Android完全退出应用程序的方法

package com.hrtx.dd.activity;
import java.util.LinkedList;
import java.util.List;
import android.app.Activity;
import android.app.Application;
public class MyApplication extends Application {
  private List activitys = null;
  private static MyApplication instance;
  private MyApplication() {
    activitys = new LinkedList();
  }
  /**
   * 单例模式中获取唯一的MyApplication实例
   * 
   * @return
   */
  public static MyApplication getInstance() {
    if (null == instance) {
      instance = new MyApplication();
    }
    return instance;
  }
  // 添加Activity到容器中
  public void addActivity(Activity activity) {
    if (activitys != null && activitys.size() > 0) {
      if(!activitys.contains(activity)){
        activitys.add(activity);
      }
    }else{
      activitys.add(activity);
    }
  }
  // 遍历所有Activity并finish
  public void exit() {
    if (activitys != null && activitys.size() > 0) {
      for (Activity activity : activitys) {
        activity.finish();
      }
    }
    System.exit(0);
  }
}