Develop Reference

how to set a package name without Website?

I've recently started creating an app for Android. but i do not have a Website.
how to get Package name. Can i make it com.example.(app name) ?
and how to know if it is available or not ?

package name have no concern with any website. It is used to identify your application uniquely in your mobile phone and in any App store. You can set any name or string in your package name i.e com.example.myfirstapplication
it is not necessary that there should be com.example in the package name you can change them also i.e my.first.application.packagename

You can set whatever package name you like, as long as it isnt already taken when you publish it on the playstore.
More good reading: https://stackoverflow.com/a/6273935/5644761

Can i publish a new app by using existing app package name

I want to know that can I publish a new android app with existing app package name?
i.e I have a photo editor and its package name is com.myapps.photoeditor. now I want to make new version but i will publish VPN. so the new app will totally different.
VPN will be ok with existing package name com.myapps.photoeditor?
plz guide me

No, you cannot do this. Package name basically I unique ID for an app. When a package name is associated with an app and it is published over google play this is a unique ID for that app. You cannot publish new app with the same package name but you can update the app.

You can't publish two apps with same package name as google consider it as primary key but story doesn't end here. There are two things you can do:
you can update your app, here updates mean you can remove existing feature, you can add new feature, you can modify existing feature but you have to handle it properly.
But you want the new feature in a separate app then you should change clone your existing code and add new feature and change package name before publishing. (Not Recommended)

If you mean by package name as the applicationId in build.gradle, then the answer is NO. Your new VPN app will overwrite your existing Photo editor app.
If you mean by package name as the package name used by your source codes, then the answer is IT DEPENDS. YES you can use same package name in source code provided that you provide different name for applicationId. NO you cannot if you provide the same name for applicationId regardless of what you use as the package name used by your source codes.
Know the difference here: https://developer.android.com/studio/build/application-id

If you want to release a new app with applicationId (from app/build.gradle) same as any apps before, you can't do it.
If you want to update an existing app with complete new ideas or feature or name or type, having access to that app's signing keys and developer account, yes you can do it. Just release with version code greater than before

Reinstalling android app without uninstaling

I have developed my first app using eclipse and uploaded to Amazon app store. Later, I realized some bugs, fixed them and re-uploaded. But, when I want to reinstall app over same/newer version, it says:
"The application you are installing will replace another application. All previous data will be saved" as other normal apks.
"And then it says application not installed." I have signed both versions using same key.

when you try to run the app(Let the new project name be AndroidAPP_v2) with all the bugs fixed ,then In eclipse It will show like this :"The application you are installing will replace another application".that means It will replace your old project(let it be AndroidAPP_v1) with new project(AndroidAPP_v2).If you click OK ,it wil automaticall install new app .
This is because both applications have the same package name. If the package name of the two applications are different, you won't get this message.
In fact, if the package name in the Manifest file (in the tag) matches the package name of another installed application, it will replace it.

Package name matches an existing developer's application (but it doesn't)

I am trying to submit an application to the marketplace but when I try to upload I get a message:
The package name of your apk (xxx.xxxxxx.xxxxxxxxx) is the same as
the package name of another developer's application. Choose a new
package name.
My company has two apps in market already but the package names are distinct. I have queried the address https://market.android.com/details?id=xxx.xxxxxx.xxxxxxxx to check to make sure that it's not mistakenly identifying one of our other apps but it comes up as "Not Found" which should mean that this package name doesn't conflict.
Is there something else I should try to get this to work? I am going to try a small change to the package name and see if it will pass but this error shouldn't have occurred in the first place.

Maybe the package id does exist, but the application is in an unpublished status at the moment.

Is better to rename the package in this way you avoid having problems.

Change the package name or contact google support

Changing the package name of an upgraded Android application

During the upgrade of my android application, I changed the package name. But Android market doesn't allow to upload the changed package name application as an upgrade. If I upload the application as a new application, will the user have two applications on his/her device? How can I make sure that the user doesn't have to download the application again from scratch without reverting the change of my package name?

two package = two different application in market place.
Once you upload one app, its package should be same. Also, the key should be same.

Android market is only concerned about the package name in your manifest, not the actual packages name in the source.
You could try to give the old package name in the manifest attribute, then for activities give the new package name instead of relative (ie .MainActivity)
Like this:
<manifest package="your.old.package" ...>
...
<application android:name="your.new.package.MainActivity" ...>
Could work..
I plan to serve two versions of my app (paid/free) this way and using same project and code.

If you change the package name, it's treated as a separate app - not just in the market, but apk's in general will only 'replace' the same package name (and only if they're both signed with the same key).
Although it's possible to phase over to a new key by signing an intermediate package with both keys, there's currently no easy way to phase over the package name.
The best that can be done is this:
New apk version is signed with the same key, but has a different package name.
When installed, the new apk arranges to use the shared_prefs with the old package name.
The data is copied across to the new package name.
The new version requests that the old version is removed, and the user sees the uninstall dialog.
Note app data is usually kept here:
/data/data/pac.kage.name/
I haven't tried this, so I can't give anymore details yet. You may also be interested in my request for a seamless way of transitioning the package name.