Default settings for BuildServiceParameters

How can I set default values for parameters in my BuildServiceParameters class? If I define an interface that implements BuildServiceParameters, then there’s no obvious place to set defaults at all. So I tried to use a class instead of an interface, and set defaults in the constructor:

abstract class MyServiceParameters implements BuildServiceParameters {

  public MyServiceParameters() {
    getFileCollection().setFrom(...);
  }

  abstract ConfigurableFileCollection getFileCollection();
}

However, the constructor’s call to getFileCollection fails with a NullPointerException. So where can I set a default that will apply to all uses of my build service and its parameters?