A DESCRIPTION OF THE PROBLEM :
ThreadLocal is a generic class but the example doesn't use the generics feature or autoboxing. In the existing documentation no type is specified for the ThreadLocal variable and the get method must use a cast to get the Integer and intValue to convert to int.

 

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
public class SerialNum {
     // The next serial number to be assigned
     private static int nextSerialNum = 0;

     private static ThreadLocal<Integer> serialNum = new ThreadLocal<Integer>() {
         protected synchronized Integer initialValue() {
             return new Integer(nextSerialNum++);
         }
     };

     public static int get() {
         return serialNum.get();
     }
 }
ACTUAL -
public class SerialNum {
     // The next serial number to be assigned
     private static int nextSerialNum = 0;

     private static ThreadLocal serialNum = new ThreadLocal() {
         protected synchronized Object initialValue() {
             return new Integer(nextSerialNum++);
         }
     };

     public static int get() {
         return ((Integer) (serialNum.get())).intValue();
     }
 }

URL OF FAULTY DOCUMENTATION :
http://java.sun.com/j2se/1.5.0/docs/api/java/lang/ThreadLocal.html