CODEWITHSUNDEEP
javaswinguser-interfaceactionlistenerjcombobox
DocBLKJK
DocBLKJK

Reputation: 23

2D JComboBox that one controling items of the other with ActionListener

I'm stuck on making two dropdown menus in Java GUI Form that the selection of first one will determine the choices in the second menu.

What I hope to achieve looks like this: enter image description here

And after I switched my choice in comboBox1, it looks like this: enter image description here

Here's my tester code:

    public static void main(String[] args) {
        Tester tester = new Tester();
        String[] flower = {"Rose", "Tulip"};
        String[] color1 = {"Yellow", "Blue", "Red"};
        String[] color2 = {"Purple", "White", "Green"};

        for (String flowerPicked : flower) {
            tester.comboBox1.addItem(flowerPicked);
        }
        tester.comboBox1.addActionListener(e -> {
            // remove previous items in comboBox2 everytime a new item in box1 is selcted
            tester.comboBox2.removeAllItems();
            String flowerChoice = tester.comboBox1.getSelectedItem().toString();
            if (flowerChoice.equalsIgnoreCase("Rose"))
                for (String colorPicked : color1) {
                    tester.comboBox2.addItem(colorPicked );
                }
            else
                for (String type : color2) {
                    tester.comboBox2.addItem(type);
                }
        });    
        tester.comboBox2.addActionListener(e -> {
            String colorChoice = tester.comboBox2.getSelectedItem().toString();
            String flowerChoice = tester.comboBox1.getSelectedItem().toString();
            system.out.println(colorChoice + " " + flowerChoice);
        });
    }

But I always run into NullPointerException at removeAllItems() and comboBox2.getSelectedItems() everytime I try to switch my choice in comboBox1.

I tried to debug it, but it seems it's because actionListener of comboBox2 was called whenever the program did removeAllItems() and comboBox2.addItem(). And I don't know how to handle this

A little help?

Upvotes: 2

Views: 24

Answers (1)

Hovercraft Full Of Eels
Hovercraft Full Of Eels

Reputation: 285405

You are right that removing all items from the JComboBox causes its ActionListener to fire and to return selection of null.

Possible solutions:

  1. Remove all ActionListeners from the JComboBox before removing all items, and then replace the listeners when done. -- or --
  2. Don't call toString() on the item returned (that's what throws the NPE -- calling toString() on a null reference) but rather cast the selected item returned as a String. A cast won't throw the NPE.

Example of the first:

ActionListener[] actionListeners = tester.comboBox2.getActionListeners();
for (ActionListener actionListener : actionListeners) {
    tester.comboBox2.removeActionListener(actionListener);
}
tester.comboBox2.removeAllItems();
String flowerChoice = tester.comboBox1.getSelectedItem().toString();
if (flowerChoice.equalsIgnoreCase("Rose"))
    for (String colorPicked : color1) {
        tester.comboBox2.addItem(colorPicked);
    }
else {
    for (String type : color2) {
        tester.comboBox2.addItem(type);
    }
}
for (ActionListener actionListener : actionListeners) {
    tester.comboBox2.addActionListener(actionListener);
}

Example of the second:

String colorChoice = (String) tester.comboBox2.getSelectedItem();
String flowerChoice = (String) tester.comboBox1.getSelectedItem();
System.out.println(colorChoice + " " + flowerChoice);

Upvotes: 1

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