java if statement doesn't work inside if(item instanceof nomclass)

Question

I have just wrote an actionPerformed for my save button, that will save data into arraylists, but before that I must be sure that all fields are not empty so if a textfield is empty I want to show a dialogbox and put all empty textfield in red background color

here is my code

//Field outside constructor
private List<Component> comp;





//inside constructor
comp = getAllComponents(this);
//method
public static List<Component> getAllComponents(final Container c) {
    Component[] comps = c.getComponents();
    List<Component> compList = new ArrayList<Component>();
    for (Component comp : comps) {
        compList.add(comp);
        if (comp instanceof Container)
            compList.addAll(getAllComponents((Container) comp));
    }
    return compList;
}

``

//actionperformed
if(e.getSource() == savebtn){
        for(Component item:comp){
            if(item.isVisible()){
                if(item instanceof JTextField){
                    JTextField txtField = (JTextField)item;
//here is my problem: with no if statement my program works fine and puts all textfields in red but I want to highlight just empty textfields;
                    if(txtField.getText() == null)
                    txtField.setBackground(Color.RED);
                }
            }
        }


    }

so how can I solve this problem? thank you very much

If you could provide a bit more body to your code, you could get some help quicker. You're kind of leaving us to guess and infer what types these are. — Makoto, Dec 01 '14 at 08:10
`instanceof` doesn't work if the class is not the same, debug what the class is where it is meant to work but doesn't. — EpicPandaForce, Dec 01 '14 at 08:11
I read the question for several times, I finally figure out the question is inside the code...and what do you mean by *not works fine*? Please try to make the question specific, it is not our responsibility to guess what your problem is. No offense. Anyway, did you print out what is `txtField.getText()`? Is it really `null` or simply empty? — Victor Wong, Dec 01 '14 at 08:20

score 2 · Answer 1 · answered Dec 01 '14 at 08:13

if(txtField.getText() == null)

Here you're not checking if it's empty, see the documentation of public String getText()¹:

Returns the text contained in this TextComponent. If the underlying document is null, will give a NullPointerException. Note that text is not a bound property, so no PropertyChangeEvent is fired when it changes. To listen for changes to the text, use DocumentListener.

I would use a DocumentListener instead, something like that:

myTextField.getDocument().addDocumentListener(new DocumentListener() {

    public void changedUpdate(DocumentEvent e) { checkIfEmpty(); }
    public void removeUpdate(DocumentEvent e) { checkIfEmpty(); }
    public void insertUpdate(DocumentEvent e) { checkIfEmpty();}

    public void checkIfEmpty() {
       if (myTextField.getText().equals("")){
           //set color to red
       } else {
           //set color back
       }   
    }
});

¹ Also note the signature of the method, it returns a String.

thank you very much, if(txtField.getText().isEmpty()) txtField.setBackground(Color.RED); works for me, but I will use your suggestion (textField.getDocument().addDocumentListener()) to get Back the default color when I type some words inside — Abderazak Elmerakchi, Dec 01 '14 at 09:50

score 0 · Accepted Answer · answered Dec 01 '14 at 09:22

Yes Maroun Maroun is right you're checking if the String is null (The Object doesn't exsit). But you want to check if the String is empty. I think the other answer solves you're Problem, but the cleaner solution is to use the isempty method.

if(txtField.getText().isEmpty())
txtField.setBackground(Color.RED);

I would have added a comment to the other answer, but my reputation is not high enough...

java if statement doesn't work inside if(item instanceof nomclass)

2 Answers2