protected final void assignId(URI id) {
        if (id == null) throw new NullPointerException("Attempt to assign id of " + this + " to null");
        this.id = id;
    }
Assigns a new or changed id to this hit.
As this is protected, reassigning isn't legal for Hits by default, however, subclasses may allow it
using this method.
container-search/src/main/java/com/yahoo/search/result/Hit.java