What does this code mean?

Question

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

Answer 1

That code appears to be cut from this sample code found in the LocalThread javadoc: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();
}The lines you're asking about declare a static ThreadLocal by the name of "serialNum". They assign to it an inline definition (extension) of LocalThread which specifies a custom initialValue() method. That initialValue() method is designed to get its value from the nextSerialNum variable (static, and incremented each time).
The neat thing that happens with the ThreadLocal instance is in the get() and set() methods. Here you see the get() method being called:return ((Integer) (serialNum.get())).intValue();That get method returns a different object for each thread which calls it. When a new thread comes into the get() method, the initialValue() method is called, creating for that thread a new serial number. When THAT thread calls get() again later, it is given that same serial number.
This one (static) instance of ThreadLocal can be accessed by any number of threads, but each thread will get()/set() a different variable, specific to that thread. This serial number implementation sets up a very simple counter for the Integer object the various threads will receive. When calling get(), the first thread will always receive an (int)0, the second thread will always receive an (int)1, the third an (int)2, and so on.