Develop Reference

Double Application on Android

I want ti install same app twice without replacing the each Other.
something like :I Want play a game from the start without losing previous game saved data.so, Is there a way i can install same apk twice without replacing the previous one. is it possible??
Will changing the apk package name & changing default installed Directory of app work?,but i think it doesn't work.

Actually you can I do that before BY:
You must have a different package name (make sure the package name in the AndroidManifest.xml file is different), there is no other way. I had two different versions of the same product (free & paid) and used: com.package.paid & com.package.free.
This means that nothing is shared between the two apps (DB included).
You can also use ANT scripts to build both apps from the same code base, I just changed it manually every time I needed to build a version. Hope this help :)

You can't do that, android only allow one version of an app exists. Do an upgrade or downgrade installation, or find another device to install the same app.

Uploading Apps to the Play Store

So I messed up pretty hard and forgot my password key store password so I ended up having to make a new application and keystore. But when I tried uploading the new apk file it said I couldn't do that because I already had an application with the same package name. So what I'm wondering is is there any easy fix to this? Can I just change the applicationid in the gradle file?
EDIT: Surprisingly enough just changing the applicationid and resyncing it worked! Thanks for the help though.

You have to provide a unique packageName, which is defined in your AndroidManifest.xml.

Your best (and fastest) solution is to use the Refactor > Rename option - within Android Studio/Eclipse IDE's - to change your project package structure to something (slightly) different.
By doing this, the IDE should take care of changes to your AndroidManifest.xml file as well.
Good Luck!!!
On Android Studio:
On Eclipse:

you must need to change the main package name of your application which must be unique . unique means with same name no any application is uploaded on playstore then only your issue will get resolve
see your problem is you forgot password of your keystore certificate see why that certificate is uses in android is to update the application every time you need to pass password to that file but unfortunately you forgot it so you have no choice other than changing pakcage name also your application will consider as a new application on playstore your previous application and this current one will be seprate application

Separate releases running on same device?

I've created an android app and installed it on my device.
Now I have changed the name and the icon for this app.
Is it possible to install this next to the "previous" app ?
When I try this it overwrites the previous one.
(I would prefer not to change the package name(s) since want to do this more then once).

No it is not possible the way you want. Name of application is irrelevant (it is just for humans). Package name (package attribute of <manifest> is what matters, so you need to name packages of your both builds differently to make this happen. There's no other way.

You will have to change the package name.

Replace a system application with your own modified application on android

I downloaded the source code of OI Notepad and did some modifications to it. And then made an .apk file out of it. But when I try to install the .apk it shows
This is a system application.
Do you still want to replace it?
When i say yes it moves forward but does not install it..
I have this post My application tries replace another my application and the answer given there the package name of both the apps cannot be same.
But is there any way in which I can manage to install modified OI Notepad without any hassles of replacing the older one. It happens in Emulator..
Thanks

If you are using eclipse you can right click the package tree and use the refactor option to quickly change the package. This, in most cases, correctly updates the Application Manifest for you. This 'should' resolve the error you are having keeping in mind that you will now have two versions of the application so it may be in your best interest to change the application name of your custom version so you can easily distinguish between the two.

non standard package name - will it cause trouble?

I have rather stupidly published a game on the android market with a package name of mick.game.tangletwister, whereas I should have called it com.rai.tangletwister (where rai is the name of my company)... it all seems to work fine - but could it cause some trouble in the future - should I change it? And if I do change it, will it cause problems for existing users that grab the updated version?

It will be fine if you keep the existing package name. Nothing wrong with that. The TLD.company.appname format is just a convention. As long as you don't enter some special characters in your packagename (which you did not), it will work anyway.
But if you try to change it you will run into trouble. See Things That Cannot Change.

If you change the package, update notification will not work any more. Moreover, when a user with the old package name installs the updated game, he/she will have both installed, because the update will replace the old version only if package/class are the same.
Depending on how many installs you have, you can, of course, remove the existing app from the Market - this should force-remove it from devices where it's installed. Then you create another app (same metadata, naturally) and use a correct package name.

1) Change the package name
2) Create a new app in the android market with that new package name
3) Update the old app, and add a pop-up saying that users should install a new version. Possible with a link to the new app
4) After some months, you should remove the old app.
Like user287351 said, it is possible to keep using the current package name, but i would prefer to have it listed under the correct package name