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();
 }

}

Output is: [{-1=[Test3, Test2, Test1, Test], 10=[Test3, Test2, Test1, Test], 11=[Test3, Test2, Test1, Test]}]

I'm trying to get it to be designated to each ID, such that if someone enters ID "10 Test" "10 Test2" "100 Test3" only 10 will be 10=[Test2, Test] and 100=[Test3]

Create a new ArrayList for each student inside the loop. It seems you are reusing and appending to the same arraylist. — yogidilip
– yogidilip, Commented May 4, 2017 at 21:29

Vasu · Accepted Answer · 2017-05-04 21:33:32Z

2

You need to get the existing ArrayList with the ID from the HashMap and then add the new element to it as shown below (follow the comments):

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

answered May 4, 2017 at 21:33

Vasu

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