Is a static counter thread safe in multithreaded application?

Question

public class counting
{
  private static int counter = 0;

  public void boolean counterCheck(){
  counter++;
  if(counter==10)
  counter=0;
  }
}

Answer 1

It's clearly not thread-safe. Consider two threads that run in perfect parallel. If the counter is 9, they'll each increment the counter, resulting in the counter being 11. Neither of them will then see that counter equal to 10, so the counter will keep incrementing from then on rather than wrapping as intended.