如果想要弹出一个AlertDialog，要写如下的代码
AlertDialog.Builder alert =new AlertDialog.Builder(this);
alert.setTitle("Warning");
alert.setMessage("Wrong time!");
alert.show();
这里构造方法的原型是AlertDialog.Builder(Context arg) 需要一个Context类的对象作为参数，一般我们都在Activity里写，所以用this，表示在当前的会话中弹出AlertDialog。
在我的一个程序里，我自定义了一个接口Public interface CustomPickerListener，在实现这个接口的方法时我需要弹出来一个AlertDialog，这里，参数表里填写this的话会提示错误：The constructor AlertDialog.Builder(new CustomPickerListener(){}) is undefined.
其实这个地方写this很明显是错误的，但是要写什么才能达成目的呢？
官方文档上对context的解释是Interface to global information about an application environment. This is an abstract class whose implementation is provided by the Android system. It allows access to application-specific resources and classes, as well as up-calls for application-level operations such as launching activities, broadcasting and receiving intents, etc.
于是我就试着写上了我程序的 包.目标类.this ，如下
AlertDialog.Builder alert =new AlertDialog.Builder(com.android.alcoholtest.AlcoholTest.this);
然后就成功了