Develop Reference

Which compileSdkVersion should I use in my android project?

I'm using latest dependencies in my project. My minSdkVersion is set as 15 and targetSdkVersion is 26. Which build tools and compileSdkVersion should I use so that the app can be run even in Adnroid 5.
Here are the details-
compileSdkVersion 28
buildToolsVersion '28.0.3'
defaultConfig {
applicationId "com.app.shoppingbull"
minSdkVersion 15
targetSdkVersion 26
versionCode 1
versionName "1.0"
}
Will these work fine in all devices or I have to reduce the CompileSdkVersion?

Always follow this,
minSdkVersion <= targetSdkVersion <= compileSdkVersion
Because, when you develop app always target the latest SDK so that you provide the latest features of Google to your user.

According to source.android.com if you want to make your App can be run in Adnroid 5 and newer, you should set the minSdkVersion to 21
tip: don't make your minSdkVersion lower than 21

Your application will work in the range of minSdkVersion and targetSdkVersion api level.
You should target your app to the latest version of Android SDK version (see SDK Platform release notes) to make sure you've covered all the Android version. It also force you to check if your code is working correctly with the latest version.
So, change your app build.gradle to something like this:
compileSdkVersion 28
buildToolsVersion '28.0.3'
defaultConfig {
applicationId "com.app.shoppingbull"
minSdkVersion 15
targetSdkVersion 28 // latest stdk
versionCode 1
versionName "1.0"

Ideally, your targetSdkVersion and compileSdkVersion should be the same, and the latest. That's 28 at the current time of writing.
minSdkVersion dictates the minimum version your app will support, so having a compile/target of 28 will still let you run on 15.

Can i set targetSDK lesser than buildToolsVersion & compileSdkVersion

I have upgraded google-play-services to 15.0.0, that uses android.support:26.1.0, so i am planning to upgrade compileSdkVersion and buildToolsVersion to version 26.
But there are some external libraries(.aar libraries) that requires targetSdkVersion to be set to version 25.
So if i set compileSdkVersion to 26 and buildToolsVersion to "26.0.0" and keep targetSdk to version 25, then what will happen

AFAIK, targetSdkVersion should be <= compileSdkVersion. As the Android Framework Developer said in this blog, ideally to have a steady state this should be the relationship of the compileSdkVersion, minSdkVersion, and targetSdkVersion:
minSdkVersion (lowest possible) <=
targetSdkVersion == compileSdkVersion (latest SDK)
Hope this helps.

Launcher Activity Not getting called on marshmallow Android

I have developed an application 6 months back for target version 22.
build.Gradle
android {
compileSdkVersion 22
buildToolsVersion "23.0.1"
defaultConfig {
applicationId "com.sample.mobile.test"
minSdkVersion 17
targetSdkVersion 22
versionCode 1
versionName "2.3"
multiDexEnabled true
}
}
so now when i am trying to run the application on marshmallow by just changing the
compileSdkVersion 23
targetSdkVersion 23
it is not calling the launcher activity of an application and also the strange thing is, it is not throwing any error.
I am not getting where the problem is, so please help me to resolve this issue,
Thanks in advance.

Running different version in Android library versus your app

I have the following in my main app:
...
android {
compileSdkVersion 23
buildToolsVersion "23.0.1"
defaultConfig {
minSdkVersion 8
targetSdkVersion 23
versionCode 19
versionName "1.0.5"
}
...
In a library I use, this is defined:
...
android {
compileSdkVersion 17
buildToolsVersion "19.1.0"
defaultConfig {
minSdkVersion 7
targetSdkVersion 7
}
...
I have uppgraded my API target etc. in my app. The library I have, use the same API target etc. (haven't changed the code). If I upgrade my app and use libraries, can I leave them as is? Or do I need to sync the two in respect with compileSdkVersion, buildToolsVersion, minSdkVersion and targetSdkVersion?

My app's minSdkVersion is 14 and targetSdkVersion is 21 but Google Play judge it just support Android 4.0~4.4

compileSdkVersion 21
buildToolsVersion "21.1.2"
defaultConfig {
applicationId "me.drakeet.seashell"
minSdkVersion 14
targetSdkVersion 21
versionCode 80
versionName "2.2"
}
Above code is in my gradle.build, and my app run perfectly in Android Lollipop (5.0), so I am puzzled that why Play judge and limit it just support Android 4.0~4.4, why not 5.0+?
Thanks!

I have fix it.
just add:
maxSdkVersion 21
in the gradle.build nearby minSdkVersion.