Java Collection - 如何在HashMap中使用空值作为键

import java.util.HashMap;
import java.util.Map;

public class Main {
  public static final void main(String[] args) {
    Map<String,String> map = new HashMap<>();
    map.put("one", "a");
    System.out.println("Size = " + map.size());
    map.put(null, "b");
    System.out.println("Size = " + map.size());
    System.out.println("map.get(null) = " + map.get(null));
  }
}