Convert a String (formatted just like an ArrayList) to an actual ArrayList

Question

I am trying to convert a String into an ArrayList. For example, my Struts2 webapp returns this String named row in a format similar to this:

[A, BB, CCC, DDDD, 1, 0, 1] (something along those lines)

I need to convert them into an ArrayList so I can prepopulate some forms in another JSP page. I hardcoded a method to convert such Strings into list form:

        StringBuffer rowBuffer = new StringBuffer(row);
        int startIndex = 0;
        int endIndex = rowBuffer.indexOf(",") - 1;
        rowBuffer.deleteCharAt(rowBuffer.indexOf("["));
        rowBuffer.deleteCharAt(rowBuffer.indexOf("]"));
        while(startIndex != -1 && endIndex != -1 && startIndex < endIndex)
        {
          String subString = rowBuffer.substring(startIndex, endIndex);

       if(subString.contains(","))
       {
          rowList.add(" ");
          startIndex = endIndex + 1;
          endIndex = rowBuffer.indexOf(",", startIndex);
       }

       else
       {
        rowList.add(subString);
        startIndex = endIndex + 2;
        endIndex = rowBuffer.indexOf(",", startIndex + 1);
       }

       if(endIndex == -1)
       {
          rowList.add(rowBuffer.substring(startIndex));
          break;
       }
    }

This works fine in cases where all the fields are populated. However, lets say I have a String that looks like this: [A, BB, , , 1, 0, 0] (the 3rd and 4th fields are missing), then I get something that doesn't work (the blank elements don't register correctly, and the size of the list is 6, when it should be 7). Is there a more elegant solution than hardcoding? If not, could someone point me in the right direction on how to handle cases with blank fields? Thanks!

Let's take a step back: why is your Struts2 webapp returning those values in this format? It would make more sense to me if it returned each value as a separate parameter. Is a `List` been printed plain in a hidden input field with the purpose to retain it in the subsequent request? You could then just print multiple hidden fields with the same name but each a different value. This is perfectly valid in HTML/HTTP. I don't do Struts2, but the Servlet API would return it as `String[]` by `request.getParameterValues()`. — BalusC, Aug 11 '10 at 22:17
Nevermind, I got it figured out (w/ some more hard coding). I am using hidden fields, but I am not a HTML / JSP / Struts2 / web programming expert by any means, so I don't want to change all my code and end up with something that ends up not working. Thanks though! — Raymond, Aug 11 '10 at 23:27
Much good luck with this project anyway. I spotted your another Struts2 question about the buttons in a table. This is insanely easy with plain HTML/JSP/Servlet or using JSF (the Java EE-provided competitor of Struts2). However, I don't do Struts2, so I couldn't answer it. There are also not really much Struts2 experts around here at Stackoverflow. You might have better luck at Struts2 mailinglist. Try not to fiddle with hacks and nasty workarounds instead of easy, clean and correct solutions. This would only hurt more on long term. — BalusC, Aug 12 '10 at 13:10

score 1 · Answer 1 · edited May 23 '17 at 12:01

1

Try this please:

import java.util.regex.*;

// ...

// Working code    
rowBuffer.deleteCharAt(rowBuffer.indexOf("["));
rowBuffer.deleteCharAt(rowBuffer.indexOf("]"));
// Create a pattern to match breaks
Pattern p = Pattern.compile("\\s*,\\s*");
// Split input with the pattern
String[] result = p.split(rowBuffer);
rowList = new ArrayList(Arrays.asList(result));

NOTE: this pre-supposes that the strings themselves do not contain commas and are not quoted. If you want to parse real CSV with commas in the fields and quoted values, do NOT use regular expressions and split; and instead use a dedicated state machine CSV parser (here's one example: http://opencsv.sourceforge.net/ - or you can roll your own, like BalusC example here)

edited May 23 '17 at 12:01

Community

1
1

answered Aug 11 '10 at 22:22

DVK

126,886
32
213
327

Hi DVK, In some cases, I have spaces between Strings (example: [AAA, BB BBB, CCC, DDDD, 1, 0, 1]), but I want to get "BB BBB", not "BB", and "BBB". I don't really know regular expressions yet... so could you help me? – Raymond Aug 11 '10 at 22:37
Looks to me like @DVK's code handles spaces immediately before and after the comma delimiter... --JA – andersoj Aug 11 '10 at 23:35
@Raymond Chang - `\s*,\s*` means match a comma surrounded on both left and right by optional whitespace. Exactly what you need. – DVK Aug 11 '10 at 23:46
@DVK: You wrote `compile("[\\s*,\\s*]+")`; you misunderstood how character class `[...]` works. This will match asterisks. – polygenelubricants Aug 12 '10 at 09:17
What if the strings contain `commas` themselves? :/ – st0le Aug 12 '10 at 09:45
@polygenelubricants - will fix, you're correct of course *facepalm* – DVK Aug 12 '10 at 13:13
@st0le - if the string contains commas, using regular expressions to parse it is a Bad Idea and you should use a dedicated CSV parser (write your own if Java doesn't provide anything like Perl's Text::CSV_XS - it's messy and tricky but doable and easier than many other parsers). I noted that in updated answer – DVK Aug 12 '10 at 13:15

score 0 · Accepted Answer · answered Aug 11 '10 at 23:28

Well I changed my code a bit and made it work (it's probably far from the most elegant solution, but it works for me...

    StringBuffer rowBuffer = new StringBuffer(row);
    int startIndex = 0;
    int endIndex = rowBuffer.indexOf(",") - 1;
    rowBuffer.deleteCharAt(rowBuffer.indexOf("["));
    rowBuffer.deleteCharAt(rowBuffer.indexOf("]"));
    while(startIndex != -1 && endIndex != -1 && startIndex < endIndex)
    {
      String subString = rowBuffer.substring(startIndex, endIndex);

       if(subString.contains(","))
       {
          rowList.add(" ");
          startIndex = endIndex - 1;
          endIndex = rowBuffer.indexOf(", ", startIndex + 1);
       }

       else
       {
         if(subString.equals("1"))
             rowList.add("True");
         else if(subString.equals("0"))
             rowList.add("False");
         else
          rowList.add(subString);
        startIndex = endIndex + 2;
        endIndex = rowBuffer.indexOf(",", startIndex + 1);
       }

       if(endIndex == -1)
       {
          if(subString.equals("1"))
             rowList.add("True");
          else if(subString.equals("0"))
             rowList.add("False");
          break;
       }
    }

score 0 · Answer 3 · answered Aug 12 '10 at 09:26

Assuming that the format is as specified by AbstractCollection.toString(), then you can simply:

Remove the surrounding brackets (with simple substring)
Then split on ", " (comma and space)
Wrap the String[] into a List<String> using Arrays.asList
- Use that to populate an ArrayList<String> if necessary

Note that this will break if the elements themselves can contain ", ". For this to work, that string must be a delimiter, and never part of the actual token.

Here's a snippet to illustrate:

    String s = "[A, BB, CCC, DDDD, 1, 0, 1]";
    String[] parts = s.substring(1, s.length() - 1).split(", ");
    List<String> list = new ArrayList(Arrays.asList(parts));

    for (String part : list) {
        System.out.print("{" + part + "} ");
    } // {A} {BB} {CCC} {DDDD} {1} {0} {1}

Convert a String (formatted just like an ArrayList) to an actual ArrayList

3 Answers3