java - 编译问题: Unreachable statement

Question

大家晚上好，

我在介绍性 java 类的一些代码中遇到了编译问题。手头的应用程序创建了一个计算器。当尝试编译时，我收到一条错误，指出我有一个“无法访问的语句”，这让我相信我陷入了某个循环(同样，我正在学习入门类(class)，所以我的假设可能是错误的)。我检查了无数次代码，但找不到问题所在。编译器指向第 99 行，keypad.add(keys[12]);//减去，作为错误的来源。我还在该行末尾添加了“/ERROR/”以帮助指出。除了帮助定位此编译错误的来源之外，您是否还可以告诉我，除了仔细梳理代码之外，我是否还应该采用任何技术来查找此类错误？

import java.awt.*;
import java.awt.event.*;
import java.awt.datatransfer.*;
import java.text.DecimalFormat;
import javax.swing.JOptionPane;

public class Calculator extends Frame implements ActionListener
{
    private Button keys[];
    private Panel keypad;
    private TextField lcd;
    private double opl;
    private boolean first;
    private boolean foundKey;
    private boolean clearText;
    private int lastOp;
    private DecimalFormat calcPattern;

    public Calculator()
    {
        // create an instance of the menu
        MenuBar mnuBar = new MenuBar();
        setMenuBar(mnuBar);

        // construct and populate the File menu
        Menu mnuFile = new Menu("File", true);
        mnuBar.add(mnuFile);
            MenuItem mnuFileExit = new MenuItem("Exit");
            mnuFile.add(mnuFileExit);

        // construct and populate the Edit menu
        Menu mnuEdit = new Menu("Edit", true);
        mnuBar.add(mnuEdit);
            MenuItem mnuEditClear = new MenuItem("Clear");
            mnuEdit.add(mnuEditClear);
            mnuEdit.insertSeparator(1);
            MenuItem mnuEditCopy = new MenuItem("Copy");
            mnuEdit.add(mnuEditCopy);
            MenuItem mnuEditPaste = new MenuItem("Paste");
            mnuEdit.add(mnuEditPaste);

        // construct and populate the About menu
        Menu mnuAbout = new Menu("About", true);
            mnuBar.add(mnuAbout);
            MenuItem mnuAboutCalculator = new MenuItem("About Calculator");
            mnuAbout.add(mnuAboutCalculator);

        // add the ActionListener to each menu item
        mnuFileExit.addActionListener(this);
        mnuEditClear.addActionListener(this);
        mnuEditCopy.addActionListener(this);
        mnuEditPaste.addActionListener(this);
        mnuAboutCalculator.addActionListener(this);

        // assign an ActionCommand to each menu item
        mnuFileExit.setActionCommand("Exit");
        mnuEditClear.setActionCommand("Clear");
        mnuEditCopy.setActionCommand("Copy");
        mnuEditPaste.setActionCommand("Paste");
        mnuAboutCalculator.setActionCommand("About");

        // constuct components and initialize beginning values
        lcd = new TextField(20);
            lcd.setEditable(false);
        keypad = new Panel();
        keys = new Button[16];
        first = true;
        opl = 0.0;
        clearText = true;
        lastOp = 0;
        calcPattern = new DecimalFormat("########.########");

        // consturct and assign captions to the Buttons
        for (int i=0; i<=9; i++)
            keys[i] = new Button(String.valueOf(i));

        keys[10] = new Button("/");
        keys[11] = new Button("*");
        keys[12] = new Button("-");
        keys[13] = new Button("+");
        keys[14] = new Button("=");
        keys[15] = new Button(".");

        // set Frame and keypad layout to grid layout
        setLayout(new BorderLayout());
        keypad.setLayout(new GridLayout(4,4,10,10));

        for (int i=7; i<=10; i++) // 7, 8, 9, divide
            keypad.add(keys[i]);

        for (int i=4; i<=6; i++) // 4, 5, 6
            keypad.add(keys[i]);

        keypad.add(keys[11]); // multiply

        for (int i=1; 1<=3; i++) // 1, 2, 3
            keypad.add(keys[i]);

        keypad.add(keys[12]); // subtract /*ERROR*/

        keypad.add(keys[0]); // 0 key

        for (int i=15; i>=13; i--) // decimal point, =, +
            keypad.add(keys[i]);

        for (int i=0; i<keys.length; i++)
            keys[i].addActionListener(this);

        add(lcd, BorderLayout.NORTH);
        add(keypad, BorderLayout.CENTER);

        addWindowListener(
            new WindowAdapter()
                {
                public void windowClosing(WindowEvent e)
                    {
                        System.exit(0);
                    }
                }
        );

    } // end of constructor method

    public void actionPerformed(ActionEvent e)
    {
        // test for menu item clicks
        String arg = e.getActionCommand();
        if (arg == "Exit")
            System.exit(0);

        if (arg == "Clear")
        {
            clearText = true;
            first = true;
            opl = 0.0;
            lcd.setText("");
            lcd.requestFocus();
        }

        if (arg == "Copy")
        {
            Clipboard cb = Toolkit.getDefaultToolkit().getSystemClipboard();
            StringSelection contents = new StringSelection(lcd.getText());
            cb.setContents(contents, null);
        }

        if (arg == "Paste")
        {
            Clipboard cb = Toolkit.getDefaultToolkit().getSystemClipboard();
            Transferable content = cb.getContents(this);
            try
            {
                String s = (String)content.getTransferData(DataFlavor.stringFlavor);
                lcd.setText(calcPattern.format(Double.parseDouble(s)));
            }
            catch (Throwable exc)
            {
                lcd.setText("");
            }
        }

        if (arg == "About")
        {
            String message = "Calculator ver.1.0\nOpenExhibit Softwar\nCopyright 2007\nAll rights Reserved";
            JOptionPane.showMessageDialog(null,message,"About Calculator", JOptionPane.INFORMATION_MESSAGE);
        }

        // test for button clicks
        foundKey = false;

        // search for the clicked key
        for (int i=0; i<keys.length && !foundKey; i++)
        {
            if(e.getSource() == keys[i])
            {
                foundKey = true;
                switch(i)
                {
                    // number and decimal point buttons
                    case 0: case 1: case 2: case 3: case 4: case 5: case 6: case 7: case 8: case 9: case 15:
                    if(clearText)
                    {
                        lcd.setText("");
                        clearText = false;
                    }
                    lcd.setText(lcd.getText() + keys[i].getLabel());
                    break;

                    // operator buttons
                    case 10: case 11: case 12: case 13: case 14:
                        clearText = true;

                        if (first) // first operand
                        {
                            if(lcd.getText().length()==0) opl = 0.0;
                            else opl = Double.parseDouble(lcd.getText());

                            first = false;
                            clearText = true;
                            lastOp = i; // save the last operator
                        }
                        else // second operand
                        {
                            switch(lastOp)
                            {
                                case 10: // divide button
                                    opl /= Double.parseDouble(lcd.getText());
                                    break;
                                case 11: // multiply button
                                    opl *= Double.parseDouble(lcd.getText());
                                    break;
                                case 12: // minus button
                                    opl -= Double.parseDouble(lcd.getText());
                                    break;
                                case 13: // plus button
                                    opl += Double.parseDouble(lcd.getText());
                                    break;
                            } // end of switch(lastOp)
                            lcd.setText(calcPattern.format(opl));
                            clearText = true;

                            if(i==14) first = true; // equal button
                            else lastOp = i; // save last operator
                        } // end else
                        break;
                } // end of switch(i)
            } // end of if
        } // end of for
    } // end of actionPerformed

    public static void main(String args[])
    {
        // set frame properties
        Calculator f = new Calculator ();
        f.setTitle("Calculator Application");
        f.setBounds(200,200,300,300);
        f.setVisible(true);

        // set image properties and add to frame
        Image icon = Toolkit.getDefaultToolkit().getImage("calcImage.gif");
        f.setIconImage(icon);

    } // end of main
} // end of class

一如既往，感谢您的帮助。

杰瑞

Answer 1

该语句之前的循环中有一个小拼写错误。看看你是否能发现它:

for (int i=1; 1<=3; i++) // 1, 2, 3
    keypad.add(keys[i]);

您的测试是1<=3 ，这总是正确的，因为 1 总是小于或等于 3。我认为你的意思是:

for (int i=1, i<3; i++)
    keypad.add(keys[i]);