What operations in Java are considered atomic?

• all assignments of primitive types except for long and double
• all assignments of references
• all assignments of volatile variables
• all operations of java.concurrent.Atomic* classes

and maybe something more. Look at the jls.

As noted in the comments, atomicity does not imply visibility. So while another thread is guaranteed not to see a partially written int, it may never see the new value.

The operations on long and double are on common 64 bit CPUs atomic as well, although there's no guarantee. See also this feature request.

In Java, the reading and writing of 32-bit or smaller quantities are guaranteed to be atomic.
By atomic, we mean each action takes place in one step and cannot be interrupted. Thus, when we have multithreaded applications, the read and write operations are thread-safe and need not be made synchronized.

For example, the following code is thread safe:

public class ThreadSafe   
  {  
    private int x;  
    public void setX(int x)  
          {
           this.x = x;
           } 
  }