reading a CSV file into different types of arrays and then into an Arraylist

Question

I have imported a CSV file into java and then have read each column of the CSV file into separate arrays. I then wanted to place all those arrays into an arraylist. The following is my code.

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;
import java.sql.*;
import oracle.jdbc.driver.*;


 public class verbindung { 

     public static void main(String[] args) {
        String filename = "betreuen_4.csv";
        File file = new File(filename);

        ArrayList<caring> betreuen = new ArrayList<caring>();

        try {
            Scanner inputStream = new Scanner(file);
            while(inputStream.hasNext()) {
                String data = inputStream.next();
                String[] values = data.split(",");
                int PInummer = Integer.parseInt(values[1]);
                String MNummer = values[0];
                String KundenID = values[2];
                System.out.println(MNummer);
            }
            inputStream.close();
        }catch(FileNotFoundException e) {
            e.printStackTrace();
        }
        betreuen[0] = MNummer;


    }


}  

I am getting an error by the last code line where it says

betreuen[0] = MNummer;

The error states:

Exception in thread "main" java.lang.Error: Unresolved compilation problems: The type of the expression must be an array type but it resolved to ArrayList MNummer cannot be resolved to a variable

caring is a class created by the following code

public class caring {
String MNr;  
int PINr;
String KID;

public caring(String MNummer,int PInummer, String KundenID ) {
    this.MNr = MNummer;
    this.PINr = PInummer;
    this.KID = KundenID;
}
}

I have typed this error into google and I believe I might understand why I am getting this error. The problem is I dont know how to get my desired outcome although I have researched alot online.

In short I would like that my Arraylist would contain the arrays (MNummer, PInummer, KundenID ). I need it to be an arraylist for code that I am going to write for inserting a batch into a databank through Java.

Answer 1

You are trying to access ArrayList<caring> betreuen as a standard array.

an ArrayList<Type> is single instance(object) of a class, however, this class stores an array of data.

to store and retrieve data you must use the appropriate methods of the class add,remove,set etc.

To add to the ArrayList<caring> betruen you need to call the Method add() with the parameter you wish to add, in this case betruen.add(MNummer);

This however will not work as MNummer is a String object and not a caring object, thus you must first create an object of type caring to add to betruen.

We can create this by calling caring _caring = new caring(MNummer, PInummer, KundenID);

Please note that Object classes should usually begin with a uppercase letter and variables should usually start with a lowercase letter, so that you can for example write code like this Caring caring = new Caring() rather than caring _caring = new caring() or caring caring = new caring() , the latter won't run as the JVM will get confused with the variable name being the same as a class name.

Now that we have our caring object we can now add it to the ArrayList by replacing betreuen[0] = MNummer; with betreuen.add(caring); if you try running this it will more than likely give you another compilation error, as you are trying to access a variable outside of it's scope.

take the following code as an example, we define obj1 before the opening curly brace "{" of the if statement, thus anything inside of the if statement can access obj1.

public void test()
{
    String obj1 = "hello";
    if(obj1.equals("hello"))
    {
        obj1 = "bye";
    }
}

the following code will however not work, we define obj2 inside the opening curly brace "{" of the if statement, thus anything outside of the if statement cant access obj1.

public void test()
{
    String obj1 = "hello";
    if(obj1.equals("hello"))
    {
        String obj2 = "bye";
    }
    obj2 = "hello";
}

To combine the above together your code should be as follows: As an aside: your code may not run correctly if any of the CSV fields have a comma in them, it is recommended to use a pre-existing CSV library, however, if you still wish to parse the CSV yourself you may wish to read the RFC 4178 Common Format and MIME Type for Comma-Separated Values (CSV) Files for guidance on the format of a CSV file

 public class verbindung { 

     public static void main(String[] args) {
        String filename = "betreuen_4.csv";
        File file = new File(filename);

        ArrayList<caring> betreuen = new ArrayList<caring>();

        try {
            Scanner inputStream = new Scanner(file);
            while(inputStream.hasNext()) {
                String data = inputStream.next();
                String[] values = data.split(",");
                int PInummer = Integer.parseInt(values[1]);
                String MNummer = values[0];
                String KundenID = values[2];
                System.out.println(MNummer);
                //create the caring object with the required paramaters
                caring _caring = new caring(MNummer, PInummer, KundenID);
                //add _caring object to the betreuen array here as it is within the same scope.
                betreuen.add(_caring);
            }
            inputStream.close();
        }catch(FileNotFoundException e) {
            e.printStackTrace();
        }
        //this will cause a compilation error
        //betreuen[0] = MNummer;

        //this will also cause a compilation error because it is out of the scope of _caring
        //betreuen.add(_caring);
    }
} 

Answer 2

you read csv line and assign values to variables but you neither create caring instances nor add them to the arraylist betreuen

int PInummer = Integer.parseInt(values[1]);
String MNummer = values[0];
String KundenID = values[2];
System.out.println(MNummer);
caring aCaring = new caring(MNummer, PInummer, KundenID); /// create an instance of caring from line values
betreuen.add(aCaring); /// add instance to the arraylist

Answer 3

In short I would like that my Arraylist would contain the arrays (MNummer, PInummer, KundenID ). I need it to be an arraylist for code that I am going to write for inserting a batch into a databank through Java.

From what I understand, you need an ArrayList/Bean of all the columns where each column is an array.

// List of all elements in a column
List<Integer> PInummer = new ArrayList<Integer>();
List<String> MNummer = new ArrayList<String>();
List<String> KundenID = new ArrayList<String>();

while(inputStream.hasNext()) 
{
    String[] values = inputStream.next().split(",");
    PInummer.add(Integer.parseInt(values[1]));
    MNummer.add(values[0]);
    KundenID.add(values[2])
}

Integer[] PInummerArray = PInummer.toArray(new Integer[PInummer.size()]);
String[] MNummerArray = MNummer.toArray(new String[MNummer.size()]);
String[] KundenIDArray = KundenID.toArray(new String[KundenID.size()]);

// List of all columns (the complete cSV)
List<Object> caring = new ArrayList<Object>();
// Adding to caring list
caring.add(PInummerArray);
caring.add(MNummerArray);    
caring.add(KundenIDArray);

// Or if you prefer to save the arrays into Caring bean
Caring _caring = new Caring(PInummerArray, MNummerArray, KundenIDArray);