Make Hashtable immutable and read-only in Java

For security reasons, it is sometimes necessary to change access rights to avoid a faulty modification. That's why Java has put a method that makes our Hashtable inaccessible for writing or in other words, a Hashtable that cannot be modified in read-only.

To change the access rights, you just have to call the method of the Collections class java.util.Collections.unmodifiableMap(Map map).

public static < K,V> Map< K,V> unmodifiableMap(Map m): This method returns an immutable Hashtable view that cannot be edited.

The following example shows the use of the java.util.Collections.unmodifiableMap().

import java.util.Collections; 
import java.util.Hashtable; 
import java.util.Map; 

public class Unmodifiable_Hashtable {
 public static void main(String[] s) {
 Hashtable hashtable = new Hashtable(); 
 hashtable.put("1", "val1"); 
 hashtable.put("2", "val2"); 
 hashtable.put("3", "val3"); 

 Map m = Collections.unmodifiableMap(hashtable); 

 m.put("4", "val4"); 

 System.out.println(m); 
 }
}

After compiling and executing the above code, this will produce the following result:

Exception in thread "main" java.lang.UnsupportedOperationException
 at java.util.Collections$UnmodifiableMap.put(Unknown Source)
 at HashTable.Unmodifiable_Hashtable.main(Unmodifiable_Hashtable.java:16)

References:
java.util.Collections.unmodifiableMap() Method