Android treats all implicit intents passed to能接受隐式Intent的Activity必须包含android.intent.category.DEFAULT,但LAUNCHER和MAIN例外，可以不用加，也可以加上但没有这个必要。startActivity()as if they contained at least one category: "android.intent.category.DEFAULT" (theCATEGORY_DEFAULTconstant). Therefore, activities that are willing to receive implicit intents must include "android.intent.category.DEFAULT" in their intent filters. (Filters with "android.intent.action.MAIN" and "android.intent.category.LAUNCHER" settings are the exception. They mark activities that begin new tasks and that are represented on the launcher screen. They can include "android.intent.category.DEFAULT" in the list of categories, but don‘t need to.) See Using intent matching, later, for more on these filters.)
隐式Intent找不到Activity:android.content.ActivityNotFoundException
原文：http://my.oschina.net/javaTechLover/blog/341690