Working with a List of Lists in Java

Question

I m trying to read a CSV file into a list of lists  of strings   pass it around for getting some data from a database  build a new list of lists of new data  then pass that list of lists so it can be written to a new CSV file  I ve looked all over  and I can t seem to find an example on how to do it    I d rather not use simple arrays since the files will vary in size and I won t know what to use for the dimensions of the arrays  I have no issues dealing with the files  I m just not sure how to deal with the list of lists   Most of the examples I ve found will create multi-dimensional arrays or perform actions inside the loop that s reading the data from the file  I know I can do that  but I want to write object-oriented code  If you could provide some example code or point me to a reference  that would be great

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Also this is an example of how to print List of List using advanced for loop   public static void main String   args           int   a  1 3  7  8  3  9  2  4  10           List lt List lt Integer gt  gt  triplets          triplets sumOfThreeNaive a  13           for  List lt Integer gt  list   triplets               for  int triplet  list                   System out print triplet                                 System out println

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I d second what xrath said - you re better off using an existing library to handle reading   writing CSV   If you do plan on rolling your own framework  I d also suggest not using List lt List lt String gt  gt  as your implementation - you d probably be better off implementing CSVDocument and CSVRow classes  that may internally uses a List lt CSVRow gt  or List lt String gt  respectively   though for users  only expose an immutable List or an array     Simply using List lt List lt String gt  gt  leaves too many unchecked edge cases and relying on implementation details - like  are headers stored separately from the data   or are they in the first row of the List lt List lt String gt  gt    What if I want to access data by column header from the row rather than by index   what happens when you call things like       reads CSV data  5 rows  5 columns  List lt List lt String gt  gt  csvData   readCSVData     csvData get 1  add  extraDataAfterColumn        now row 1 has a value in  nonexistant  column 6 csvData get 2  remove 3       values in columns 4 and 5 moved to columns 3 and 4      attempting to access column 5 now throws an IndexOutOfBoundsException    You could attempt to validate all this when writing out the CSV file  and this may work in some cases    but in others  you ll be alerting the user of an exception far away from where the erroneous change was made  resulting in difficult debugging

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The example provided by  tster shows how to create a list of list  I will provide an example for iterating over such a list   Iterator lt List lt String gt  gt  iter   listOlist iterator    while iter hasNext         Iterator lt String gt  siter   iter next   iterator        while siter hasNext              String s   siter next             System out println s

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Here s an example that reads a list of CSV strings into a list of lists and then loops through that list of lists and prints the CSV strings back out to the console   import java util ArrayList  import java util List   public class ListExample       public static void main final String   args                  sample CSV strings   pretend they came from a file         String   csvStrings   new String                      abc def ghi jkl mno                    pqr stu vwx yz                    123 345 678 90                      List lt List lt String gt  gt  csvList   new ArrayList lt List lt String gt  gt                pretend you re looping through lines in a file here         for String line   csvStrings                        String   linePieces   line split                   List lt String gt  csvPieces   new ArrayList lt String gt  linePieces length               for String piece   linePieces                                csvPieces add piece                             csvList add csvPieces                        write the CSV back out to the console         for List lt String gt  csv   csvList                          dumb logic to place the commas correctly             if  csv isEmpty                                  System out print csv get 0                    for int i 1  i  lt  csv size    i                                          System out print       csv get i                                                System out print   n                        Pretty straightforward I think   Just a couple points to notice    I recommend using  List  instead of  ArrayList  on the left side when creating list objects   It s better to pass around the interface  List  because then if later you need to change to using something like Vector  e g  you now need synchronized lists   you only need to change the line with the  new  statement  No matter what implementation of list you use  e g  Vector or ArrayList  you still always just pass around List lt String gt   In the ArrayList constructor  you can leave the list empty and it will default to a certain size and then grow dynamically as needed   But if you  know how big your list might be  you can sometimes save some performance   For instance  if you knew there were always going to be 500 lines in your file  then you could do     List lt List lt String gt  gt  csvList   new ArrayList lt List lt String gt  gt  500    That way you would never waste processing time waiting for your list to grow dynamically grow   This is why I pass  linePieces length  to the constructor   Not usually a big deal  but helpful sometimes   Hope that helps

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ArrayList lt ArrayList lt String gt  gt  listOLists   new ArrayList lt ArrayList lt String gt  gt     ArrayList lt String gt  singleList   new ArrayList lt String gt     singleList add  hello    singleList add  world    listOLists add singleList    ArrayList lt String gt  anotherList   new ArrayList lt String gt     anotherList add  this is another list    listOLists add anotherList

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public class TEst        public static void main String   args             List lt Integer gt  ls new ArrayList lt  gt             ls add 1           ls add 2           List lt Integer gt  ls1 new ArrayList lt  gt             ls1 add 3           ls1 add 4           List lt List lt Integer gt  gt  ls2 new ArrayList lt  gt             ls2 add ls           ls2 add ls1            List lt List lt List lt Integer gt  gt  gt  ls3 new ArrayList lt  gt             ls3 add ls2             methodRecursion ls3              private static void methodRecursion List ls3            for Object ls4 ls3                         if ls4 instanceof List                                     methodRecursion  List ls4                 else                    System out print ls4

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Something like this would work for reading   String filename    something csv   BufferedReader input   null  List lt List lt String gt  gt  csvData   new ArrayList lt List lt String gt  gt     try        input    new BufferedReader new FileReader filename        String line   null      while    line   input readLine       null                String   data   line split               csvData add Arrays toList data            catch  Exception ex          ex printStackTrace      finally        if input    null                input close

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If you are really like to know that handle CSV files perfectly in Java  it s not good to try to implement CSV reader writer by yourself  Check below out    http   opencsv sourceforge net   When your CSV document includes double-quotes or newlines  you will face difficulties    To learn object-oriented approach at first  seeing other implementation  by Java  will help you  And I think it s not good way to manage one row in a List  CSV doesn t allow you to have difference column size

[java] Working with a List of Lists in Java

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