Java Hashmap ArrayList

Question

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import javax.swing.JOptionPane;

public class HashMapDemo {
 public static double runProcess;
 public static int ID = 0;
 public static void processHashMap() {
  HashMap < Integer, ArrayList < String >> students = new HashMap < > ();
  List < String > arr = new ArrayList <  > (100);
  int x = 0;
  while (ID != -1) {
   String uData = JOptionPane.showInputDialog("Please enter your Student ID and Course Number (Seperated by a space) or enter -1 to view list: ");
   String[] splitter = uData.split(" ");
   ID = Integer.parseInt(splitter[0]);
   arr.add(0, splitter[1]);
   students.put(ID, (ArrayList < String > ) arr);
   x++;
  }
  System.out.println(Arrays.asList(students));
 }
 public static void main(String[] args) {
  processHashMap();
 }
}

输出为：[{-1 = [test3，test2，test1，test]，10 = [test3，test2，test1，test]，11 = [test3，test2，test1，test1，test]}]

我试图将其指定为每个ID，因此，如果某人输入ID" 10 test" 10 test2" 100 test3"，只有10个= [test2，test]和100 = [test3]

Answer 1

您需要从HashMap获得现有的ArrayList，然后使用add，然后CC_4如下所示(遵循注释(：

String[] splitter = uData.split(" ");
ID = Integer.parseInt(splitter[0]);
ArrayList<String> studentsList = students.get(ID);//get the existing list from Map
if(studentsList == null) {//if no list for the ID, then create one
    studentsList= new ArrayList<>();
}
studentsList.add(0, splitter[1]);//add to list
students.put(ID, studentsList);//put the list inside map