Variable access

Now that we can start threads and create code that runs parallel, it is time to talk a little bit about how these threads can exchange data between each other. At first glimpse, it seems fairly simple. The threads use the same shared memory; therefore, they all can read and write all the variables that the Java access protection allows them. This is true, except that some threads may just decide not to read the memory. After all, if they have just recently read the value of some variable, why read it again from the memory to the registers if it was not modified? Who would have modified them? Let's see the following short example:

package packt.java9.by.example.thread; 
  
 public class VolatileDemonstration implements Runnable { 
     private Object o = null; 
     private static final Object NON_NULL = new Object(); 
     @Override 
     public void run() { 
         while( o == null ); 
         System.out.println("o is not null"); 
     } 
     public static void main(String...