 
        else if 
(categoriesList.getSelectedValue().equals("Underground")) {
            UndergroundSymbol us = new UndergroundSymbol(x,y);
            mp.add(us);
        }
        else if (categoriesList.getSelectedValue().equals("Train")) {
            TrainSymbol ts = new TrainSymbol(x,y);
            mp.add(ts);
        }
        else if (categoriesList.getSelectedValue().equals(null)) {

            NoCategorySymbol ncs = new NoCategorySymbol(x,y);
            mp.add(ncs);
        }

        mp.validate();
        mp.repaint();

That's because null has no .equals method. If something might be null, you want to check it with == null (or ideally with another method ; isn't there a hasSelectedValue() method on that API ?) — Aaron

Aaron Aaron · Accepted Answer · 2017-05-10T09:36:03

You should use JList.isSelectionEmpty() as a condition instead of testing against the value that might be null.

You should either make this test first, and/or change all the other tests which have this unfortunate syntax :

mightBeNull.equals(neverNull)

As a general rule it is generally better to avoid this syntax and use neverNull.equals(mightBeNull) instead, as it avoids attempting to invoke the inexistant null.equals() method.

I propose the following code :

if (categoriesList.isSelectionEmpty()) {
    mp.add(new NoCategorySymbol(x,y));
} else {
    // here we know categoriesList.getSelectedValue() isn't null
    String selectedValue = categoriesList.getSelectedValue());
    if ("Underground".equals(selectedValue)) {
       mp.add(new UndergroundSymbol(x,y));
    } else if ("Train".equals(selectedValue)) {
       mp.add(new TrainSymbol(x, y));
    }
}
mp.validate();
mp.repaint();

If nothing is selected in Jlist

3 Answers