What is the difference between HashMap and Hashtable in Java?

Hi everyone! I've just started learning the Java Collections Framework and got confused about two similar classes — HashMap and Hashtable. They both implement the Map interface and seem to do the same thing: store key-value pairs. But I've heard there are important differences between them, and Hashtable is considered obsolete. I wrote a small test code but don't see the difference: ```java import java.util.*; public class TestMap { public static void main(String[] args) { // HashMap Map hashMap = new HashMap<>(); hashMap.put("key1", "value1"); hashMap.put(null, "nullValue"); // works System.out.println("HashMap: " + hashMap); // Hashtable Map hashtable = new Hashtable<>(); hashtable.put("key1", "value