Re: how to make this code thread safer?

The simplest fix is to just add "synchronized" to the method
declarations. This will cause each method to acquire the instance's
monitor during their execution, ensuring that only one thread can be
executing either method at a time.

Another alternative would be to use the "synchronized" statement to the
code inside each method. Assuming the "isAbsent()" method isn't
affected by the "other code" or the "doA()", "doB()", or "doC()"
methods, that could look like this:

public class MyClass
{
   private Person tim = new Person("Tim");
   private Person tom = new Person("Tom");
   private final Object lock = new Object();

   public void doThis
   {
     this.checkSomething();

     synchronized (lock)
     {...
     }
     ...
   }

}
...
Note the use of a dedicated object reference for synchronization. In
general, it is poor practice to use the "this" reference for
synchronization, though that's what declaring a method as "synchronized"
does. It's not the end of the world to use "this", but doing so "leaks=

some of your implementation, and introduces the possibility of some
other code not related to your own taking the same monitor, increasing
contention as well as the complexity of the locking scenarios.

Those are the simplest techniques available in Java, and for the example
you've given are probably the most appropriate anyway. As you learn
more about concurrent programming, you'll probably want to look at the
other synchronization features in Java, found mainly in the
java.util.concurrent package.