How do I make defensive copies of a Mutable Object which contains a mutable field in an Immutable Object?

class ImmutableObject {

  private final MutableObject immutable_field;

  ImmutableObject(MutableObject y) {
    this.immutable_field = y;
  }

  MutableObject return_immutable_field() {
    return immutable_field;
  }
}

class MutableObject {

  public int mutable_field;
}

• The MutableObject does not have a constructor that lets me set the field.
• The MutableObject's current state should be captured in the Immutable Object and never changed.