MainActivity.java

// method to loop through results trying to find an operator
private char getOperatorFromResult(ArrayList<String> results) {
    for (String str : results) {
        if (getCharOperatorFromText(str) != '0') {
            return getCharOperatorFromText(str);
        }
    }
    return '0';
}

// method to convert string operator to char
private char getCharOperatorFromText(String strOper) {
    switch (strOper) {
        case "plus":
        case "add":
            return '+';
        case "minus":
        case "subtract":
            return '-';
        case "times":
        case "multiply":
            return '*';
        case "divided by":
        case "divide":
            return '/';
        case "power":
        case "raised to":
            return '^';
    }
    return '0';
}