jCombobox NullPointer异常

Question

我创建一个包含3个组合框的框架，每个框架依赖于另一个，第3个依赖于第2个，第2个依赖于第1个。 问题是当我在第3次更改第一次获得NullPointer异常时，因为它的操作是第2次更改操作。

我的问题是如何防止第3个jComboBox上的项目更改动作＆＃34; jCombobox2＆＃34;当我改变第一个jComboBox＆＃34; jComboBox0＆＃34; ？

这是我的代码：

    private void jComboBox0ItemItemStateChanged(ItemEvent event) {
    jComboBox1.removeAllItems();
    ComboItem cat = (ComboItem) jComboBox0.getSelectedItem();


    String requete = "from Subcategory where Fk_Category = " + cat.getValue();

    Collection subcategories = Subcategory.getListeSubcategory(requete);

    for (Iterator i = subcategories.iterator(); i.hasNext();) {
        Subcategory item = new Subcategory();
        item = (Subcategory) i.next();
        System.out.println(item.getId());

        jComboBox1.addItem(new ComboItem(item.getNom(), (int) item.getId()));
    }

    // System.out.println("tbdlat a lkhra ! : "+listCategory.get(0));

}

private void jComboBox1ItemItemStateChanged(ItemEvent event) {
    // nda2
    jComboBox2.removeAllItems();
    ComboItem cat = (ComboItem) jComboBox1.getSelectedItem();


    String requete = "from Area  where fk_Subcategory = " + cat.getValue()+" group by Nom_Area";

    Collection areas = Area.getListeArea(requete);

    for (Iterator i = areas.iterator(); i.hasNext();) {
        Area item = new Area();
        item = (Area) i.next();
        System.out.println(item.getId());

        jComboBox2.addItem(new ComboItem(item.getNom(), (int) item.getId()));
    }

}

private void jComboBox2ItemItemStateChanged(ItemEvent event) {
    // i'll do some code here
}

Answer 1

您的代码的问题似乎是您没有检查是否选择了元素。如果选择了某个项，则应该在监听器内部进行检查，如果不进行检查，则会调用itemStateChange方法两次。

我在下面发布了一个代码，其中包含三个组合框和一个用于所有三个组合的侦听器：

    import java.awt.FlowLayout;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;

import javax.swing.JComboBox;
import javax.swing.JFrame;

public class MainWindow1 extends JFrame implements ItemListener {

    public MainWindow1() {

        String[] petStrings = { "Bird", "Cat", "Dog", "Rabbit", "Pig" };

        //Create the combo box, select item at index 4.
        //Indices start at 0, so 4 specifies the pig.

        setLayout(new FlowLayout());

        JComboBox petList1 = new JComboBox(petStrings);
        //petList1.setSelectedIndex(4);
        petList1.addItemListener(this);
        petList1.setName("petList1");

        JComboBox petList2 = new JComboBox(petStrings);
        petList2.setSelectedIndex(4);
        petList2.addItemListener(this);
        petList2.setName("petList2");

        JComboBox petList3 = new JComboBox(petStrings);
        petList3.setSelectedIndex(4);
        petList3.addItemListener(this);
        petList3.setName("petList3");

        getContentPane().add(petList1);
        getContentPane().add(petList2);
        getContentPane().add(petList3);


        setSize(800, 600);
        setVisible(true);
    }

    public static void main(String[] args) {
        new MainWindow1();

    }

    @Override
    public void itemStateChanged(ItemEvent e) {
        if (e != null && e.getSource().toString() != null && e.getSource().toString().contains("petList1") && e.getStateChange() == ItemEvent.SELECTED) {

            System.out.println( "1" + e.getSource());
        } else if (e != null && e.getSource().toString() != null && e.getSource().toString().contains("petList2") && e.getStateChange() == ItemEvent.SELECTED) {
            System.out.println( "2" + e.getSource());
        } else if (e != null && e.getSource().toString() != null && e.getSource().toString().contains("petList3") && e.getStateChange() == ItemEvent.SELECTED) {

            System.out.println("3" + e.getSource());
        }

    }

}