Java Opposite of Generics

[sharpdust]

I am trying to extend the java.util.Hashtable class. However, I would like it so that when somebody tries to use my CustomHashtable class, they can only construct it use Strings for the types for the key/value pair.

Here is my code:


import java.util.Hashtable;

public class CustomHashtable <K, V> extends Hashtable
{
public static void main(String [] args)
{

// Should be okay
Hashtable myStringTable = new CustomHashtable();
myStringTable.put("Entry 1", "Entry 2");
System.out.println(myStringTable);

// Not okay
Hashtable myIntTable = new CustomHashtable();
myIntTable.put(5, 6);
System.out.println(myIntTable);
}

}

So in the example, I want the first CustomHashtable construction to be okay, but for the second to not to be okay.
I realize that in general, classes should be generic as possible, but in this case I want to force the programmer to only construct Hashtables with key/value pairs of type String.
In other words, I don't want any casting to be done in the main method. I was hoping there was a way to cast <K, V> to be something like <(String) K, (String) V>, but java doesn't like that.

Sorry if this is a simple question, I haven't done java programming in a long time.

[Reiger]

You should not extend the raw type Hashtable but rather a parametrised type:


public class CustomHashtable extends Hashtable<String,String>


You may want to read up on generics here: http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf (is a PDF file).

[sharpdust]

Thank you Reiger, that's exactly what I wanted.

[Some Penguin]

As a side note, if you don't need concurrency control at the level of the map you're going to be better off with HashMap instead of HashTable, and if you *do* need concurrency control at the level of the map you're still likely to be better off with ConcurrentHashMap.

More related, if you wanted to do something more flexible, Java does accept significantly more complicated syntax such as



class Tree<V extends Serializable & Comparable<V>>