package cn.netkiller;

interface Callback {
    void call();
}

abstract class Task {
    public final void executeWith(Callback callback) {
        this.execute();
        if (callback != null)
            callback.call();
    }

    protected abstract void execute();
}

class SimpleTask extends Task {
    protected void execute() {
        System.out.println("Do some tasks before the callback method.");
    }
}

public class Main {
    public static void main(String[] args) {

        System.out.println("Hello world!");

        Task task = new SimpleTask();
        Callback callback = new Callback() {
            public void call() {
                System.out.println("The callback method has been called!");
            }
        };
        task.executeWith(callback);
    }
}