1.Package name and certificate

Sometimes a developer will make a change to an application that has surprising results when installed as an update to a previous version — shortcuts break, widgets disappear, or it can’t even be installed at all. There are certain parts of an application that are immutable once you publish it, and you can avoid surprises by understanding them.

The most obvious and visible of these is the “manifest package name,” the unique name you give to your application in its AndroidManifest.xml. The name uses a Java-language-style naming convention, with Internet domain ownership helping to avoid name collisions. 

knowledge extension - "Java-language-style naming convention"
>>>>The domain-name-backwards convention is there to prevent name collisions. Two different companies with the same product name will have different namespaces so everything works fine.If you don't have a domain then you need to choose a name that is meaningful to you and will not collide with anything else. That's OK; it just means very slightly more work for you to make sure that there isn't an existing product with the name you want, and there may be difficulties if there ever is a name collision.

1.The package name of your application

If the manifest package name has changed, the new application will be installed alongside the old application, so they both co-exist on the user’s device at the same time.

2.The certificate of your application

If the signing certificate changes, trying to install the new application on to the device will fail until the old version is uninstalled.

If you change the signing certificate of your application, you should always change its manifest package name as well to avoid failures when it’s installed. In other words, the application coming from a different author makes it a different application, and its package name should be changed appropriately to reflect that. (Of course it’s fine to use the same package name for the development builds of your app signed with your test keys, because these are not published.)

2.AndroidManifest.xml

More than just your package name that is immutable. A major function of the AndroidManifest.xml is essentially to declare a public API from your application for use by other applications and the Android system. Every component you declare in the manifest that is not private (that is whose android:exported state is true) should be treated as a public API and never changed in a way that breaks compatibility.

A subtle but important aspect of what constitutes a break in compatibility is the android:name attribute of your activity, service, and receiver components. This can be surprising because we think of android:name as pointing to the private code implementing our application, but it is also (in combination with the manifest package name) the official unique public name for that component, as represented by the ComponentName class.

1.If the name of a main activity of your application is changed, any shortcuts the user made to it will no longer work. A shortcut is an Intent that directly specifies the ComponentName it should run.

2.If the name of a service implementing a Live Wallpaper changes, then a user who has enabled your Live Wallpaper will have their wallpaper revert to the system default when getting the new version of your app. The same is true for Input Methods, Accessibility Services, Honeycomb’s new advanced Widgets, and so on.

3.If the name of a receiver implementing a Device Admin changes, then as with the live wallpaper example, the device admin will be disabled when the application is updated. This also applies to other kinds of receivers, such as App Widgets.

There are two main kinds of Intents:

>>Implicit Intents only specify “what” they should match, using actions, categories, data, MIME types, and so on. The exact components that they will find are only determined at run-time, by the Package Manager matching it against the current applications.

>>Explicit Intents specify a single explicit “who” they should match, through a ComponentName. Regardless of whatever else is in the Intent, it is only associated with the exact manifest package name and class name as given in its ComponentName.

Example:

To let the user pick a live wallpaper, the first thing Android must do is show them a list of the available live wallpaper services. It does this by building an implicit Intent with the appropriate action for a live wallpaper and asking the Package Manager for all services that support this Intent. The result is then the list of live wallpapers shown to the user.

When the user actually selects a specific live wallpaper they want to use, however, Android now must build an explicit Intent that identifies that particular live wallpaper. This is what is handed to the WallpaperManager to tell it which wallpaper to show.

This is why changing the name of the component in your manifest will cause the wallpaper to disappear: the explicit Intent that was previously saved is now invalid because the ComponentName it references no longer exists. There is no information available to indicate what the new name of the component is. (For example consider if your application had two different live wallpaper services the user could select.) Instead, Android must treat that live wallpaper as uninstalled and revert to its default wallpaper.

This is how input methods, device administrators, account managers, app widgets, and even application shortcuts work. The ComponentName is the public unique name of the components you declare in your manifest, and must not change if they are visible to other applications.

There are some parts of your application that can not change. Please be careful.