driver-legacy/src/main/com/mongodb/DBCursor.java
@Nullable
    public DBObject one() {
        DBCursor findOneCursor = copy().limit(-1);
        try {
            return findOneCursor.hasNext() ? findOneCursor.next() : null;
        } finally {
            findOneCursor.close();
        }
    }
@Nullable
    public DBObject one() {
        DBCursor findOneCursor = copy().limit(-1);
        try {
            return findOneCursor.hasNext() ? findOneCursor.next() : null;
        } finally {
            findOneCursor.close();
        }
    }
Returns the first document that matches the query.

@return the first matching document
@since 2.12