[hardwired]

The lines/snippet you posted compile(s) and run(s) okay.
You mentioned an error for the return value of the put method. If I modify your code to explore this

Code:

Hashtable<String, Integer> numbers= new Hashtable<String, Integer>();
numbers.put("one", 1);
int m = numbers.put("two", 2);        // runtime error
Integer n = numbers.put("three", 3);  // okay

the line with "int m = ..." (line 7) throws a NullPointerException:

Code:

C:\jexp>java Test
Exception in thread "main" java.lang.NullPointerException
        at Test.main(test.java:7)

j2se 1.5+ has autoboxing which can cause trouble. The put method takes Objects for both key and value. Your Hashtable declaration specifies String(key) and Integer(value) which, according to the javadoc for the class api, will then return an Integer. So if you want a return value from the put method you will need an Integer (vis–a–vis an int primitive data type) to receive it (as shown above).