Java 如何将HashMap写入CSV？

我有一个方法

countOcc（）

，它打印一个列表（如下）

方法：

public void countOcc(ArrayList<Integer> list) {

    String aout = new String();
    System.out.println("\n");

    Integer[] numbers = list.toArray(new Integer[list.size()]);

    Map<Integer, Integer> map = new HashMap<Integer, Integer>();
    for (int i = 0; i < numbers.length; i++) {
        int key = numbers[i];
        if (map.containsKey(key)) {
            int occurrence = map.get(key);
            occurrence++;
            map.put(key, occurrence);
        } else {
            map.put(key, 1);
        }
    }

    Iterator iterator = map.keySet().iterator();
    while (iterator.hasNext()) {
        int key = (Integer) iterator.next();
        int occurrence = map.get(key);

         System.out.println(key+":00"+ " ==> " + occurrence + " hits(s)");


    }        

}

我知道opencsv，但我真的不知道如何在HashMap中使用它。

，写入CSV文件就像写入文本文件一样

public static void write()
{
     StringBuilder out = new StringBuilder();
     for (int key : map.keySet())
     {
         out.append(key + ":00,");
     }

     // remove last ',' from line
     out = new StringBuilder(out.substring(0, out.length() - 1));

     for (int occurence : map.values())
     {
         out.append(occurence + ",");
     }

     // remove last ',' from line
     out = new StringBuilder(out.substring(0, out.length() - 1));

     try (FileWriter fw = new FileWriter("my.csv"))
     {
         writer.append(out.toString());
     }
     catch (IOException e)
     {
         e.printStackTrace();
     }
}

抄袭：

Map values=newhashmap（）；
用于（字符串行：行）{
String[]parts=line.split（“”）；
字符串时间=部分[0]；
字符串值=零件[2]；
价值。投入（时间、价值）；
}
StringWriter编写器=新建StringWriter（）；
CSVWriter writer=新的CSVWriter（writer）；
writerText（values.keySet（）.toArray（））；
writerText（values.values（）.toArray（））；
writer.close（）；
System.out.println（writer.toString（））；

---

这将提供您的输出。

以下是写入CSV文件的示例，您可以相应地对其进行自定义：

import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;


    public class CsvFileWriter {


    //Delimiter used in CSV file
    private static final String COMMA_DELIMITER = ",";
    private static final String NEW_LINE_SEPARATOR = "\n";

    //CSV file header
    private static final String FILE_HEADER = "id,firstName,lastName,gender,age";

    public static void writeCsvFile(String fileName) {

        //Create new students objects
        Student student1 = new Student(1, "Ahmed", "Mohamed", "M", 25);
        Student student2 = new Student(2, "Sara", "Said", "F", 23);
        Student student3 = new Student(3, "Ali", "Hassan", "M", 24);
        Student student4 = new Student(4, "Sama", "Karim", "F", 20);
        Student student5 = new Student(5, "Khaled", "Mohamed", "M", 22);
        Student student6 = new Student(6, "Ghada", "Sarhan", "F", 21);

        //Create a new list of student objects
        List students = new ArrayList();
        students.add(student1);
        students.add(student2);
        students.add(student3);
        students.add(student4);
        students.add(student5);
        students.add(student6);

        FileWriter fileWriter = null;

        try {
            fileWriter = new FileWriter(fileName);

            //Write the CSV file header
            fileWriter.append(FILE_HEADER.toString());

            //Add a new line separator after the header
            fileWriter.append(NEW_LINE_SEPARATOR);

            //Write a new student object list to the CSV file
            for (Student student : students) {
                fileWriter.append(String.valueOf(student.getId()));
                fileWriter.append(COMMA_DELIMITER);
                fileWriter.append(student.getFirstName());
                fileWriter.append(COMMA_DELIMITER);
                fileWriter.append(student.getLastName());
                fileWriter.append(COMMA_DELIMITER);
                fileWriter.append(student.getGender());
                fileWriter.append(COMMA_DELIMITER);
                fileWriter.append(String.valueOf(student.getAge()));
                fileWriter.append(NEW_LINE_SEPARATOR);
            }



            System.out.println("CSV file was created successfully !!!");

        } catch (Exception e) {
            System.out.println("Error in CsvFileWriter !!!");
            e.printStackTrace();
        } finally {

            try {
                fileWriter.flush();
                fileWriter.close();
            } catch (IOException e) {
                System.out.println("Error while flushing/closing fileWriter !!!");
                e.printStackTrace();
            }

        }
    }
    }

---

你调查过吗？该框架具有内置的CsvMapWriter：

基本上，您可以将头定义为一个数组（可以使用映射的排序键集），然后使用以下命令编写映射：

mapWriter.write(map, header, processors);

您可能需要：

  public static void countOcc(List<Integer> list) {
        Map<Integer, Integer> map = new HashMap<>();
        for (Integer key : list) {
            if (map.containsKey(key)) {
                map.put(key, map.get(key) + 1);
            } else {
                map.put(key, 1);
            }
        }

        String csv = "d://data.csv";
        try (CSVWriter writer = new CSVWriter(new FileWriter(csv))) {
            String[] keysArray = new String[map.keySet().size()];
            String[] valuesArray = new String[map.values().size()];
            int counter = 0;
            for (Entry<Integer, Integer> entry : map.entrySet()) {
                keysArray[counter] = entry.getKey() + ":00";
                valuesArray[counter] = entry.getValue() + "";
                counter++;
            }
            writer.writeNext(keysArray);
            writer.writeNext(valuesArray);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

publicstaticvoidcountocc（列表）{
Map Map=newhashmap（）；
for（整型键：列表）{
if（地图容器（图例））{
map.put（键，map.get（键）+1）；
}否则{
地图放置（图例1）；
}
}
String csv=“d://data.csv”；
try（CSVWriter writer=new-CSVWriter（new-FileWriter（csv）））{
String[]keysArray=新字符串[map.keySet（）.size（）]；
String[]valuesArray=新字符串[map.values（）.size（）]；
int计数器=0；
for（条目：map.entrySet（））{
KeyArray[计数器]=entry.getKey（）+“：00”；
valuesArray[counter]=entry.getValue（）+“”；
计数器++；
}
writerText（keysArray）；
writer.writeNext（valuesArray）；
}捕获（IOE异常）{
e、 printStackTrace（）；
}
}

keySet（）和values（）方法调用的顺序不能保证相同。因此，您可能会得到与内容行不匹配的标题-而且您的标题也不会被排序。它给了我OP显示的顺序，并且始终如此。如果可以重用现有的、经过良好测试的框架，shrugI不明白为什么要自己编写这篇文章。另外：您的代码不应该使用硬编码的行分隔符-您应该使用系统的值-否则您的代码只能在Windows中工作，而不能在例如Linux（或其他方式）中工作。请参阅Stackflow thread中的解决方案

import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;


    public class CsvFileWriter {


    //Delimiter used in CSV file
    private static final String COMMA_DELIMITER = ",";
    private static final String NEW_LINE_SEPARATOR = "\n";

    //CSV file header
    private static final String FILE_HEADER = "id,firstName,lastName,gender,age";

    public static void writeCsvFile(String fileName) {

        //Create new students objects
        Student student1 = new Student(1, "Ahmed", "Mohamed", "M", 25);
        Student student2 = new Student(2, "Sara", "Said", "F", 23);
        Student student3 = new Student(3, "Ali", "Hassan", "M", 24);
        Student student4 = new Student(4, "Sama", "Karim", "F", 20);
        Student student5 = new Student(5, "Khaled", "Mohamed", "M", 22);
        Student student6 = new Student(6, "Ghada", "Sarhan", "F", 21);

        //Create a new list of student objects
        List students = new ArrayList();
        students.add(student1);
        students.add(student2);
        students.add(student3);
        students.add(student4);
        students.add(student5);
        students.add(student6);

        FileWriter fileWriter = null;

        try {
            fileWriter = new FileWriter(fileName);

            //Write the CSV file header
            fileWriter.append(FILE_HEADER.toString());

            //Add a new line separator after the header
            fileWriter.append(NEW_LINE_SEPARATOR);

            //Write a new student object list to the CSV file
            for (Student student : students) {
                fileWriter.append(String.valueOf(student.getId()));
                fileWriter.append(COMMA_DELIMITER);
                fileWriter.append(student.getFirstName());
                fileWriter.append(COMMA_DELIMITER);
                fileWriter.append(student.getLastName());
                fileWriter.append(COMMA_DELIMITER);
                fileWriter.append(student.getGender());
                fileWriter.append(COMMA_DELIMITER);
                fileWriter.append(String.valueOf(student.getAge()));
                fileWriter.append(NEW_LINE_SEPARATOR);
            }



            System.out.println("CSV file was created successfully !!!");

        } catch (Exception e) {
            System.out.println("Error in CsvFileWriter !!!");
            e.printStackTrace();
        } finally {

            try {
                fileWriter.flush();
                fileWriter.close();
            } catch (IOException e) {
                System.out.println("Error while flushing/closing fileWriter !!!");
                e.printStackTrace();
            }

        }
    }
    }

mapWriter.write(map, header, processors);

  public static void countOcc(List<Integer> list) {
        Map<Integer, Integer> map = new HashMap<>();
        for (Integer key : list) {
            if (map.containsKey(key)) {
                map.put(key, map.get(key) + 1);
            } else {
                map.put(key, 1);
            }
        }

        String csv = "d://data.csv";
        try (CSVWriter writer = new CSVWriter(new FileWriter(csv))) {
            String[] keysArray = new String[map.keySet().size()];
            String[] valuesArray = new String[map.values().size()];
            int counter = 0;
            for (Entry<Integer, Integer> entry : map.entrySet()) {
                keysArray[counter] = entry.getKey() + ":00";
                valuesArray[counter] = entry.getValue() + "";
                counter++;
            }
            writer.writeNext(keysArray);
            writer.writeNext(valuesArray);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }