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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: How to parse an array?
Date: Tue, 20 Jun 2017 16:23:46 -0400
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On 6/20/2017 3:18 PM, Eric Douglas wrote:
> I'm writing a method to take 2 arrays and return the subset of the first array.  Is there a more efficient way than creating a new array and iterating?

     It would remove some few obstacles to understanding if you
were to stop treating `ArrayList' and `array' as synonyms.  In
return, I will forswear using `4' to mean `for'.  Deal?

> ArrayList<String> list1 = new ArrayList(); > list1.add("AA");
> list1.add("CC");
> list1.add("DD");
> list1.add("BB");
> list1.add("EE");
> ArrayList<Integer> list2 = new ArrayList();
> list2.add(1);
> list2.add(3);
> ArrayList<String> retval = new ArrayList();

     `new ArrayList<>(list2.size())' might be a bit more efficient,
as it uses the minimum memory and avoids any expansions of too-short
intermediate lists.

> for (int i : list2) {
>       retval.add(list1.get(i));

     Aside: This might throw IndexOutOfBoundsException.  If you
"know things" about the length of `list1' and the contents of
`list2', that might not be a concern.

> }
> Collections.sort(retval);

     Aside: You could get a NullPointerException during the sort
if `list1' contains any nulls that are selected by the contents
of `list2'.  Again, this may not be a concern if you have
sufficient prior knowledge.

> System.out.println(retval);

     There are certainly other ways to write this -- as a stream
pipeline, for example.  Whether the alternatives would be more or
less efficient is something you'd need to measure for yourself.
My guess (*no* hard evidence, mind you) is that "manual" methods
would win if `list2' is short, streams would win if it's Really
Long (because you could parallelize), and there'd probably be a
rather wide intermediate range of lengths where differences might
be difficult to measure.

     And, of course, you could always use arrays instead.

-- 
esosman@comcast-dot-net.invalid
Thirteen hundred ten days to go.