swing - Java JRE 1.7 JCombobox setSelectedItem(Object anObject) 不能正常工作？

Question

请看下面的代码片段，

    String[] choices = {"Apple", "Banana", "Custard"};  
    JComboBox<String> fruits = new JComboBox<String>(choices);        
    fruits.setSelectedItem("Custard");

它抛出空指针异常。见下文，

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
     at java.awt.EventQueue.getCurrentEventImpl(Unknown Source)
     at java.awt.EventQueue.getCurrentEvent(Unknown Source)
     at javax.swing.JComboBox.fireActionEvent(Unknown Source)
     at javax.swing.JComboBox.setSelectedItem(Unknown Source)

同样的问题也发生在 setSelectedIndex() 上。如果 Java JRE 1.7 有任何问题，请为这个问题提出好的解决方法或建议我。

Answer 1

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at java.awt.EventQueue.getCurrentEventImpl(Unknown Source) at java.awt.EventQueue.getCurrentEvent(Unknown Source) at javax.swing.JComboBox.fireActionEvent(Unknown Source) at javax.swing.JComboBox.setSelectedItem(Unknown Source)

来自

String[] choices = {"Apple", "Banana", "Custard"};  
JComboBox<String> fruits = new JComboBox<String>(choices);        
fruits.setSelectedItem("Custard");

仅在调用 setSelectedItem 之前添加 Action/ItemListener 的情况下才有可能生成(调试那个，什么是 XxxListener 触发) , 改为

String[] choices = {"Apple", "Banana", "Custard"};  
JComboBox<String> fruits = new JComboBox<String>(choices);        
fruits.setSelectedItem("Custard");
fruits.addAction / ItemListener(new Action / ItemListener)

Java6 中同样的问题

@sanjay wrote if i add that actionlistener to the combobox. it gives the same error. But it working properly in java 1.6 without that Combobox generic type.

• 不，我不是在说，你可以从这段代码中生成这个异常

  1. 注释代码行(//)mainComboBox.setSelectedItem("Fruit");

  2. 并取消注释//mainComboBox.setSelectedItem("Shape");

然后这段代码触发了同样的异常，对于 JComboBox 来说是很常见的问题，在 Java6 中也是同样的问题(通过从 JComboBox 定义中删除泛型)

import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;
import java.util.ArrayList;
import java.util.Hashtable;
import javax.swing.DefaultComboBoxModel;
import javax.swing.JComboBox;
import javax.swing.JFrame;

public class FruitAndVedg extends JFrame implements ActionListener, ItemListener {

    private static final long serialVersionUID = 4L;
    private JComboBox<String> mainComboBox;
    private JComboBox<String> subComboBox;
    private ArrayList item;
    private Hashtable<Object, Object> subItems = new Hashtable<>();

    public FruitAndVedg() {
        item = new ArrayList();
        item.add("Select Item");
        item.add("Fruit");
        item.add("Vedg");
        String[] items = {"Select Item", "Color", "Shape", "Fruit"};
        mainComboBox = new JComboBox<>(items/*item.toArray()*/);
        mainComboBox.setSelectedItem("Fruit");
        mainComboBox.addActionListener(this);
        mainComboBox.addItemListener(this);
        //mainComboBox.setSelectedItem("Shape");
        add(mainComboBox, BorderLayout.WEST);
        subComboBox = new JComboBox<>();
        subComboBox.setPrototypeDisplayValue("XXXXXXXXXX");
        add(subComboBox, BorderLayout.CENTER);
        String[] subItems1 = {"Select Fruit", "Apple", "Plum"};
        subItems.put(items, subItems1);
        String[] subItems2 = {"Select Vedg", "Carrot", "Peas"};
        subItems.put(items, subItems2);
    }

    @Override
    public void actionPerformed(ActionEvent ae) {
        String item = (String) mainComboBox.getSelectedItem();
        Object o = subItems.get(item);
        if (o == null) {
            subComboBox.setModel(new DefaultComboBoxModel());
        } else {
            subComboBox.setModel(new DefaultComboBoxModel((String[]) o));
        }
    }

    @Override
    public void itemStateChanged(ItemEvent ie) {
        if (ie.getStateChange() == ItemEvent.SELECTED) {
            if (ie.getSource() == mainComboBox) {
                if (mainComboBox.getSelectedIndex() != 0) {
                }
            }
        }
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new FruitAndVedg();
                frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
}