如何解决 Java 中的“令牌“else”的语法错误”？

Question

我是新来的，但让我们直接回答这个问题：当我为一个类项目计算器编写以下代码时，我遇到了一个“令牌错误”。这是完整的错误消息：

 Exception in thread "main" java.lang.Error: Unresolved compilation problem:
Syntax error on token "else", delete this token

at calculatorApplet.main(calculatorApplet.java:42)

我写了这段代码：

 import java.util.Scanner;
import javax.swing.JOptionPane;
public class calculatorApplet
{
    Scanner sc = new Scanner(System.in);
    public static void main(String[] args)
    {
        JOptionPane.showMessageDialog(null, "Welcome to the Calculator!");
        String option = JOptionPane.showInputDialog(null, "Which calculator mode do you want?");
        if (option.equals("Addition"))
        {
            Double add1 = Double.parseDouble(JOptionPane.showInputDialog(null, "Okay type the first number(s) of your addition problem."));
            Double add2 = Double.parseDouble(JOptionPane.showInputDialog(null, "Now type the second number(s) of your addition problem."));
            Double preAdd = add1+add2;
            Double Add = preAdd;
            JOptionPane.showMessageDialog(null, "The sum is " + Add + ".");
        }
        else
        {
            JOptionPane.showMessageDialog(null, "Huh?");
        }
        if (option.equals("Subtraction"))
        {
            Double sub1 = Double.parseDouble(JOptionPane.showInputDialog(null, "Okay type the first number(s) of your subtraction problem."));
            Double sub2 = Double.parseDouble(JOptionPane.showInputDialog(null, "Now type the second number(s) of your subtraction problem."));
            Double preSub = sub1-sub2;
            Double Sub = preSub;
            JOptionPane.showMessageDialog(null, "The difference is " + Sub + ".");
        }
        else
        {
            JOptionPane.showMessageDialog(null, "Huh?");
        }
        if (option.equals("Multiplication"));
        {
            Double mult1 = Double.parseDouble(JOptionPane.showInputDialog(null, "Okay type the first number(s) of your multiplication problem."));
            Double mult2 = Double.parseDouble(JOptionPane.showInputDialog(null, "Now type the second number(s) of your multiplication problem."));
            Double preMult = mult1*mult2;
            Double Mult = preMult;
            JOptionPane.showMessageDialog(null, "The product is " + Mult + ".");
        }
        else //Here is the error.
        {
            JOptionPane.showMessageDialog(null, "Huh?");
        } //Here ends the error.
        if (option.equals("Division"))
        {
            Double div1 = Double.parseDouble(JOptionPane.showInputDialog(null, "Okay type the first number(s) of your division problem."));
            Double div2 = Double.parseDouble(JOptionPane.showInputDialog(null, "Now type the second number(s) of your division problem."));
            Double preDiv = div1/div2;
            Double Div = preDiv;
            JOptionPane.showMessageDialog(null, "The quotient is " + Div + ".");
        }
        else
        {
            JOptionPane.showMessageDialog(null, "Huh?");
        }
        //End of if statements.
    }

}

那是代码，我评论了令牌错误所在的区域。也许解决方案就在我面前。我希望我的代码成为一个计算器，所以如果你们可以修复我的代码，使其不与数学运算冲突。谢谢，我很感激任何帮助。

原文由 Carlos Lombardii 发布，翻译遵循 CC BY-SA 4.0 许可协议

Answer 1

问题是 if 之后的分号。

The grammar is if ( condition ) expression-or-block { else expression-or-block }

由于 ; 是一个空表达式，这适用于 if 子句并且在 then 子句中没有任何反应 - 在您的情况下语法上没有 else 子句，因为您的 if 语句语句以分号结尾，然后是一个简单的块。然后分词器检测到一个悬空的 else 令牌并抱怨。

原文由 Sebastian 发布，翻译遵循 CC BY-SA 3.0 许可协议

Answer 2

从此处的行尾删除 ; ：

if (option.equals("Multiplication"));

原文由 Rápli András 发布，翻译遵循 CC BY-SA 3.0 许可协议