Java 为什么 JComboBox 上的 itemStateChanged 在更改时被调用两次？

Why is itemStateChanged on JComboBox is called twice when changed?

提问by Nicks

I'm using a JComboBox with an ItemListener on it. When the value is changed, the itemStateChanged event is called twice. The first call, the ItemEvent is showing the original item selected. On the second time, it is showing the item that has been just selected by the user. Here's some tester code:

我正在使用带有 ItemListener 的 JComboBox。当值改变时，itemStateChanged 事件被调用两次。第一次调用，ItemEvent 显示所选的原始项目。第二次，它显示用户刚刚选择的项目。这是一些测试代码：

public Tester(){

    JComboBox box = new JComboBox();
    box.addItem("One");
    box.addItem("Two");
    box.addItem("Three");
    box.addItem("Four");

    box.addItemListener(new ItemListener(){
        public void itemStateChanged(ItemEvent e){
            System.out.println(e.getItem());
        }
    });

    JFrame frame = new JFrame();
    frame.getContentPane().add(box);
    frame.pack();
    frame.setVisible(true);
}

So when I changed the Combo box once from "One" to "Three" the console shows:

因此，当我将组合框从“一”更改为“三”时，控制台显示：

One
Three

Is there a way I can tell using the ItemEvent maybe, that it's the second item (ie. the user selected item)? And if someone can explain why it gets called twice, that would be nice too!

有没有办法我可以使用 ItemEvent 判断它是第二个项目（即用户选择的项目）？如果有人能解释为什么它被调用两次，那也很好！

Thanks

谢谢

采纳答案by kgiannakakis

Have a look at this source:

看看这个来源：

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Tester {

    public Tester(){

        JComboBox box = new JComboBox();
        box.addItem("One");
        box.addItem("Two");
        box.addItem("Three");
        box.addItem("Four");

        box.addItemListener(new ItemListener(){
            public void itemStateChanged(ItemEvent e){
                System.out.println(e.getItem() + " " + e.getStateChange() );
            }
        });

        JFrame frame = new JFrame();
        frame.getContentPane().add(box);
        frame.pack();
        frame.setVisible(true);
    }

    public static void main(String [] args) {
        Tester tester = new Tester();
    }
}

Use the getStateChange to determine if an item is selected or deselected

使用 getStateChange 来确定项目是被选中还是被取消选中

回答by VonC

According to this thread,

根据这个线程，

It gets tripped when you leave one result and then called again when set to another result

Don't listen for itemStateChanged. Use an ActionListener instead, which is good for handling events of the combo.
You need a ItemStateListener if you need to separately handle deselection / selection depending on the item involved.

Changing the state of the item within itemStateChanged causes itemStateChanged to be fired... this called "reentrance".

当您离开一个结果时它会被绊倒，然后在设置为另一个结果时再次调用它

不要监听 itemStateChanged。使用 ActionListener 代替，这有利于处理组合的事件。
如果您需要根据所涉及的项目单独处理取消选择/选择，则需要一个 ItemStateListener。

在 itemStateChanged 中更改项目的状态会导致 itemStateChanged 被触发……这称为“重入”。

回答by VonC

Yo can do it like this:

你可以这样做：

import java.awt.event.*;

jComboBox1.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            System.out.println("Hello");
        }
    });

回答by Abdullah

JComboBox.setFocusable(false)will do the trick.

JComboBox.setFocusable(false)会做的伎俩。

回答by Hussain

The code is:

代码是：

public class Tester {

    private JComboBox box;

    public Tester() {

        box = new JComboBox();
        box.addItem("One");
        box.addItem("Two");
        box.addItem("Three");
        box.addItem("Four");

        box.addItemListener(new ItemListener() {

            public void itemStateChanged(ItemEvent e) {
                if (e.getStateChange() == 1) {

                    JOptionPane.showMessageDialog(box, e.getItem());
                    System.out.println(e.getItem());
                }
            }
        });

        JFrame frame = new JFrame();
        frame.getContentPane().add(box);
        frame.pack();
        frame.setVisible(true);
    }
}

回答by qizer

I wanted to get the index string after selected and set in combobox

我想在选择并在组合框中设置后获取索引字符串

        comboBox1.addItemListener(new ItemListener() {
            @Override
            public void itemStateChanged(ItemEvent e) {
                if(e.getStateChange() == ItemEvent.SELECTED) {
                    comboBox1ItemStateChanged();
                }
            }
        });

回答by 21stking

private void dropDown_nameItemStateChanged(java.awt.event.ItemEvent evt) {                                                 


    if(evt.getStateChange() == ItemEvent.SELECTED)
    {
        String item = (String) evt.getItem();
        System.out.println(item);
    }

}

Good Luck!

祝你好运！

回答by Alex

Quote from Java Tutorial:

引用自Java 教程：

"Only one item at a time can be selected in a combo box, so when the user makes a new selection the previously selected item becomes unselected. Thus two item events are fired each time the user selects a different item from the menu. If the user chooses the same item, no item events are fired."

“在组合框中一次只能选择一个项目，因此当用户进行新选择时，先前选择的项目将变为未选择状态。因此，每次用户从菜单中选择不同的项目时，都会触发两个项目事件。如果用户选择相同的项目，不会触发任何项目事件。”

回答by Kuldeep Melligeri

1. When the anyitem is selected from the combo box, it internally triggers selection change, i.e. it will call the function setSelectedItem.
2. If an explicit itemStateChanged event listener is implemented, the setSelectedItem will call itemStateChanged. So, when an item is selected it calls setSelectedItem then it calls itemStateChanged.
3. As the value of the combo box changes, even that too triggers itemStateChanged and hence itemStateChanged gets called.
4. I had written listener for item change to handle change in value of combo box when set internally from the code and that caused the function getting called twice.

1. 当从组合框中选择 anyitem 时，它会在内部触发选择更改，即它将调用函数 setSelectedItem。
2. 如果实现了显式 itemStateChanged 事件侦听器，则 setSelectedItem 将调用 itemStateChanged。因此，当一个项目被选中时，它会调用 setSelectedItem，然后它会调用 itemStateChanged。
3. 随着组合框的值发生变化，即使这样也会触发 itemStateChanged，因此 itemStateChanged 被调用。
4. 我已经为项目更改编写了侦听器，以处理从代码内部设置时组合框值的更改，这导致函数被调用两次。

Here are the 2 back traces, which gets invoked when a value is selected from combo box.

这是 2 个回溯，当从组合框中选择一个值时会调用它们。

1st time on actual value change:

第一次实际值变化：

dataMgr.MainInterface.jComboBoxPaymentStatusValueChangeHandle(MainInterface.java:1431), 
dataMgr.MainInterface.jComboBoxPaymentStatusItemStateChanged(MainInterface.java:1676), 
dataMgr.MainInterface.access0(MainInterface.java:28), 
dataMgr.MainInterface.itemStateChanged(MainInterface.java:437), 
javax.swing.JComboBox.fireItemStateChanged(JComboBox.java:1223), 
javax.swing.JComboBox.selectedItemChanged(JComboBox.java:1271), 
javax.swing.JComboBox.contentsChanged(JComboBox.java:1330), 
javax.swing.AbstractListModel.fireContentsChanged(AbstractListModel.java:118), 
javax.swing.DefaultComboBoxModel.setSelectedItem(DefaultComboBoxModel.java:93), 
javax.swing.JComboBox.setSelectedItem(JComboBox.java:576), javax.swing.JComboBox.setSelectedIndex(JComboBox.java:622), javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(BasicComboPopup.java:852), java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290), java.awt.Component.processMouseEvent(Component.java:6533), javax.swing.JComponent.processMouseEvent(JComponent.java:3324), javax.swing.plaf.basic.BasicComboPopup.processMouseEvent(BasicComboPopup.java:501), java.awt.Component.processEvent(Component.java:6298), java.awt.Container.processEvent(Container.java:2236), java.awt.Component.dispatchEventImpl(Component.java:4889), java.awt.Container.dispatchEventImpl(Container.java:2294), java.awt.Component.dispatchEvent(Component.java:4711), java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888), java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525), java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466), java.awt.Container.dispatchEventImpl(Container.java:2280), java.awt.Window.dispatchEventImpl(Window.java:2746), java.awt.Component.dispatchEvent(Component.java:4711), java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758), java.awt.EventQueue.access0(EventQueue.java:97), java.awt.EventQueue.run(EventQueue.java:709), java.awt.EventQueue.run(EventQueue.java:703), java.security.AccessController.doPrivileged(Native Method), java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76), java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86), java.awt.EventQueue.run(EventQueue.java:731), java.awt.EventQueue.run(EventQueue.java:729), java.security.AccessController.doPrivileged(Native Method), java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76), java.awt.EventQueue.dispatchEvent(EventQueue.java:728), java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201), java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116), java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105), java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101), java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93), java.awt.EventDispatchThread.run(EventDispatchThread.java:82)]

2nd time from the due to operation on combobox

由于在组合框上操作，第二次

dataMgr.MainInterface.jComboBoxPaymentStatusValueChangeHandle(MainInterface.java:1431), 
dataMgr.MainInterface.jComboBoxPaymentStatusItemStateChanged(MainInterface.java:1676), 
dataMgr.MainInterface.access0(MainInterface.java:28), 
dataMgr.MainInterface.itemStateChanged(MainInterface.java:437), 
javax.swing.JComboBox.fireItemStateChanged(JComboBox.java:1223), 
javax.swing.JComboBox.selectedItemChanged(JComboBox.java:1280), 
javax.swing.JComboBox.contentsChanged(JComboBox.java:1330), 
javax.swing.AbstractListModel.fireContentsChanged(AbstractListModel.java:118), 
javax.swing.DefaultComboBoxModel.setSelectedItem(DefaultComboBoxModel.java:93), 
javax.swing.JComboBox.setSelectedItem(JComboBox.java:576), 
javax.swing.JComboBox.setSelectedIndex(JComboBox.java:622), 
javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(BasicComboPopup.java:852), 
java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290), 
java.awt.Component.processMouseEvent(Component.java:6533), 
javax.swing.JComponent.processMouseEvent(JComponent.java:3324), 
javax.swing.plaf.basic.BasicComboPopup.processMouseEvent(BasicComboPopup.java:501), 
java.awt.Component.processEvent(Component.java:6298), java.awt.Container.processEvent(Container.java:2236), 
java.awt.Component.dispatchEventImpl(Component.java:4889), java.awt.Container.dispatchEventImpl(Container.java:2294), 
java.awt.Component.dispatchEvent(Component.java:4711), java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4888), 
java.awt.LightweightDispatcher.processMouseEvent(Container.java:4525), java.awt.LightweightDispatcher.dispatchEvent(Container.java:4466), 
java.awt.Container.dispatchEventImpl(Container.java:2280), java.awt.Window.dispatchEventImpl(Window.java:2746), 
java.awt.Component.dispatchEvent(Component.java:4711), java.awt.EventQueue.dispatchEventImpl(EventQueue.java:758),
java.awt.EventQueue.access0(EventQueue.java:97), java.awt.EventQueue.run(EventQueue.java:709), 
java.awt.EventQueue.run(EventQueue.java:703), java.security.AccessController.doPrivileged(Native Method),
java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76), 
java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:86), 
java.awt.EventQueue.run(EventQueue.java:731), java.awt.EventQueue.run(EventQueue.java:729), 
java.security.AccessController.doPrivileged(Native Method), java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(ProtectionDomain.java:76), java.awt.EventQueue.dispatchEvent(EventQueue.java:728), java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201), java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116), java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105), java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101), java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93), java.awt.EventDispatchThread.run(EventDispatchThread.java:82)]

回答by Junaid Khan

Have a look here,

看这里，

box.addItemListener(new ItemListener(){
    public void itemStateChanged(ItemEvent e){
        if(e.getStateChange()== ItemEvent.SELECTED) {
            //this will trigger once only when actually the state is changed
            JOptionPane.showMessageDialog(null, "Changed");
        }
    }
});

When you select a new option, it will only once call the JOptionPane, indicating that the code there will be called once only.

当你选择一个新选项时，它只会调用一次 JOptionPane，表示那里的代码只会被调用一次。