java.util.Collections.unmodifiableMap() 方法

package com.codingdict;

import java.util.*;

public class CollectionsDemo {
   public static void main(String[] s) {

      //object hash table
      Hashtable<String,String> table = new Hashtable<String,String>();

      // populate the table
      table.put("key1", "value1");
      table.put("key2", "value2");
      table.put("key3", "value3");

      System.out.println("Initial collection: "+table);

      // create unmodifiable map
      Map m = Collections.unmodifiableMap(table);

      // try to modify the collection
      m.put("key3", "value3");
   }
}