ArrayList or Hashset

Question

ArrayList queryParms = new ArrayList(); StringBuffer sql = new StringBuffer(); sql.append( "SELECT A.FLDREC_NUM, " + "@CHARDATE(A.FLDDATE), " + "T.FLDDESCR, @DEC(A.FLDLEVEL,3) " + " FROM @SCHEMAALCOHOL A LEFT OUTER JOIN @SCHEMADRUGCAUS T " + " ON A.FLDREASON = T.FLDCODE " + " WHERE A.FLDEMPLOYEE = ? " + " ORDER BY A.FLDDATE DESC" ); queryParms.add(new Long(empRec));

Can i use HashSet instead of ArrayList above and does it make any sense in terms of performance?.
What does the query do, do we need to append the query in StringBuffer. Why can't i directly add to ArrayList?

DON'T use a HashSet. It will not keep your parameters in the same order, so your SQL can get really funny results (if you have more than one parameter). — Thilo
– Thilo, Commented Sep 27, 2010 at 10:20
I think you really need to cover my Internal life of ArrayList tutorial and Internal life of HashSet tutorial — Volodymyr Levytskyi
– Volodymyr Levytskyi, Commented Jul 27, 2013 at 8:47

Jon Skeet · Accepted Answer · 2010-09-27 09:29:09Z

In general, you would want to use an ArrayList for query parameters - because the order matters. When you've only got a single parameter (as you have here) it's not an issue - but I'd definitely use a list for consistency.

As for using StringBuffer - you haven't shown what you're doing with sql here. If you're not appending anything else, you could use a simple String... but you wouldn't be adding it to the ArrayList, as it's the query itself, not a query parameter.

Finally, I'd recommend using generics, e.g.

ArrayList<Long> queryParameters = new ArrayList<Long>();

Or at least, use StringBuilder instead of StringBuffer, which is not synchronized and thus faster.

Sanjeev Kulkarni N · Accepted Answer · 2011-10-11 07:12:20Z

As for performance, consider:

Using StringBuilder(instead of StringBuffer) which is not synchronized and is not an issue, since you're creating a new StringBuilder() for each request. If it's the same statement used for each request, it should be a final String, globally declared.
As suggested by Jon, use ArrayList with Generics. The queryParams is used as a part of QueryService implementation used to query DB. As it appears, it's a pre-compiled sql query and order of query params is important at run-time. HashSet doesn't guarantee order.

卢声远 Shengyuan Lu · Accepted Answer · 2010-09-27 09:29:40Z

0

more efficient:

sql.append("SELECT A.FLDREC_NUM, "); sql.append("@CHARDATE(A.FLDDATE), "); ......

answered Sep 27, 2010 at 9:29

卢声远 Shengyuan Lu

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2 Comments

Jon Skeet Over a year ago

Nope, that's less efficient, because it's concatenated at execution time instead of at compile time. More efficient would be just to use a String to start with.

Thilo Over a year ago

Yes, don't use the StringBuilder unless you have if statements or loops when constructing the query.

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