如何将值加载到JComboBox中，具体取决于另一个JComboBox的选择？

Question

Here is my code:

JComboBox unity=new JComboBox();
unity.setBounds(430,280,140,25);
unity.addItem("Pakistan");
unity.addItem("China");
unity.addItem("America");
unity.addItem("Japan");
unity.addItem("Bangladesh");
unity.addItem("Srilanka");
unity.addItem("India");
unity.addItem("Turkey");
unity.addItem("UK");
unity.addItem("Afghanistan");
unity.addItem("Iran");
unity.addItem("Iraq");
unity.setEditable(true);
uy.add(unity);

JLabel city=new JLabel("City:");
city.setBounds(350,320,100,25);
city.setForeground(Color.BLACK);
uy.add(city);
JComboBox cety=new JComboBox();
cety.setBounds(430,320,140,25);

cety.addItem("");
uy.add(cety);
unity.addItemListener(new ItemListener(){
public void itemStateChanged(ItemEvent olala){
    if(unity.getSelectedItem().equals("Pakistan")){
        cety.addItem("Lahore");
        cety.addItem("Islamabad");
        cety.addItem("Karachi");
        cety.addItem("Rawalpindi");
        cety.addItem("Faisalabad");
        cety.addItem("Gujjranwala");
    }
}
});

But when i run the program,the block of if statement did not do anything. What should I do,if I want to Load Values into JComboBox , depending on selection from another JComboBox ?

Answer 1

Actually, if you choose "China" first, then "Pakistan" again, and click on the down arrow of the cety combobox, you'll see the items.

You can easily debug and see when the method is called by adding a println like so,

        ...
        unity.addItemListener(new ItemListener() {
            @Override
            public void itemStateChanged(ItemEvent olala) {
// DEBUG
System.out.println("itemStateChanged(): item = " + olala.getItem());
            ...

You'll notice that itemStateChanged is called when you select a different item. If you start the program and see the comboboxes for the first time, they haven't changed state yet, the method hasn't been called, and cety is empty. You could make unity change state, by adding

        unity.setSelectedIndex(1);
        unity.setSelectedIndex(0);

at the end of your code. Alternatively, you could just move all the unity.addItem lines to after the addItemListener so adding the first item to the list will trigger the event.

You'll need to clear the items before adding new ones, or selecting a couple of items in unity will result in a long list in cety . If you need an empty string in the list, you need to move that statement here as well,

            ...
        public void itemStateChanged(ItemEvent olala){
            cety.removeAllItems();
            cety.addItem("");
            if (unity.getSelectedItem().equals("Pakistan")) {
                cety.addItem("Lahore");
                ...

You could then use a switch instead of a ton of if statements, get the item from the event instead of unity , and get rid of the raw types:

    JComboBox<String> unity = new JComboBox<>();
    unity.setBounds(430, 280, 140, 25);
    unity.setEditable(true);
    uy.add(unity);

    JLabel city = new JLabel("City:");
    city.setBounds(350, 320, 100, 25);
    city.setForeground(Color.BLACK);
    uy.add(city);

    JComboBox<String> cety = new JComboBox<>();
    cety.setBounds(430, 320, 140, 25);
    uy.add(cety);

    unity.addItemListener(new ItemListener() {
        @Override
        public void itemStateChanged(ItemEvent event) {
            cety.removeAllItems();
            cety.addItem("");
            switch (event.getItem().toString()) {
                case "Pakistan":
                    cety.addItem("Lahore");
                    cety.addItem("Islamabad");
                    cety.addItem("Karachi");
                    cety.addItem("Rawalpindi");
                    cety.addItem("Faisalabad");
                    cety.addItem("Gujjranwala");
                    break;
                case "China":
                    cety.addItem("Beijing");
                    break;
            //  ...
            }
        }
    });
    unity.addItem("Pakistan");
    unity.addItem("China");
    unity.addItem("America");
    unity.addItem("Japan");
    unity.addItem("Bangladesh");
    unity.addItem("Srilanka");
    unity.addItem("India");
    unity.addItem("Turkey");
    unity.addItem("UK");
    unity.addItem("Afghanistan");
    unity.addItem("Iran");
    unity.addItem("Iraq");

// This would be a workaround if you don't want to move the addItems:
//  unity.setSelectedIndex(1);
//  unity.setSelectedIndex(0);