I get an error when I try to get a number from a string?

Question

Java

Hello, I am writing a calculator and I have such a problem. When I write -2 minus 2, I should output -4, but for some reason an error occurs. I just can’t understand why, I tried to remove the minus, but still.
Here is the layout

The main numbers are large this is the TextView in mainactivity.java it is written to the display variable
When the equals button is clicked the method is called

// do result of two numbers
    public void onEqual (View v) {
            display.setText(makeResult()); // set TextView result of computation
    }

And the makeResult method itself, which looks for two numbers in the TextView and performs the necessary operation with them (+, -, /, etc.)
There is also an operator variable, it is static and stores the current operator entered by the user, so that you can then do what you need calculation of two numbers, krch here is the code of the method

// make result of two numbers
    private String makeResult () {
        double one; // one number
        double two; // two number
        double result; // result of two numbers

        String dis = display.getText().toString(); // take text from TextView for more comfortable(manipulation)

        one = Double.parseDouble(dis.substring(0, dis.indexOf(operator) - 1)); // берём первое число от начала строки до оператора(оператор не берём)
        two = Double.parseDouble(dis.substring(dis.indexOf(operator) + 2, dis.length())); // берём второе число после оператора и до конца строки

        switch (operator) {
            case "+":
                result = one + two;
                break;

            case "-":
                result = one - two;
                break;

            case "/":
                result = one / two;
                break;

            case "x":
                result = one * two;
                break;

            default:
                result = 0;
        }

        operator = "";

        return new DecimalFormat("#.##########").format(result); // do format of result ( 5.0 -> 5 )
    }

Also, each operator (+, -, /, x) between numbers has 1 space on each side, for greater beauty and to make it easier to pull out 2 numbers, etc., but this is no longer important.
So why does an error occur, why does he not want to eat a number with a negative sign and make a minus with it with the second number? (-2 - 2) will return the application crash

Answer 1

So why does an error occur, why does he not want to eat a number with a negative sign and make a minus with it with the second number? (-2 - 2) will return the application crash

because the operator in this case is "-"
and now take a closer look at this line "-2 - 2" that the user entered and find the operator "-" in it. Found? What is his index? Zero is correct. now let's do
take a substring starting from character 0 to -1.
Funny right? :)