Basic addition and subtraction with choices

Please help with this. I am very new to this, and I need this badly. I need to create a program where it lets you choose from addition or subtraction. This is my current program:

import javax.swing.JOptionPane;

public class RationalZ {

    public static void main(String args[]) {

        JOptionPane.showMessageDialog(null,
            "Enter the letter A for Addition and B Subtraction");

        String choice = JOptionPane.showInputDialog(
      开发者_运维百科      "A=Addition B=Subtraction");

        if (choice == "a") {

            String strNum1 = JOptionPane.showInputDialog(
                "Enter a Number");
            String strNum2 = JOptionPane.showInputDialog(
                "Enter a second number");

            int aNum1 = Integer.parseInt(strNum1);
            int aNum2 = Integer.parseInt(strNum2);

            JOptionPane.showMessageDialog(null, aNum1 + aNum2);

            System.exit(0);

        } else {
            System.exit(0);
        }
    }
}

What's wrong? Even in the first step I can't get it.

You might want to review the difference between the == operator and the equals() method.

Addendum: It's fairly easy to find good information on Java String comparison methods; it's a little harder to find a good explanation of why == with String is usually wrong.

Addendum:

Do I use another if statement?

You've got a good if-then statement; now you need to expand it to an if-then-else statement. Notice how you can add more if-then statements after the else.