问Java教程.isEmpty()方法

EN

import printing.Printer;

public class HelloWorld {

public static void main(String[] args) 
{
    Printer myPrinter = new Printer(true, "MY PRINTER");
    //myPrinter.loadPaper(3);
    myPrinter.print(2);
}
}

package printing;

public class Printer extends Machine
{
    private String modelNumber;
    private PaperTray paperTray = new PaperTray(); //printer now HAS paper tray

public Printer(boolean isOn, String modelNumber)
{
    super(isOn); //calls the constructor of the machine class - Super calls inherited parent
    this.modelNumber = modelNumber;
}

public void print(int copies) 
{
    String onStatus = "";
    if(isOn)
    {
        onStatus = " is On!";
    }
    else
    {
        onStatus = " is Off!";
    }

    String textToPrint = modelNumber + onStatus;

    while(copies > 0 && !paperTray.isEmpty())
    {
        System.out.println(textToPrint);
        copies--;
        paperTray.usePage();
    }

    if(paperTray.isEmpty())
    {
        System.out.println("Load more paper!");
    }   
}

//for each 
public void printColours()
{
    String[] colours = new String[] {"Red", "Blue", "Green", "Yellow" };

    //for current colour "in" colours
    for(String currentColour : colours)
    {
        if("Green".equals(currentColour))
        {
            continue;
        }
        System.out.println(currentColour);
    }
}

public void print(String text) 
{
    System.out.println(text);
}

public String getModelNumber()
{
    return modelNumber;
}

public void loadPaper(int count) 
{
    paperTray.addPaper(count);
}

public class PaperTray {

int pages = 0;

public void addPaper(int count) 
{
    pages += count;
}

public void usePage()
{
    pages--;
}

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

public class Machine {

protected boolean isOn;

public Machine(boolean isOn)
{
    this.isOn = isOn;
}

public void TurnOn()
{
    isOn = true;
}

public void TurnOff()
{
    isOn = false;
}