New Java programmer, basic java composition

Question

I am a new computer programming student. I watched a video about Java, basic composition, and the guy in the video made an example about this topic like this:

public class PaperTray
{
  int pages = 0;
  ....
  public boolean isEmpty()
  {
    return pages > 0;
  }
}

---

public class Printer extends Machine
{
  private PaperTray paperTray = new PaperTray();
  ....
  public void print(int copies)
  {
  ....
  while(copies > 0 && !paperTray.isEmpty() )
  {
    System.out.println("some text to print");
    copies--;
  }
  if(paperTray.isEmpty())
  {
    System.out.println("load paper");
  }
}

My question is if the paper tray is empty, then in class PaperTray the method isEmpty() will return false. Therefore, the if statement in the class Printer will not be executed. And if the paper tray is not empty, the method isEmpty() in class PaperTray will return true, so the while statement in the class Printer will not be executed. Am I wrong, or the instructor in the video clip made some mistakes?

Thank you

Answer 1

The logic of the isEmpty does not make sense: I wold expect either

public boolean isEmpty() {
    return pages == 0;
}

or

public boolean isNotEmpty() {
    return pages > 0;
}

Answer 2

if the paper tray is empty, then in class PaperTray the method isEmpty() will return false

It should return true (for any sensible implementation, that is :-). For a method called isEmpty(), common sense dictates that it returns true when the enclosing object / collection is empty, and false when it is not empty.

In other words, the implementation you show above has a bug.