如何使用回调机制？

我必须实施一项信用卡申请，其中我必须只处理一个信用卡帐户。类似的操作credit(), debit(), pinChange().

但对我来说问题是我必须使用“JAVA CALLBACK”机制在两种情况下通知用户：

1. 引脚更改时
2. 当余额低于 5000 时。

如何使用这些通知的回调？

这里更关注 CALLBACKS 的使用。

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);
    }
}

更改 Pin 码并降低余额是 CreditCard 的行为，因此 CreditCard 对象是事件生成者而不是事件侦听器，因此您不希望将 CreditCard 设为侦听器（回调）。

实际上，当 pinChange 或 lowBalance 等事件发生时，您的信用卡将回调您想要通知的侦听器的方法。

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);
    }
}

希望这会清楚...