Java：从 CSV 传递到 HashMap 并打印键和值

Question

I have a CSV file with 5 rows, and three pieces of data separated by comas on each row. Row Example: Drew, 37, 150.5 I am trying to read the data from the CSV, into a HashMap and then print out the HashMap Keys and Values. Right now I'm only getting empty brackets to print out. Code below, I'm taking an online course and having trouble reaching the professor, and have tried several online resources, appreciate anyone's advice so I can learn. TY!

import java.io.FileNotFoundException;  // Import this class to handle errors
import java.util.Scanner; // Import the Scanner class to read text files
import java.util.HashMap;
import java.util.Map;

public class Main {
  public static void main(String[] args) {
    Map<String, Person> myPeople = new HashMap();
    try {
      File myObj = new File("people.csv");
      Scanner myReader = new Scanner(myObj);
      while (myReader.hasNextLine()) {
        String data = myReader.nextLine();
        
        System.out.println(myPeople);
      }
      myReader.close();
    } catch (FileNotFoundException e) {
      System.out.println("An error occurred.");
      e.printStackTrace();
    }
  }
}```
My current output is below, without keys or values.
 java -classpath .:/run_dir/junit-4.12.jar:target/dependency/* Main
{}
{}
{}
{}
{}
 

Answer 1

You should be printing data rather than myPeople, as you have not added anything to myPeople yet.

You can use myPeople.put(key, value) to add elements to your map, see https://www.geeksforgeeks.org/hashmap-put-method-in-java/ .

Then you can use myPeople.get(key) to retrieve the value, see https://www.geeksforgeeks.org/java-util-hashmap-in-java-with-examples/ .

Answer 2

You haven't added anything to your map, that's why it is just printing empty map ({}) on each iteration. Here is an example of how to do this:

public class Main {

    static class Person {
        String name;
        int age;
        int height;

        public Person(String name, int age, int height) {
            this.name = name;
            this.age = age;
            this.height = height;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            Person person = (Person) o;
            return age == person.age && height == person.height && name.equals(person.name);
        }

        @Override
        public int hashCode() {
            return Objects.hash(name, age, height);
        }

        @Override
        public String toString() {
            return "Person{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    ", height=" + height +
                    '}';
        }
    }

    private static final String csv =
            "Drew, 37, 168\n" +
                    "Bob, 30, 170";

    public static void main(String[] args) {
        Map<String, Person> map = new HashMap<>();
        try (Scanner scanner = new Scanner(csv)) {
            while (scanner.hasNextLine()) {
                String line = scanner.nextLine();
                String[] lineParts = line.split("\\s*,\\s*");
                map.put(lineParts[0], new Person(lineParts[0], Integer.parseInt(lineParts[1]), Integer.parseInt(lineParts[2])));
            }
        }
        System.out.println(map);
    }
}

Output: {Bob=Person{name='Bob', age=30, height=170}, Jack=Person{name='Jack', age=37, height=168}}