Java 教程 .isEmpty() 方法

Question

我有 3 个类(Printer、PaperTray 和 Machine)和我的主 Java 类(HelloWorld)。

我期待看到“装入更多纸张!”作为调用“myPrinter.print(2)”时的输出，但输出显示“我的打印机已打开!”两次，就好像托盘中有纸一样。如果有足够的纸张被传递到 loadPaper 中，我应该只会看到显示打印机两次的输出(暂时已被注释掉，直到我使加载纸张正常工作):

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);
}
}

来 self 的打印机类:

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);
}

}

当调用“.isEmpty()”时，pages的值为-2，但我相信“.isEmpty()”仅在该值等于0时才有效？所以这就是我猜测我的问题所在。然而我的代码(99％确定)与导师的示例相同。

PaperTray 类是:

public class PaperTray {

int pages = 0;

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

public void usePage()
{
    pages--;
}

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

机器类别:

public class Machine {

protected boolean isOn;

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

public void TurnOn()
{
    isOn = true;
}

public void TurnOff()
{
    isOn = false;
}

}

感谢任何帮助。

Answer 1

您的isEmpty() PaperTray中的方法是错误的，现在返回 true如果pages > 0 (当它已满时)，并返回 false如果pages <= 0 (当它为空时)。

您需要将条件切换为pages == 0或者绝对确定pages <= 0 .