基于JComboBox值的Java JButton.setVisible

Question

Ok, I'm stymied. I know I'm a Java noob, and I've done something stupid, but I hate to run to StackOverflow every time I'm stumped. Anyway, here goes:

I'm building a simple little app with a form. Fill out the form and it calculates some values for you. I have a JComboBox that sets some default values in a series of JTextField's. There are a couple of buttons (that at this point, don't do anything) that I want to either be visible or hidden based on the value in the JComboBox. No biggie, right?

Currently, I'm able to see and manipulate the JTextField's in the JComboBox's actionPerformed() method. But if I try to do change the setVisible(true) on a JButton, I get a NullPointerException.

I'll post some code, but not all of it. I've been doing this in Eclipse Kepler with WindowBuilder (I know, I know...) so the code is fairly massive.

Here's where one of the JTextFields and the JButton are defined:

    txtAC = new JTextField();
    txtAC.setFont(new Font("Tahoma", Font.BOLD, 12));
    txtAC.setHorizontalAlignment(SwingConstants.CENTER);
    txtAC.setToolTipText("Enter the percentage of facility electric usage resulting from air conditioning.");
    txtAC.setForeground(new Color(0, 102, 204));
    txtAC.setBounds(143, 180, 53, 20);
    contentPane.add(txtAC);
    txtAC.setColumns(10);

    JButton btnAdd = new JButton("Add");
    btnAdd.setToolTipText("Click the Add button to include the custom facility type to the Facility Type dropdown list.");
    btnAdd.setBounds(573, 60, 89, 20);
    btnAdd.setVisible(false);
    contentPane.add(btnAdd);

Here's the actionPerformed method on the JComboBox:

    final JComboBox cboFacilityType = new JComboBox();
    cboFacilityType.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent e) {

            if (isInitCBO)
                return;

            String sql = "SELECT * FROM tblDefaultIndustry WHERE INDUSTRY = '" + cboFacilityType.getSelectedItem() +"'";


            try
            {
                rs = stmt.executeQuery(sql);
                while (rs.next())
                {
                    txtLighting.setText("");
                    txtAC.setText("");
                    txtRefrig.setText("");
                    txtEquip.setText("");
                    txtResistive.setText("");

                    if (rs.getString("INDUSTRY").equals("<User Defined>"))
                    {
                        usageFieldsEditable(true);
                        btnAdd.setVisible(true);
                        txtLighting.requestFocusInWindow();
                    }else{
                        txtLighting.setText(Integer.toString(rs.getInt(2)));
                        txtAC.setText(Integer.toString(rs.getInt(3)));
                        txtRefrig.setText(Integer.toString(rs.getInt(4)));
                        txtEquip.setText(Integer.toString(rs.getInt(5)));
                        txtResistive.setText(Integer.toString(rs.getInt(6)));
                        usageFieldsEditable(false);
                        btnAdd.setVisible(false);

                    }
                }
            }catch (SQLException e1)
            {
                e1.printStackTrace();
            }

        }

        private void usageFieldsEditable(boolean b) {
            // TODO Auto-generated method stub
            txtLighting.setEditable(b);
            txtAC.setEditable(b);
            txtRefrig.setEditable(b);
            txtEquip.setEditable(b);
            txtResistive.setEditable(b);
        }
    });

At first, the btnAdd acted like it didn't have any variable associated with it. I noticed that WindowBuilder had created private variables for the JTextFields but not the JButton. So I added one to see if that would work. No dice. Didn't make sense anyway.

Hopefully you have enough to go on here. It just doesn't make sense that the actionPerformed() method can see and manipulate a JTextField but not a button. They are in the same Panel and everything.

Thanks for your sage wisdom.

Answer 1

It definitely makes sense to have a field for the button, since the actionPerformed() method needs to access the button to meke it visible or hidden. But you get a NullPointerException, which means that the btnAdd field is not initialized. And indeed, instead of doing

this.btnAdd = new JButton("Add");

or simply

btnAdd = new JButton("Add");

which would initialize the btnAdd field, you're doing

JButton btnAdd = new JButton("Add");

That declares and initializes a local variable, which happens to have the same name as the btnAdd field, leaving this field uninitialized.

Side notes: you should learn using layout managers instead of hard-coding bounds of components, making your app ugly on machines with different settings from yours. And you should also try not mixing data access code, using JDBC, with UI code. Delegate the data access to another object.

Answer 2

You have initialize JButton as

JButton btnAdd = new JButton("Add");

which is local initialization of button variable and when you are trying to use the object of JButton it goes out of scope.

try this

Declare the JButton btnAdd; and then initialize this reference of button with

btnAdd = new JButton("Add");