如何计算csv文件的总人数？

Question

given the following code:

import java.util.Arrays;
import java.util.List;
import java.util.Map;

/**
 * siehe Zettel 0 - Aufgabe 2
 */
public class PrintCSV_Task2 {

    private static String FILENAME = "data.csv";

    public static void main(String[] args) {
        System.out.println("---(Ausgabe) Aufgabe 2.1---");
        printAll_CSV_NUM(ReadCSV_Task2.fetchAll_CSV_NUM(FILENAME));
        System.out.println("---(Ausgabe) Aufgabe 2.2---");
        printAll_CSV_ASSOC(ReadCSV_Task2.fetchAll_CSV_ASSOC(FILENAME));

        System.out.println("Compiled, no syntactical failures =)");
    }

    /**
     * Aufgabe 2.1
     * @param csvData die Daten der CSV
     */
    public static void printAll_CSV_NUM(List<String[]> csvData) {
        csvData.stream().map(entry -> Arrays.stream(entry).reduce(
                "", //init
                (e1, e2) -> (e1.equals("")) ? e2 : (e1 + "; " + e2)) //concat
        ).forEach(System.out::println);
    }

    /**
     * Aufgabe 2.2
     * @param csvData die Daten der CSV
     */
    public static void printAll_CSV_ASSOC(List<Map<String, String>> csvData) {
        csvData.stream().map(entry -> entry.entrySet().stream().reduce(
                new StringBuilder(), //init
                (s, e) -> s.append(s.length() == 0 ? "" : "; ").append(e.getKey()).append(':') .append(e.getValue()), //concat String with MapEntry
                (s1, s2) ->  s1.append(s1.length() == 0 ? "" : "; ").append(s2) //concat already converted MapEntry_ies (String) with another
        )).forEach(System.out::println);
    }
}

and:

import java.util.List;
import java.util.Map;

/**
 * siehe Zettel 0 - Aufgabe 3
 */
public class ProcessCSV_Task2 {
    private static String FILENAME = "data.csv";

    public static void main(String[] args) {
        System.out.println("---(Ausgabe) Aufgabe 2.1---");
        System.out.printf("Die Anzahl der Personen ist %d", countCsvData_CSV_NUM(ReadCSV_Task2.fetchAll_CSV_NUM(FILENAME)));
        System.out.println();
        System.out.println("---(Ausgabe) Aufgabe 2.2---");
        PrintCSV_Task2.printAll_CSV_NUM(selectCsvDataByYear_CSV_NUM(ReadCSV_Task2.fetchAll_CSV_NUM(FILENAME), 1988));
        System.out.println("---(Ausgabe) Aufgabe 2.3---");
        PrintCSV_Task2.printAll_CSV_ASSOC(selectCsvDataByValue_CSV_ASSOC(ReadCSV_Task2.fetchAll_CSV_ASSOC(FILENAME), "Stadt", "Paderklar"));
        System.out.println("---(Ausgabe) Aufgabe 2.4---");
        System.out.printf("Das Durchschnittsgeburtsjahr der Personen in %s ist %f", "Paderklar", avgCsvDataByTown_CSV_ASSOC(ReadCSV_Task2.fetchAll_CSV_ASSOC(FILENAME), "Paderklar"));
        System.out.println();

        System.out.println("Compiled, no syntactical failures =)");
    }

    /**
     * Aufgabe 3.1
     * @param csvData die Daten der CSV
     * @return die Anzahl der Personen
     */
    public static int countCsvData_CSV_NUM(List<String[]> csvData) {
        
    }

    /**
     * Aufgabe 2.2
     * @param csvData die Daten der CSV
     * @param selectedYear das ausgewählte Jahr, nach der die Daten gefiltert werden sollen
     * @return die Selektion
     */
    public static List<String[]> selectCsvDataByYear_CSV_NUM(List<String[]> csvData, int selectedYear) {

    }

    /**
     * Aufgabe 2.3
     * @param csvData die Daten der CSV
     * @param selectedKey das Attribut, nach dem selektiert werden sollst
     * @param selectedValue der bestimmte Attributwert
     * @return die Selektion
     */
    public static List<Map<String, String>> selectCsvDataByValue_CSV_ASSOC(List<Map<String, String>> csvData, String selectedKey, String selectedValue) {

    }

    /**
     * Aufgabe 2.4
     * @param csvData die Daten der CSV
     * @param selectedTown die ausgewählte Stadt
     * @return Durchschnittsswert der Geburtenjahre
     */
    public static double avgCsvDataByTown_CSV_ASSOC(List<Map<String, String>> csvData, String selectedTown) {

    }
}

How can i count the total number of persons in the data.csv file?

The data.csv file is an excel file and looks like this:

PersonID Name        Geburtsjahr  Stadt
0        Hans Müller 1988         Köln

(It contains 1000 Entries)

The function for counting is

public static int countCsvData_CSV_NUM(List<String[]> csvData)

Answer 1

The data.csv file is an excel file and looks like this:

No it isn't. An excel file ends in .xls or .xlsx and doesn't look like that, it looks like meaningless gobbledygook (it is a binary format).

Its a csv file: "Character Separated Values". And the character separating them is, hopefully, a tab character. THere is a different style of data format called a fixed-length-record file, where each record consists of an exact and equal number of spaces. So, if the bytes in that file involve a 0 , and then a bunch of spaces, and then Hans Müller , then it's not a csv file at all, and you should rename it. If that file contains a 0, then a tab character, and then Hans Müller , great, it's a CSV file (and the character used to separate, is a tab).

Given that there are 9 spaces between 1988 and Köln, you either have an extremely exotic tab stop configured, or it's a fixed-length-records file and not a CSV (nor an excel file), or you've taken some liberties when pasting it.

You need to figure out what the file format actually is before you can start writing code to read it.

NB: The rest of this answer assumes it is, after all, tab-separated values. If it turns out to be fixed-length-records, stop reading here.

A problem with CSV files is that there is no specification. Some CSV files allow records that have newlines in them (or tabs in them) by employing, for example, quotes, and some quote escape mechanism. Others do not.

Your paste doesn't include any 'interesting' data (where the data itself contains either tabs or newlines). Thus, it is impossible to tell.

Let's assume (I'll take 'Aufgabe' as a hint here) that it's homework and the course material has wished away the complications of CSV and you are just to assume the input data will not ever include tabs or newlines.

Then, your question boils down to an extremely simple: How many lines are in this file?

However, the input to countCsvData_CSV_NUM (which, by the way, is not standard java, 'we' don't name methods that way, if you are in a position to reconsider the course material or venue of education you may want to consider it, this is.. not a good start to a first-steps java course) is a 'list of string arrays' which is rather bizarre. Presumably the idea is that it is a list of each individual 'string', so then it's just.. return list.size(); . I suggest you learn how to write java, and learn how to debug. For example, write some code that just prints out, given some sample input, what that List<String[]> csvData actually contains. You can't really write code properly until you understand what you're supposed to do and what your inputs actually are, after all.