从单独的类打印私有字段

Question

我的任务是将两个类联系在一起。第一类是售票机，它允许人们购买一张票，然后将其打印出来（通过 System.out.println）。第二类是显示时间的时钟显示。

我的任务是让票类打印时钟显示类中当前显示的时间。有人告诉我，我不需要编辑 NumberDisplay 类或 ClockDisplay 类。

我最初的想法是在我的 ticketmachine 类中创建一个新的 ClockDisplay 字段，然后使用

System.out.println("Time:" + ClockDisplay.displayString);

因为 displayString 是我用来在时钟显示类中查找值的。但是，由于该字段是私有的并且我不能编辑时钟显示类，所以我不能这样做。有什么想法吗？

谢谢。到目前为止，这是我的代码，前面提到的 TicketMachine 类中的代码。

数字显示

public class NumberDisplay
{
private int limit;
private int value;

/**
 * Constructor for objects of class NumberDisplay.
 * Set the limit at which the display rolls over.
 */
public NumberDisplay(int rollOverLimit)
{
    limit = rollOverLimit;
    value = 0;
}

/**
 * Return the current value.
 */
public int getValue()
{
    return value;
}

/**
 * Return the display value (that is, the current value as a two-digit
 * String. If the value is less than ten, it will be padded with a leading
 * zero).
 */
public String getDisplayValue()
{
    if(value < 10) {
        return "0" + value;
    }
    else {
        return "" + value;
    }
}

/**
 * Set the value of the display to the new specified value. If the new
 * value is less than zero or over the limit, do nothing.
 */
public void setValue(int replacementValue)
{
    if((replacementValue >= 0) && (replacementValue < limit)) {
        value = replacementValue;
    }
}

/**
 * Increment the display value by one, rolling over to zero if the
 * limit is reached.
 */
public void increment()
{
    if ((value +1) >= limit) {
        value = 0;
    }
    else {
        value = value + 1;
    }
}
}

时钟显示

public class ClockDisplay
{
private NumberDisplay hours;
private NumberDisplay minutes;
private NumberDisplay seconds;
private String displayString;    // simulates the actual display

/**
 * Constructor for ClockDisplay objects. This constructor 
 * creates a new clock set at 12:00:00.
 */
public ClockDisplay()
{
    hours = new NumberDisplay(12); // changed from 24hour to 12 hour
    minutes = new NumberDisplay(60);
    seconds = new NumberDisplay(60);
    updateDisplay();
}

/**
 * Constructor for ClockDisplay objects. This constructor
 * creates a new clock set at the time specified by the 
 * parameters.
 */
public ClockDisplay(int hour, int minute, int second)
{
    hours = new NumberDisplay(12); //changed from 24hour to 12 hour
    minutes = new NumberDisplay(60);
    seconds = new NumberDisplay(60);
    setTime(hour, minute, second);
}

/**
 * This method should get called once every minute - it makes
 * the clock display go one minute forward.
 */
public void timeTick()
{
    minutes.increment();
    if(minutes.getValue() == 0) {  // it just rolled over!
        hours.increment();
    }
    updateDisplay();
}

/**
 * Set the time of the display to the specified hour and
 * minute and second.
 */
public void setTime(int hour, int minute, int second)
{
    if (hour == 12) { //changing the display from '00:00' to '12:00'
        hour = 0;
    }
    hours.setValue(hour);
    minutes.setValue(minute);
    seconds.setValue(second);
    updateDisplay();
}

/**
 * Return the current time of this display in the format HH:MM:SS.
 */
public String getTime()
{
    return displayString;
}

/**
 * Update the internal string that represents the display.
 */
private void updateDisplay()
{
    int hour = hours.getValue(); //changes the display to from showing '00:00' to '12:00'
    if (hour == 0) {
        hour = 12;
    }
    displayString = hour + ":" + 
                    minutes.getDisplayValue() + ":" + seconds.getDisplayValue();
}

}

售票机

public class TicketMachine
{
// The price of a ticket from this machine.
private int price;
// The amount of money entered by a customer so far.
private int balance;
// The total amount of money collected by this machine.
private int total;
// The time from the clockdisplay class
private ClockDisplay time;

/**
 * Create a machine that issues tickets of the given price.
 */
public TicketMachine(int cost)
{
    price = cost;
    balance = 0;
    total = 0;
}

/**
 * @Return The price of a ticket.
 */
public int getPrice()
{
    return price;
}

/**
 * Return The amount of money already inserted for the
 * next ticket.
 */
public int getBalance()
{
    return balance;
}

/**
 * Receive an amount of money from a customer.
 * Check that the amount is sensible.
 */
public void insertMoney(int amount)
{
    if(amount > 0) {
        balance = balance + amount;
    }
    else {
        System.out.println("Use a positive amount rather than: " +
                           amount);
    }
}

/**
 * Print a ticket if enough money has been inserted, and
 * reduce the current balance by the ticket price. Print
 * an error message if more money is required.
 */
public void printTicket()
{
    if(balance >= price) {
        // Simulate the printing of a ticket.
        System.out.println("##################");
        System.out.println("# The BlueJ Line");
        System.out.println("# Ticket");
        System.out.println("# " + price + " cents.");
        System.out.println("##################");
        System.out.println();

        // Update the total collected with the price.
        total = total + price;
        // Reduce the balance by the prince.
        balance = balance - price;
        // Print the current time from the NumberDisplay class.
        System.out.println("Time:" + ClockDisplay.displayString);
    }
    else {
        System.out.println("You must insert at least: " +
                           (price - balance) + " more cents.");

    }
}

/**
 * Return the money in the balance.
 * The balance is cleared.
 */
public int refundBalance()
{
    int amountToRefund;
    amountToRefund = balance;
    balance = 0;
    return amountToRefund;
}

}

Answer 1

使用提供的方法getTime() 来获取它。喜欢，

System.out.println("Time:" + ClockDisplay.getTime());

另外（按照惯例），Java 变量应该以小写字母开头。

ClockDisplay clockDisplay;
// Set-up the variable....
System.out.println("Time:" + clockDisplay.getTime());

Answer 2

如果您发现只有字段是私有的。要访问这些私有字段，您有公共的 getter-setter 方法。这是基本 OOP 概念之一 - 封装。

要访问您的 getter setter，您必须实例化您的对象，如您所见，这些不是静态方法。

您可以在要打印时间的类中创建 ClockDisplay 对象。

ClockDisplay clockDisplay = new ClockDisplay();
System.out.print(clockDisplay.getTime());

Answer 3

在您的 TicketMachine 类中，您忘记实例化 ClockDisplay。在TicketMachine 的printTicket() 方法中试试这个：

time = new ClockDisplay();        
System.out.println("Time:" + time.getTime());

更新

以下是设置时间的方法：

time = new ClockDisplay();
time.setTime(3,9,34);
System.out.println("Time: " + time.getTime());

这是输出：

##################
# The BlueJ Line
# Ticket
# -2 cents.
##################

Time: 3:09:34