String 将此JOptionPane程序的字符串转换为字符

我正在尝试将我的操作变量转换为char，这样它就可以与if语句一起工作。我该怎么做呢？以下是我的代码：

import java.util.Scanner;
import javax.swing.JOptionPane;

public class Java_Calculator
{
    public static void main(String[] args)
    {
        String firstnumber, secondnumber, operation;

        double num1;
        double num2;
        double sum;
        double sub;
        double div;
        double mul;
        double mod;

        firstnumber = 
            JOptionPane.showInputDialog ("Enter the first Operand: ");

        secondnumber =
            JOptionPane.showInputDialog ("Enter the second Operand: ");

        operation =
            JOptionPane.showInputDialog ("Enter the Operator: ");

         num1 = Double.parseDouble (firstnumber);
         num2 = Double.parseDouble (secondnumber);

         sum= num1 + num2;
         sub= num1 - num2;
         div= num1 / num2;
         mul= num1 * num2;
         mod= num1 % num2;

         if (operation == '+'){
               JOptionPane.showMessageDialog (null, sum);}

         if (operation == '-'){
               JOptionPane.showMessageDialog (null, sub);
         }

         if (operation == '/'){
               JOptionPane.showMessageDialog (null, div);
         }

         if (operation == '*'){
               JOptionPane.showMessageDialog (null, mul);
         }

         if (operation == '%'){
               JOptionPane.showMessageDialog (null, mod);
         }
    }
} 

---

现在我假设我的if语句是正确的。正在寻找将该操作变量转换为字符的最佳方法。

假设输入正确，您的运算符是第一个字符：

操作。字符（0）

请添加一些关于如何解决此问题的解释

 String a=JOptionPane.showInputDialog ("Enter the Operator: ");
    char ch=a.charAt(0); //charAt(int index of string)
    if(ch=='+'){
    .......
    }
    .....
    .....
    .....