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Hashtable Immutability

import java.util.Collections; import java.util.Hashtable; import java.util.Map; public class MainClass {   public static void main(String[] s) {     Hashtable table = new Hashtable();     table.put("key1", "value1");     table.put("key2", "value2");     table.put("key3", "value3");     Map m = Collections.unmodifiableMap(table);     m.put("key3", "value3");     System.out.println(m);   } } Exception in thread "main" java.lang.UnsupportedOperationException at java.util.Collections$UnmodifiableMap.put(Collections.java:1286) at MainClass.main(MainClass.java:14)