如何使用回调机制？ [英] How to use callback mechanism?

问题描述

我必须实施信用卡申请，我必须只处理一个信用卡帐户。像 credit（）， debit（）， pinChange（）。

I have to implement a credit card application in which i have to handle only one Credit card account. Operations like credit(), debit(), pinChange().

但问题是我必须使用JAVA CALLBACK机制在两种情况下通知用户：

But the problem for me is I have to use "JAVA CALLBACK" mechanism to notify user in two cases:

1. 更换引脚


2. 当余额低于5000时。

如何对这些通知使用回调？

How do I use callbacks for these notifications?

这里更多地关注CALLBACKS的使用..

The use of CALLBACKS is more of focus here..

public interface Callback {

    public void onPinChange();
    public void onLowBalance();

    }
    import java.util.Scanner;

    public class CreditCard implements Callback{

    Callback callback;

    int pin;
    float balance;

    public CreditCard() {

        callback = this;
        this.pin = 1234; // default pin
        this.balance = 10000f; // opening balance

    }

    public void creditBalance(float amount) {
        this.balance = this.balance + amount;

    }

    public void debitBalance(float amount) {
        if (balance <= amount) {
            System.out.println("Not enough balance to debit");
        } else {
            balance = balance - amount;
        }
        if (balance < 5000) {
            callback.onLowBalance();
        }

    }

    public void changePin(int newPin) {
        System.out.println("Enter the current pin");
        Scanner scanner = new Scanner(System.in);
        int existingPin = scanner.nextInt();
        if (existingPin != pin) {
            System.out.println("Wrong pin!");
        } else {
            pin = newPin;
            callback.onPinChange();
        }
        scanner.close();
    }

    @Override
    public void onPinChange() {
        System.out.println("Pin changed");

    }

    @Override
    public void onLowBalance() {
        System.out.println("low balance");

    }

    public static void main(String[] args) {

        CreditCard card = new CreditCard();
        card.changePin(3333);
        card.debitBalance(5200);
    }
}

推荐答案

更改Pin和获得低的余额是CreditCard的行为，因此CreditCard对象是事件生产者而不是事件监听器，所以你不想让CreditCard成为一个监听器（CallBack）。

Changing Pin and getting the balance low is the behavior of CreditCard, so CreditCard object is event producer not event listener so you won't want to make CreditCard a listener (CallBack).

实际上，当发生任何类似pinChange或lowBalance的事件时，您的CreditCard将回拨您想要通知的侦听器的方法。

Actually your CreditCard will be calling back methods of the listener you want to be informed when any event like pinChange or lowBalance occurs.

您的代码应如下所示：

class CreditCard{
    int pin, balance;
    private Callback callback;
    CreditCard(Callback callback){
        this.callback=callback;
    }

    public void pinChange(int pin){
        this.pin=pin;
        //inform the listener as well
        callback.pinChanged();
    }

    public void withdraw(int amount){
        this.balance-=amount;
        //inform the the listener
        if(balance<1000)callback.lowBalance();
    }
}

class MyListener implements Callback{
    public void pinChanged(){
        //do what is needed when somebody changes pin..
       //i.e send sms to the customer 
        System.out.println("PIN changed..");
    }

    public void lowBalance(){
        //inform the customer about lowbalance.
        System.out.println("little money in card..");
    }

    main(String... args){
        CreditCard cc=new CreditCard(new MyListener());
        cc.changePin(3306);
    }
}

希望这个明白......

Hope this'll clear...

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