public void setId(int newId, DocumentType owner) {
        if (owner == null) {
            throw new NullPointerException("Can not assign an id of " + this + " without knowing the owner");
        }

        validateId(newId, owner);

        owner.removeField(getName());
        this.fieldId = newId;
        this.forcedId = true;
        owner.addField(this);
    }
Sets the id of this field. Don't do this unless you know what you are doing

@param newId the id - if this is less than 100 it will cause document to serialize
             using just one byte for this field id. 100-127 are reserved values
@param owner the owning document, this is checked for collisions and notified
             of the id change. It can not be null
document/src/main/java/com/yahoo/document/Field.java