内部类和局部变量问题

Question

为项目编写程序。我需要使用 ListSelectionListener 来从 JList 中获取当前选择，以确定其他面板使用的 CardLayout 的当前卡片。如果不是直接使用 CardLayout 类的 show 方法从侦听器中更改当前卡片，我希望至少能够由侦听器创建一个带有选择名称的字符串。最简单的方法是什么，最终会导致当前卡被更改？非常感谢您的帮助！

来源：

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

public class ClientApp extends JFrame
{
    public static void main(String[] args)
    {
        new ClientApp();
    }


    public ClientApp()
    {
        this.setSize(750,380);
        this.setTitle("Honeydukes Muggle Ordering System");
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel infoPanel = new JPanel(new CardLayout());
        JPanel invntryPanel = new JPanel();


        //Creating the array for the invntryPanel Jlist

        String[] candy = {"Acid Pops", "Bat's Blood Soup",
                          "Bertie Bott's Every Flavour Beans",
                          "Blood-flavoured Lollipops",
                          "Cauldron Cakes", "Charm Choc",
                          "Chocoballs", "Chocolate Cauldrons",
                          "Chocolate Frogs", "Chocolate Skeletons",
                          "Chocolate Wands", "Choco-Loco", "Cockroach Clusters",
                          "Nougat", "Crystallised Pineapple",
                          "Drooble's Best Blowing Gum", "Exploding Bonbons",
                          "Toffees", "Fizzing Whizzbees",
                          "Fudge Flies", "Ice Mice",
                          "Jelly Slugs", "Liquourice Wands",
                          "Pepper Imps", "Peppermint Toads",
                          "Pink Coconut Ice", "Pixie Puffs",
                          "Pumpkin Fizz", "Salt Water Taffy",
                          "Shock-o-Choc", "Skeletal Sweets",
                          "Splindle's Lick'O'Rish Spiders",
                          "Sugar Quills", "Sugared Butterfly Wings",
                          "Toothflossing Stringmints", "Tooth-Splintering Strongmints",
                          "Treacle Fudge", "Chocolates", "Wizochoc"};
        JList candyList = new JList(candy);
        candyList.setVisibleRowCount(18);
        candyList.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

        ListSelectionListener sl = new ListSelectionListener() {
           public void valueChanged(ListSelectionEvent e) {
            if (e.getValueIsAdjusting() == false) {
                CardLayout.show(infoPanel, (String)candyList.getSelectedValue());
            }
           }
        };

        candyList.addListSelectionListener(sl);

        //Creating a scrollpane for the JList
        JScrollPane scroll = new JScrollPane(candyList, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
                                             JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
        invntryPanel.add(scroll);


        //Creating the cards
        JPanel startCard = new JPanel(new BorderLayout());
        JPanel acidPopsCard = new JPanel();
        JPanel batsBloodSoupCard = new JPanel();
        JPanel bertieBottsCard = new JPanel();
        JPanel bloodPopsCard = new JPanel();
        JPanel cauldronCakesCard = new JPanel();
        JPanel charmChocCard = new JPanel();
        JPanel chocoballsCard = new JPanel();
        JPanel chocCauldronsCard = new JPanel();
        JPanel chocFrogsCard = new JPanel();
        JPanel chocSkeleCard = new JPanel();
        JPanel chocWands = new JPanel();
        JPanel chocolocoCard = new JPanel();
        JPanel roachClustersCard = new JPanel();
        JPanel nougatCard = new JPanel();
        JPanel crystalPineappleCard = new JPanel();
        JPanel droobleGumCard = new JPanel();
        JPanel explodeBonbonsCard = new JPanel();
        JPanel toffeesCard = new JPanel();
        JPanel fizzWhizCard = new JPanel();
        JPanel fudgeFliesCard = new JPanel();
        JPanel iceMiceCard = new JPanel();
        JPanel jellySlugsCard = new JPanel();
        JPanel liquorWandsCard = new JPanel();
        JPanel pepImpsCard = new JPanel();
        JPanel pepToadsCard = new JPanel();
        JPanel pinkCocoIceCard = new JPanel();
        JPanel pixiePuffsCard = new JPanel();
        JPanel pumpkFizzCard = new JPanel();
        JPanel saltTaffeyCard = new JPanel();
        JPanel shockChocCard = new JPanel();
        JPanel skeleSweetsCard = new JPanel();
        JPanel spindleSpidersCard = new JPanel();
        JPanel sugarQuillsCard = new JPanel();
        JPanel sugarWingsCard = new JPanel();
        JPanel flossMintsCard = new JPanel();
        JPanel splintMintsCard = new JPanel();
        JPanel treacleFudgeCard = new JPanel();
        JPanel chocolatesCard = new JPanel();
        JPanel wizochocCard = new JPanel();

        //Adding the cards to the infoPanel
        infoPanel.add(startCard, "Start");
        infoPanel.add(acidPopsCard, "Acid Pops");
        infoPanel.add(batsBloodSoupCard, "Bat's Blood Soup");
        infoPanel.add(bertieBottsCard, "Bertie Bott's Every Flavour Beans");
        infoPanel.add(bloodPopsCard, "Blood-flavoured Lollipops");
        infoPanel.add(cauldronCakesCard, "Cauldron Cakes");
        infoPanel.add(charmChocCard, "Charm Choc");
        infoPanel.add(chocoballsCard, "Chocoballs");
        infoPanel.add(chocCauldronsCard, "Chocolate Cauldrons");
        infoPanel.add(chocFrogsCard, "Chocolate Frogs");
        infoPanel.add(chocSkeleCard, "Chocolate Skeletons");
        infoPanel.add(chocWands, "Chocolate Wands");
        infoPanel.add(chocolocoCard, "Choco-Loco");
        infoPanel.add(roachClustersCard, "Cockroach Clusters");
        infoPanel.add(nougatCard, "Nougat");
        infoPanel.add(crystalPineappleCard, "Crystallised Pineapple");
        infoPanel.add(droobleGumCard, "Drooble's Best Blowing Gum");
        infoPanel.add(explodeBonbonsCard, "Exploding Bonbons");
        infoPanel.add(toffeesCard, "Toffees");
        infoPanel.add(fizzWhizCard, "Fizzing Whizzbees");
        infoPanel.add(fudgeFliesCard, "Fudge Flies");
        infoPanel.add(iceMiceCard, "Ice Mice");
        infoPanel.add(jellySlugsCard, "Jelly Slugs");
        infoPanel.add(liquorWandsCard, "Liquourice Wands");
        infoPanel.add(pepImpsCard, "Pepper Imps");
        infoPanel.add(pepToadsCard, "Peppermint Toads");
        infoPanel.add(pinkCocoIceCard, "Pink Coconut Ice");
        infoPanel.add(pixiePuffsCard, "Pixie Puffs");
        infoPanel.add(pumpkFizzCard, "Pumpkin Fizz");
        infoPanel.add(saltTaffeyCard, "Salt Water Taffy");
        infoPanel.add(shockChocCard, "Shock-o-Choc");
        infoPanel.add(skeleSweetsCard, "Skeletal Sweets");
        infoPanel.add(spindleSpidersCard, "Splindle's Lick'O'Rish Spiders");
        infoPanel.add(sugarQuillsCard, "Sugar Quills");
        infoPanel.add(sugarWingsCard, "Sugared Butterfly Wings");
        infoPanel.add(flossMintsCard, "Toothflossing Stringmints");
        infoPanel.add(splintMintsCard, "Tooth-Splintering Strongmints");
        infoPanel.add(treacleFudgeCard, "Treacle Fudge");
        infoPanel.add(chocolatesCard, "Chocolates");
        infoPanel.add(wizochocCard, "Wizochoc");

        //startCard building
        JLabel startLbl = new JLabel("<html><center>Welcome to the Honeydukes Muggle Ordering System!<br />Please select from one of our products to the left to begin!</center></html>");
        startCard.add(startLbl, BorderLayout.CENTER);

        this.add(invntryPanel, BorderLayout.LINE_START);
        this.add(infoPanel, BorderLayout.CENTER);
        this.setVisible(true);
    }
}

错误：

ClientApp.java:54: error: local variable infoPanel is accessed from within inner
 class; needs to be declared final
                CardLayout.show(infoPanel, (String)candyList.getSelectedValue())
;
                                ^
ClientApp.java:54: error: local variable candyList is accessed from within inner
 class; needs to be declared final
                CardLayout.show(infoPanel, (String)candyList.getSelectedValue())
;
                                                   ^
ClientApp.java:54: error: non-static method show(Container,String) cannot be ref
erenced from a static context
                CardLayout.show(infoPanel, (String)candyList.getSelectedValue())
;
                          ^
Note: ClientApp.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
3 errors

Answer 1

我能想到几种方法。首先，将candyList的引用设为final

final JList candyList = new JList(candy);

这应该允许内部类看到列表。就个人而言，我不喜欢这种方法，但这只是我自己。

您可能会尝试的另一件事是通过事件对象引用列表

ListSelectionListener sl = new ListSelectionListener() {
    public void valueChanged(ListSelectionEvent e) {
        JList candyList = (JList)e.getSource();
        if (e.getValueIsAdjusting() == false) {
            CardLayout.show(infoPanel, (String)candyList.getSelectedValue());
        }
       }
    };

或者构造一个新的内部类，可以将列表作为参数传递给它

public class MyInnerListener implements ListSelectionListener {

    private JList list;

    public MyInnerListener(JList list) {

        this.list = list;

    }
    public void valueChanged(ListSelectionEvent e) {
        if (e.getValueIsAdjusting() == false) {
            CardLayout.show(infoPanel, (String)list.getSelectedValue());
        }
    }

}

只是一些想法；）