java My String变量未由Double初始化。两倍，但只取决于吃多少和多少汉堡，不回答

Question

我在这些行中遇到错误：

meat = Double.parseDouble(howMuchMeat);

及

burgers = Double.parseDouble(howManyBurgers); 

它说howMuchMeat和howManyBurgers未初始化，如何使用用户输入初始化它们

import javax.swing.JOptionPane;
public class HamburgerHelperProgram {
    public static void main(String[] args) {
        String answer;
        String howManyBurgers;
        String howMuchMeat;
        double a;
        double burgers, meat;
        double burgers1, meat1;
        
        answer = JOptionPane.showInputDialog("I am your hamburger helper! What question do you need answered? "
                + "\nAnswer 1 to find how many burgers can be made from a certain amount of meat, "
                + "\nand 2 to find how much meat is needed to make a specific number of burgers. ");
        a = Double.parseDouble(answer);
        
        if ( a == 1 )
            howManyBurgers = JOptionPane.showInputDialog("How much meat do you have? (Enter in pounds): ");
        burgers = Double.parseDouble(howManyBurgers);
        
        burgers1 = burgers % ( 1 / 4 );
        JOptionPane.showMessageDialog(null, "You can make " + burgers1 + " burgers.");
        
        if ( a == 2 )
            howMuchMeat = JOptionPane.showInputDialog("How many burgers do you want to make? (Enter whole numbers): ");
        meat = Double.parseDouble(howMuchMeat);
        meat1 = meat * ( 1 / 4 );
        JOptionPane.showMessageDialog(null, "You need " + meat1 + " pounds of meat to make " + meat + " burgers.");
            
        if ( a != 2 )
            JOptionPane.showMessageDialog(null, "Invalid, try again."); 
                else if ( a != 1 )
            JOptionPane.showMessageDialog(null, "Invalid, try again.");
        }
    }
}