将字符串从数组列表字符串转换为数组列表字符串

Question

I'm working on a way to save an ArrayList String into a .txt document, then open it up, modify the ArrayList String then close it etc.

Currently, I'm using this to open the ArrayList up again:

ArrayList.add(fileIn.nextLine())

This works, but not as expected because after some saves of the Array List this happens:

[ [ [ [ ], 7,68, 9,71, 11,73, 13,72, 12,70], 9,72, 11,73, 13,72, 12,70, 10,69, 9,71], 6,76, 6,78, 8,77, 8,75, 7,74]

The array list 'overlaps'. Is there any way to save or load the array list without it overlapping?

Thanks!

Answer 1

You have a few problems here:

1. You shouldn't capitalize a variable name in Java (by convention.) Doing so makes your code much harder for other people to read/debug. Therefore, you should change your code sample to be 'someArrayList' or 'arrayList'. This would help clear things up.
2. You misunderstand the add method of ArrayLists and the nextLine method of whatever reader you are using. 'add' does no parsing whatsoever, and 'nextLine' also does no parsing. If your file currently is [5,1,3] and you call your code, it will simply add the whole [5,1,3] String to your ArrayList as a single entry. This is why you are seeing this nesting behavior.
3. Now, when you're talking about where the brackets even come from, we're talking about the toString method of ArrayList (which you are probably calling implicitly). The toString method of the ArrayList class prints out the entries, separated by commas, with opening and closing brackets.

To solve #2 and #3, you will need to either use a library (as suggested elsewhere) or do it yourself by looping through and printing each item in the list. If you do this, you will also need to choose and use a separator whcih you know won't be present in any of the items (comma is usually good). Finally, you will have to parse the results of the nextLine method. Here's a quick snippet for that.

String nextLine = fileIn.nextLine();
arrayList.addAll(nextLine.split(","));//I use comma here, since I assume you will be separating your list with commas

Finally, a small tip for adding separators... make sure you don't put a hanging separator after the last item in your list.

This question is, I think, unique (I can't find another question about printing and then reading an ArrayList,) although there are other questions about both printing ArrayList's and parsing files. You may be better off researching the general topics more thoroughly in the future.

Answer 2

Why do the work by hand, when you could use something like:

• org.apache.commons.io.IOUtils.readLines(java.io.InputStream, java.nio.charset.Charset)

and

• org.apache.commons.io.IOUtils.writeLines(java.util.Collection, java.lang.String, java.io.OutputStream, java.nio.charset.Charset)