@Override
    public boolean equals(Object o) {
        if (o == this) return true;
        if (!(o instanceof Document other)) return false;
        return (super.equals(o) && docId.equals(other.docId) &&
                content.equals(other.content));
    }
Returns true if the argument is a document which has the same set of values
document/src/main/java/com/yahoo/document/Document.java