它编译并运行，但是当我按下任何按钮时，它会给我一个错误信息。谢谢

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

    class cwp2
    {
        public static void main(String[] args)
        {
            MyFrame F1 = new MyFrame();
        }
    }

    class MyFrame extends JFrame 
    {
        MyFrame()
        {
        //main window
            setSize(480,300);
        MyPanel mainpanel = new MyPanel();
            mainpanel.setLocation(300,300);
        add(mainpanel);
            setTitle("Super Screen & Keyboard");
        setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        setVisible(true);
    }
    }

   class MyPanel extends JPanel implements ActionListener
    {
        private JTextArea screen;
        private JButton spacebar,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z;

        MyPanel()   
    {
        //text screen 
        JTextArea screen = new JTextArea();
        screen.setColumns(40);
        screen.setRows(9);
        screen.setWrapStyleWord(true);
        screen.setLineWrap(true);

        //buttons and actions
        spacebar = new JButton("spacebar");
        spacebar.addActionListener(this);
        a = new JButton("a");
        a.addActionListener(this);
        b = new JButton("b");
        b.addActionListener(this);
        c = new JButton("c");
        c.addActionListener(this);
        d = new JButton("d");
        d.addActionListener(this);
        e = new JButton("e");
        e.addActionListener(this);
        f = new JButton("f");
        f.addActionListener(this);
        g = new JButton("g");
        g.addActionListener(this);
        h = new JButton("h");
        h.addActionListener(this);
        i = new JButton("i");
        i.addActionListener(this);
        j = new JButton("j");
        j.addActionListener(this);
        k = new JButton("k");
        k.addActionListener(this);
        l = new JButton("l");
        l.addActionListener(this);
        m = new JButton("m");
        m.addActionListener(this);
        n = new JButton("n");
        n.addActionListener(this);
        o = new JButton("o");
        o.addActionListener(this);
        p = new JButton("p");
        p.addActionListener(this);
        q = new JButton("q");
        q.addActionListener(this);
        r = new JButton("r");
        r.addActionListener(this);
        s = new JButton("s");
        s.addActionListener(this);
        t = new JButton("t");
        t.addActionListener(this);
        u = new JButton("u");
        u.addActionListener(this);
        v = new JButton("v");
        v.addActionListener(this);
        w = new JButton("w");
        w.addActionListener(this);
        x = new JButton("x");
        x.addActionListener(this);
        y = new JButton("y");
        y.addActionListener(this);
        z = new JButton("z");
        z.addActionListener(this);

        add(q);
        add(w);
        add(e);
        add(r);
        add(t);
        add(y);
        add(u);
        add(i);
        add(o);
        add(p);
        add(a);
        add(s);
        add(d);
        add(f);
        add(g);
        add(h);
        add(j);
        add(k);
        add(l);
        add(z);
        add(x);
        add(c);
        add(v);
        add(b);
        add(n);
        add(m);
        add(spacebar);
        add(screen);
    }

    public void actionPerformed(ActionEvent e)
    {
        if (e.getSource() == spacebar)
        {
            screen.append(" ");
        }

        else 
        {
            screen.append(e.getSource.getActionCommand());
        }
    }

When I press a button I get an error message on the command prompt.

当我按一个按钮时，我在命令提示符上得到一个错误消息。

1 个解决方案

#1

This is wrong:

这是错误的:

e.getSource.getActionCommand()

The getSource() method returns an object of Object type, and Object has no getActionCommand() method.

getSource()方法返回对象类型的对象，对象没有getActionCommand()方法。

It's simply

这仅仅是

e.getActionCommand()

Since e is an ActionEvent variable, and this type in fact has this method.

因为e是一个ActionEvent变量，而这个类型实际上有这个方法。

Suggestion: next time you have a similar question, please show all complete error or exception messages and indicate in a comment which line throws it.

建议:下次你有类似的问题时，请显示所有的错误或异常信息，并在评论中指出哪一条线抛出。

Edit

编辑

Your new problem is due to your shadowing the screen variable.

你的新问题是由于你的遮蔽屏幕变量。

Your code:

你的代码:

class MyPanel extends JPanel implements ActionListener {  
    private JTextArea screen;  // screen declared here
    private JButton spacebar,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z;

    MyPanel() {
      //text screen 
      JTextArea screen = new JTextArea();  // but re-declared here!

This means that the screen variable that you initialize in the constructor is a local variable and not the field that was declared in the class.

这意味着在构造函数中初始化的屏幕变量是一个局部变量，而不是类中声明的字段。

This can be fixed by not redeclaring the variable:

这可以通过不重新声明变量来确定:

class MyPanel extends JPanel implements ActionListener {  
    private JTextArea screen;  // screen declared here
    private JButton spacebar,a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z;

    MyPanel() {
      //text screen 
      // JTextArea screen = new JTextArea();  // but re-declared here!
      screen = new JTextArea();   // note the difference?

#1