为什么我不能做 hashmap.put()

Question

Whenever i try to run my error is this "hmap.put(id, b0);" am i doing it correctly? im trying to do a user input and insert it in a hashmap. it says: no suitable method found for put(Integer,Student) method Map.put(Integer,String) is not applicable (argument mismatch; Student cannot be converted to String) method AbstractMap.put(Integer,String) is not applicable (argument mismatch; Student cannot be converted to String) method HashMap.put(Integer,String) is not applicable

(argument mismatch; Student cannot be converted to String)

package javaapplication30;
import java.util.*;
import java.util.Scanner;
import java.util.Map;
import java.util.HashMap;

class Student {
  int id;
  String sn, cor;

  public Student(int id, String sn, String cor) {
    this.id = id;
    this.sn = sn;
    this.cor = cor;

  }
}

public class JavaApplication30 {
  public static void main(String[] args) {
    HashMap < Integer, String > hmap = new HashMap < Integer, String > ();
    Scanner sc = new Scanner(System.in);

    for (int i = 0; i < 2; i++) {
      System.out.print("id: ");
      Integer id = sc.nextInt();
      System.out.print("name: ");
      String sn = sc.next();
      System.out.print("course: ");
      String cor = sc.next();

      Student b0 = new Student(id, sn, cor);

      hmap.put(id, b0);

    }

    for (Map.Entry m: hmap.entrySet()) {
      System.out.println(m.getKey() + " " + m.getValue());
    }
  }
}

Answer 1

You have declared the map to be a HashMap<Integer, String> . In other words, the key type is Integer and the value type is String .

But then you do this:

  Student b0 = new Student(id, sn, cor);
  hmap.put(id, b0);

That is attempting to add a map entry where the value is a Student .

Student is not a subclass of String so that is not legal.

---

Here is what the error message says, and how to interpret it:

no suitable method found for put(Integer,Student) 

That corresponds to this call put(id, b0) . Observe that id is declared to be an Integer and b0 is a Student

method Map.put(Integer,String) is not applicable 

The compiler has found a put method in the Map interface whose signature is put(Integer,String) . It has the correct name and the correct number of parameters. But...

(argument mismatch; Student cannot be converted to String)

The compiler has tried to find a legal way to use that put method. The first argument is compatible, but there is no conversion that will convert a Student (which is what the argument is) to a String (which is what the method requires).

The solution is to change the declaration of hmap :

HashMap<Integer, Student> hmap = new HashMap<>();

(The <> tells the compiler to infer (ie work out) the generic type parameters from the context.)

Answer 2

You declared your Hasmap as: new HashMap<Integer, String>();

Then your HashMap expects Integer as key and String as value. Your object b0 is not a String, it's a Student object.

Then you should change your HashMap as new HashMap<Integer, Student>(); ( or you can call toString() function on b0, it depends on what do you want to do)