How to sort data in a CSV file using a particular field in Java?

Question

I want to read a CSV file in Java and sort it using a particular column. My CSV file looks like this:

 ABC,DEF,11,GHI....
 JKL,MNO,10,PQR....
 STU,VWX,12,XYZ....

Considering I want to sort it using the third column, my output should look like:

 JKL,MNO,10,PQR....
 ABC,DEF,11,GHI....
 STU,VWX,12,XYZ....

After some research on what data structure to use to hold the data of CSV, people here suggested to use Map data structure with Integer and List as key and value pairs in this question:

 Map<Integer, List<String>>
 where the value, List<String> = {[ABC,DEF,11,GHI....], [JKL,MNO,10,PQR....],[STU,VWX,12,XYZ....]...}
 And the key will be an auto-incremented integer starting from 0.

So could anyone please suggest a way to sort this Map using an element in the 'List' in Java? Also if you think this choice of data structure is bad, please feel free to suggest an easier data structure to do this.

Thank you.

Answer 1

I would use an ArrayList of ArrayList of String:

ArrayList<ArrayList<String>>

Each entry is one line, which is a list of strings. You initialize the list by:

List<ArrayList<String>> csvLines = new ArrayList<ArrayList<String>>();

To get the nth line:

List<String> line = csvLines.get(n);

To sort you write a custom Comparator. In the Constructor of that comparator you can pass the field position used to sort.

The compare method then gets the String value on stored position and converts it to a primitive ava type depending on the position. E.g you know that at position 2 in the csv there is an Integer, then convert the String to an int. This is neccessary for corretcly sorting. You may also pass an ArrayList of Class to the constructor such that it knows which field is what type.
Then use String.compareTo() or Integer.compare(), depending on column position etc.

Edit example of working code:

List<ArrayList<String>> csvLines = new ArrayList<ArrayList<String>>();
Comparator<ArrayList<String>> comp = new Comparator<ArrayList<String>>() {
    public int compare(ArrayList<String> csvLine1, ArrayList<String> csvLine2) {
        // TODO here convert to Integer depending on field.
        // example is for numeric field 2
        return Integer.valueOf(csvLine1.get(2)).compareTo(Integer.valueOf(csvLine2.get(2)));
    }
};
Collections.sort(csvLines, comp);