Don't do this: Share names

The following program is valid Java, even though the Runnable on line 1 is a completely different symbol than Runnable on line 3:

public class Refrigerator implements Runnable {
  public void run() {
    new Runnable().freeze();
  }

  public class Runnable {
    void freeze() {
      System.out.println("cold and refreshing");
    }
  }

  public static void main(String[] args) {
    new Thread(new Refrigerator()).start();
  }
}

I'm not quite sure if this is shadowing, obscuring or hiding, but it's certainly not good. Writing a compiler must be pretty difficult!