How to read from particular header in opencsv?

Question

I have a csv file. I want to extract particular column from it.For example: Say, I have csv:

id1,caste1,salary,name1
63,Graham,101153.06,Abraham
103,Joseph,12245         


        

Answer 1

I had a task to remove several columns from existing csv, example of csv:

FirstName, LastName, City, County, Zip
Steve,Hopkins,London,Greater London,15554
James,Bond,Vilnius,Vilniaus,03250

I needed only FirstName and LastName columns with values and it is very important that order should be the same - default rd.readMap() does not preserve the order, code for this task:

        String[] COLUMN_NAMES_TO_REMOVE = new String[]{"", "City", "County", "Zip"};
        CSVReaderHeaderAware rd = new CSVReaderHeaderAware(new StringReader(old.csv));
        CSVWriter writer = new CSVWriter((new FileWriter(new.csv)),
                CSVWriter.DEFAULT_SEPARATOR, CSVWriter.NO_QUOTE_CHARACTER, CSVWriter.NO_ESCAPE_CHARACTER, CSVWriter.DEFAULT_LINE_END);

        // let's get private field
        Field privateField = CSVReaderHeaderAware.class.getDeclaredField("headerIndex");
        privateField.setAccessible(true);
        Map headerIndex = (Map) privateField.get(rd);

        // do ordering in natural order - 0, 1, 2 ... n
        Map sortedInNaturalOrder = headerIndex.entrySet().stream()
                .sorted(Map.Entry.comparingByValue(Comparator.naturalOrder()))
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
                        (oldValue, newValue) -> oldValue, LinkedHashMap::new));

        // let's get headers in natural order
        List headers = sortedInNaturalOrder.keySet().stream().distinct().collect(Collectors.toList());

        // let's remove headers
        List removedColumns = new ArrayList(Arrays.asList(COLUMN_NAMES_TO_REMOVE));
        headers.removeAll(removedColumns);
        // save column names           
        writer.writeNext(headers.toArray(new String[headers.size()]));
   
        List keys = new ArrayList<>();
        Map values;
        while ((values = rd.readMap()) != null) {
            for (String key : headers) {
                keys.add(values.get(key));
                if (keys.size() == headers.size()) {
                    String[] itemsArray = new String[headers.size()];
                    itemsArray = keys.toArray(itemsArray);
                    // save values                       
                    writer.writeNext(itemsArray);
                    keys.clear();
                }
            }
        }
        writer.flush();

Output:

FirstName, LastName
Steve,Hopkins
James,Bond