Question

我有这个代码，我试图修改，以添加一个菜单栏与'文件'等。添加它们一直没有问题，添加它们的听众证明是一个问题。每当我尝试使用语法fileMenu1.addActionListener(this);时，我都会收到错误“无法在静态上下文中使用它”。有什么建议？我相信我即将结束这一切。这是一个多课程。如果需要，会把别人放在其他地方

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


public class BingoMain extends JFrame implements ActionListener { //I ADDED THE LISTENER HERE

    private static final int ROWS = 5;
    private static final int COLS = 5;
    private static final int MAX_BINGO = 15 * COLS;     //15*5 = 75. Max number of bingo numbers
    private static JMenuItem fileMenu1 = new JMenuItem("Play");
    private static JMenuItem fileMenu2 = new JMenuItem("Quit");
    /**
     * @param args
     *
     */
    public static void main (String[] args)  {

        //Ask for how number of players, take the input, parse it, create that many bingo cards
        String players = JOptionPane.showInputDialog(null, "How many players? (1 to 5 players)");
        int playerNums= Integer.parseInt(players);

        JFrame myBingoGUI=new JFrame(); //frame
        myBingoGUI.setSize(900, 400);
        myBingoGUI.setLocation(100, 100);
        myBingoGUI.setTitle("BINGO");
        myBingoGUI.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Container myContentPane = myBingoGUI.getContentPane();

        JMenuBar bar = new JMenuBar();  //create menu bar
        JMenu fileMenu = new JMenu("File"); //create the file item in the bar

        bar.add(fileMenu);

        fileMenu.add(fileMenu1);
        fileMenu.add(fileMenu2);
        myBingoGUI.setJMenuBar(bar);

        fileMenu1.addActionListener(this);     //ERROR!
        fileMenu2.addActionListener(this);     //Same error

        myContentPane.setLayout(new GridLayout(0, playerNums));

        BingoCard[] cards = new BingoCard[playerNums];
        for (int i = 0; i < cards.length; i++) {
            cards[i] = new BingoCard("Card " + (i + 1), COLS, ROWS, MAX_BINGO / COLS);
            BingoGUI bingoCard = new BingoGUI();
            cards[i].addListener(bingoCard);
            myContentPane.add(bingoCard);
        }

        myBingoGUI.setVisible(true);

        System.out.println(cards[0]);   //print the cards on the console
        System.out.println();
        /*
         *   Play the game:   
         */
        boolean winner = false;     //default false value for every player
        while (!winner) {
           String error = "";
            int calledValue = -1;
            int calledColumn=-1;
            do {
               String calledNumber = JOptionPane.showInputDialog(null, error + " Enter a BINGO call:");
                error = "";
                calledColumn = -1;
                calledValue = -1;


                /*
                 *   The first character of the input string is converted to a column number between 0 and 4
                 */
                if (Character.toUpperCase(calledNumber.charAt(0))=='B') calledColumn=0;
                if (Character.toUpperCase(calledNumber.charAt(0))=='I') calledColumn=1;
                if (Character.toUpperCase(calledNumber.charAt(0))=='N') calledColumn=2;
                if (Character.toUpperCase(calledNumber.charAt(0))=='G') calledColumn=3;
                if (Character.toUpperCase(calledNumber.charAt(0))=='O') calledColumn=4;
                if (calledColumn < 0) {
                    error = "Called Column '" + Character.toUpperCase(calledNumber.charAt(0)) + "' must be on the BINGO card";      //if first character is not a B, I, N, G, O show message
                } else {    //error catching
                    /*
                     *  The remainder of the input string is converted to an integer
                     */
                    //try catch block to catch any illegal numerical values (A legal column with an illegal value within the range will still be accepted)
                    try {
                        calledValue = Integer.parseInt(calledNumber.substring(1,calledNumber.length()));
                        if (calledValue < 1 || calledValue > MAX_BINGO) {
                            error = "Value not legal " + calledValue + " (1 <= value <= " + MAX_BINGO + ")";    //error if <0 or >75 is input, values dont exist in Bingo
                        }
                    } catch (NumberFormatException nfe) {
                        error = "Illegal number " + calledNumber.substring(1,calledNumber.length());    //error if format is wrong (i.e B9g or N5t) cant mix letters with numbers
                    }
                }
            } while (error.length() != 0);
            /*
             *   The array of called numbers is updated to show the number has been called.
             */
            for (BingoCard card : cards) {
                if (card.called(calledColumn, calledValue)) {       
                    winner = true;
                }
            }
            if (winner) {
                for (BingoCard card : cards) {
                    JOptionPane.showInputDialog(null, "BINGO");
                    card.gameOver();
                }
            }
            System.out.println(cards[0]);
            System.out.println();

        } // while

    } // main

}

Answer 1

代码应该是BingoMain的实例。 main方法是static，这意味着它不与类的任何实例相关联。关键字static表示方法，字段等与类本身相关联，而不是类的实例。代码错误地假定static main方法可以引用类本身的实例，这是静态的。

 public static void main (String[] args)  {
        /* Omitted*/
        BingoMain main = new BingoMain();
        fileMenu1.addActionListener(main);     //NO ERROR!

        /* Omitted*/
}

Answer 2

基本上this在static方法中没有上下文，因为没有“this”可用。

相反，您需要实现ActionListener的类的实例，奇怪的是，您永远不会创建它。

您使用...

创建一个类

public class BingoMain extends JFrame implements ActionListener {...

但您使用...

创建主框架

JFrame myBingoGUI=new JFrame(); //frame

这是一种更好的方法，但却无法从JFrame延伸。

相反，我建议删除extends JFrame部分并创建一个构造函数来初始化主程序，然后将其添加到框架中......

例如......

public class BingoMain implements ActionListener { //I ADDED THE LISTENER HERE

    private static final int ROWS = 5;
    private static final int COLS = 5;
    private static final int MAX_BINGO = 15 * COLS;     //15*5 = 75. Max number of bingo numbers
    private JMenuItem fileMenu1 = new JMenuItem("Play");
    private JMenuItem fileMenu2 = new JMenuItem("Quit");

    public BingoMain() {

        //Ask for how number of players, take the input, parse it, create that many bingo cards
        String players = JOptionPane.showInputDialog(null, "How many players? (1 to 5 players)");
        int playerNums= Integer.parseInt(players);

        JFrame myBingoGUI=new JFrame(); //frame
        myBingoGUI.setSize(900, 400);
        myBingoGUI.setLocation(100, 100);
        myBingoGUI.setTitle("BINGO");
        myBingoGUI.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        Container myContentPane = myBingoGUI.getContentPane();

        JMenuBar bar = new JMenuBar();  //create menu bar
        JMenu fileMenu = new JMenu("File"); //create the file item in the bar

        bar.add(fileMenu);

        fileMenu.add(fileMenu1);
        fileMenu.add(fileMenu2);
        myBingoGUI.setJMenuBar(bar);

        fileMenu1.addActionListener(this);     //ERROR!
        fileMenu2.addActionListener(this);     //Same error
        //...
    }

    /**
     * @param args
     *
     */
    public static void main (String[] args)  {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                BingoMain main = new BingoMain();
            }
        });

    } // main

}

尝试并避免使用static变量，您可能希望使用多个变量实例

<强>更新

正如@peeskillet所指出的，代码示例中似乎没有实现actionPerformed。这可能只是你的疏忽，或者你将要承担的下一个问题......

确保添加

@Override
public void actionPerformed(ActionEvent evt) {
}

到您的班级和import java.event.ActionEvent您的导入......

Answer 3

    fileMenu1.addActionListener(this);     //ERROR!
    fileMenu2.addActionListener(this);     //Same error

main方法是静态方法，静态上下文中没有this。您必须创建BingoMain类的实例，并在其中传递该类的实例。

Answer 4

你应该在类的构造函数中构建你的接口，如：

public class BingoMain extends JFrame implements ActionListener {
    public BingoMain() {
        // code to build your UI 
        fileMenu1.addActionListener(this);
        // some more code
    }

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

如何为此实现ActionListener？

4 个答案: