Java menu item enabling within event listener

问题

Hello im trying to enable my JMenuItem from within an event listener but it seems to be out of scope. im new to java so how would i properly go about this. the said event listener will change to a new view and enable the disabled menu items.

    //Create and add MenuItems
    JMenuItem fileItem0 = new JMenuItem("Load");
    collMenu.add(fileItem0);

    JMenuItem fileItem1 = new JMenuItem("Add");
    fileItem1.setEnabled(false);
    collMenu.add(fileItem1);

    JMenuItem fileItem2 = new JMenuItem("Delete");
    fileItem2.setEnabled(false);
    collMenu.add(fileItem2);

    //Add Menu bar to frame
    menubar.add(collMenu);

    //Menu Item Functions
    fileItem0.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent actionEvent) {
            JOptionPane.showMessageDialog(null,"You selected: Load.");
            //Enable fileitem1 & 2 here ?? 
        }
    });

回答1:

I hope this small example will help you clear your doubts...

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

public class MenuExample extends JFrame {
    JMenuBar menuBar = new JMenuBar();
    JMenu menu = new JMenu("File");
    JMenu menu1 = new JMenu("Edit");
    JMenuItem item1 = new JMenuItem("New");
    JMenuItem item2 = new JMenuItem("Open");

    public MenuExample() {
        setJMenuBar(menuBar);
        setVisible(true);
        setSize(400, 200);
        menuBar.add(menu);
        menuBar.add(menu1);
        menu.add(item1);
        menu.add(item2);
        item1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JFileChooser chooser = new JFileChooser();
                chooser.showOpenDialog(null);

            }

        });
        item2.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                JFileChooser chooser = new JFileChooser();
                chooser.showOpenDialog(null);

            }

        });

    }

    public static void main(String[] args) {
        MenuExample ex = new MenuExample();
    }

}

回答2:

Don't declare these menu items (fileitem1 and fileitem2) in the same method. Just Declare your fileitem1 and fileitem2 outside of your method.

This solves your problem...

回答3:

Delcare the JMenuItems you are trying to access as final to be accessed from within an inner class (addActionListener(new ActionListener() {...}).

Inner class needs JMenuItems accessed within ActionListener to be final. To avoid strange side-effects with closures in java variables referenced by an anonymous delegates.

Here is an example:

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

public class Test {

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                new Test().createAndShowUI();
            }
        });
    }

    private void createAndShowUI() {
        JFrame frame = new JFrame("Test");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        initComponents(frame);
        frame.pack();
        frame.setVisible(true);
    }

    private void initComponents(JFrame frame) {
        JMenuBar menuBar = new JMenuBar();

        JMenu menu1 = new JMenu("File");

        JMenuItem itemLoad = new JMenuItem("Load");
        //delcare them as final to be accessed from within an inner class
        final JMenuItem itemAdd = new JMenuItem("Add");
        final JMenuItem itemDel = new JMenuItem("Del");

        itemAdd.setEnabled(false);
        itemDel.setEnabled(false);

        itemLoad.addActionListener(new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                itemAdd.setEnabled(true);
                itemDel.setEnabled(true);
            }
        });

        menu1.add(itemLoad);
        menu1.add(itemAdd);
        menu1.add(itemDel);
        menuBar.add(menu1);
        frame.setJMenuBar(menuBar);
    }
}

来源：https://stackoverflow.com/questions/12543206/java-menu-item-enabling-within-event-listener

标签

java

swing

event-listener

jmenuitem