How to intercept a method call in Play Framework (Not http call)

Question

I have a class generates and return some string. I need to append a common string value to the return value of the method.

Example

// Generates a string
public class StringGenerator {
    public static String getString(){
        return "abc";
    }
}

// An interceptor intercepts the method call and append string to the return value (sudo code)
public class InterceptorClass {
    public static String interceptStringGeneratorMethod (){
        // append "def" to return value of the method
    }
}

// Calling string generation method
public Result index(Http.Request request) throws IOException {
    return ok(StringGenerator.getString());
    // Expected output of the method call is "abcdef"
}

I am using latest play framework 2.8.12