How to properly call a String converted to a Double method?

I have created a method which converts a string value into a new Double. I want to create an if statement that tests whether said method returns null which it is suppose to do. This is the code so far:

Content class

public class Content 
{
        Double x;

 String string = "b";

        public void testParsing() 
        { 
         if (//call xMethod == null) {
            System.out.println("Ovalid operator Success");}
            else {
                    System.out.println("Invalid operator Fail");
                    }
        }

        /*
        * Chops up input on ' ' then decides whether to add or multiply.
        * If the string does not contain a valid format returns null.
        */
        public Double x(String x)
        {

    String[] parsed;
    if (x.contains("*")) 
    {
        // * Is a special character in regex
        parsed = x.split("\\*");

        return Double.parseDouble(parsed[0]) * Double.parseDouble(parsed[1]);
    }
    else if (x.contains("+")) 
    {
        // + is again a special character in regex
        parsed = x.split("\\+");

        return Double.parseDouble(parsed[0]) + Double.parseDouble(parsed[1]);
    }

    return null;
}
}

Main class

public class MainClass {

public static void main(String[] args) {

Content call = new Content();

call.testParsing();

}
}

I know that the following line compiles and outputs as a success: (line 9)

if (x("") == null) {

But I don't think this is doing what I am asking it to do, I am asking it to check if the outcome of the method in which x is pointing to returns null or not. Any clarification on how to properly call this method to check for that condition would be much appreciated, thanks.