Hashtable Immutability: to make the table read-only using Collections.unmodifiableMap(Map map) : Hashtable Basics « Collections « Java Tutorial

1. Java Tutorial
2. Collections
3. Hashtable Basics

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)