Beginner radian/degree converter program
Nowadays is common to omit the generic type on the right hand side of an assignment so instead of

 ArrayList&lt;Integer&gt; divisors = new ArrayList&lt;Integer&gt;(); 


you can write 

 ArrayList&lt;Integer&gt; divisors = new ArrayList&lt;&gt;(); 


Since you are not using any ArrayList specific functionality it is cleaner to declare divisors as List (same goes for gcd)

 List&lt;Integer&gt; divisors = new ArrayList&lt;&gt;();


The call gcd.get(gcd.size() deserves a separate method with a reasonable name. 

Regarding BufferedReader userInput ... while in this example it is harmless to not close IO resources, it is good practice to do so in general. Check out "try with resources statement": https://docs.oracle.com/javase/tutorial/essential/exceptions/tryResourceClose.html. 

It is also common to reuse the Reader and not create a new one every time you need to perform a read operation. With this in mind you will have something like this:

public static void main(String[] args) throws IOException {
    System.out.println("Do you want to convert from radicals to degrees, or degrees to radicals?");
    System.out.print("Use \"R\" for radials to degrees, and \"D\" for degrees to radicals: ");
    try (BufferedReader userInput = new BufferedReader(new InputStreamReader(System.in))) {
        String userChoice = userInput.readLine();

        if (userChoice.contains("D") || userChoice.contains("d")) {

            System.out.print("Degrees: ");
            dtr(userInput);

        } else if (userChoice.contains("R") || userChoice.contains("r")) {

            System.out.println("Radical numerator (omit pi): ");
            System.out.println("Radical denominator: ");
            rtd(userInput);

        } else {

            System.out.println("Invalid response. Please restart the program.");
        }
    }
}


Note that now rtd requires only one parameter. Because you print before reading user input, the user gets to see

 Radical numerator (omit pi): 
 Radical denominator: 


before she has an opportunity to provide any input. Those statements should move into rtd interleaved with reading user input.

 if (userChoice.contains("D") || userChoice.contains("d"))


can be simplified like so:

if (userChoice.toLowerCase().contains("d")) {