变量不正确；分数计算器；JAVA

我有我的代码在这里，我已经张贴在这里之前，并已得到帮助。这些变量似乎不正确，我不确定为什么。我输入了分数1/2+3/4，得到的答案是8/4。我知道这与变量有关，因为当我运行调试器时，它会说a=1，'b'=1，'c'=4，'d'=4a'和d'按我所希望的方式正确，从“a”到“d”从左到右。我想我知道问题在哪里，但我不知道如何解决它。我认为“b”的问题与这行代码有关

b = Integer.parseInt("1");

这是我的全部代码

导入java.util.*

public class Calculator {
public static void main(String[] args) {
System.out.println("Please enter two fractions to add, subtract, multiply, or divide\nor\nType             'quit' to exit the program.");
Boolean on = true;
Scanner console = new Scanner(System.in);
while (on) {
    String input = console.nextLine();
    if (input.equalsIgnoreCase("quit")) {
        on = false;
    } else
        System.out.println(run(input));
}
}
public static String run(String input) {
int indexOfSecondSpace = 0;
int indexOfOperation = 0;
String firstNumber = "0";
String secondNumber = "0";
int beginning = input.indexOf(" ") + 1;
int end = input.indexOf(" ", beginning);
String operator = input.substring(beginning, end);
if (input.contains("+") == true) {
indexOfOperation = input.indexOf("+");
} else if (operator.equals("-")) {
indexOfOperation = input.indexOf("-");
} else if (operator.equals("*")) {
indexOfOperation = input.indexOf("*");
} else if (operator.equals("/")) {
indexOfOperation = input.indexOf("/");
}
firstNumber = (input.substring(0, input.indexOf(" ")));
secondNumber = (input.substring(beginning + 1));
int a = 0;
int b = 0;
int c = 0;
int d = 0;  
if (firstNumber.contains("/")) {
a = Integer.parseInt(firstNumber.substring(0,firstNumber.indexOf("/")));
b = Integer.parseInt(firstNumber.substring(0,firstNumber.indexOf("/")));
} else if (!firstNumber.contains("/"))
a = Integer.parseInt(input.substring(0, input.indexOf(" ")));
b = Integer.parseInt("1");
{
    if (secondNumber.contains("/")) {
    c = Integer.parseInt(secondNumber.substring(secondNumber.indexOf("/")+1));
    d = Integer.parseInt(secondNumber.substring(secondNumber.indexOf("/")+1));
    } else if (!secondNumber.contains("/")) {
    c = Integer.parseInt(secondNumber.substring(secondNumber.length()));
    d = Integer.parseInt("1");
    }
}
String res = calculate(input, a, b, c, d) ;
return res;
}
public static String calculate(String input, int a, int b, int c, int d){
if (input.contains ("+"))
{
    System.out.println("your answer is " + (a*d + b*c)+"/" +(b*d));
}
else if  (input.contains("-"))
{
    System.out.println("your answer is " + (a*d - b*c)+ "/" +(b*d));
}
else if (input.contains("/"))
{
    System.out.println("your answer is " + (a*d)/(b*c)+ "/" +(b*d));
}
else if (input.contains("*"))
{
    System.out.println("your answer is " + (a*c) +"/" +(b*d));
}
return input;
}
}

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您的代码中有以下内容

c = Integer.parseInt(secondNumber.substring(secondNumber.indexOf("/")+1));
d = Integer.parseInt(secondNumber.substring(secondNumber.indexOf("/")+1));

为什么你希望c和d有不同的值

也值得注意

d = Integer.parseInt("1");

可以改写为

d = 1;

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那么，我该如何解决这个问题，让它打印出我想要的输出呢。对不起，我对java还是个新手。谢谢你的帮助。谢谢