大家晚上好
我遇到了一些用于介绍性 Java 类的代码的编译问题。手头的应用程序创建了一个计算器。在尝试编译时，我收到一条错误消息，指出我有一个“无法访问的语句”，这让我相信我在某个地方陷入了循环（同样，我正在上入门课程，所以我的假设可能是错误的）。我已经无数次地查看了代码，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。我还在这一行的末尾添加了“/ ERROR /”以帮助指出这一点。除了帮助定位此编译错误的来源之外，您能否告诉我除了对代码的细致梳理之外，是否还有其他技术可以用来查找此类错误？
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