Cannot resolve symbol 'AppCompatActivity' , Cannot resolve symbol 'R', Could not download support-compat.aar

Question

I installed Android Studio 2.1.3, and successfully created a new project (just call it ProA), it runs well in AVD. That time I had just only downloaded the SDK platform android-24. I created that project with the Minimum SDK API 14. Actually , I didn't download android-14 at that time.
Then later I downloaded android-14, android-15,android-21,android-23, tried to create a new project (call it ProB), but many errors occurred there.

1. The MainActivity's superclass is AppCompatActivity , but the error is Cannot solve symbol 'AppCompatActivity'.

2. There is a red line under the @Override tag over the onCreate() method, saying that Method does not override a method from its superclass

3. In onCreate() method ,the setContentView(R.layout.activity_main) code snippet shows Cannot resolve symbol 'R'.

4. The Messages window has a error message saying
   A problem occurred configuring project ':app'.
Could not download support-compat.aar(com.android.support:-compat:24.2.0): No cached version available for offline mode

the overall picture is like this: the overall error image(click to see)

the content of app\src\build.gradle:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 24
    buildToolsVersion "24.0.1"

    defaultConfig {
        applicationId "com.example.zhangsir.aftervpn"
        minSdkVersion 14
        targetSdkVersion 24
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
    testCompile 'junit:junit:4.12'
    compile 'com.android.support:appcompat-v7:24.2.0'
}

the content of gradle\build.gradle:

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:2.1.3'

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

the screenshot of the standalone sdk manager (click to see)

---

modified at 2016 Aug,27

I have downloaded all the Android SDK Build-tools:

---

What can I do to solve this problem?

Answer 1

go to File >Invalidate Caches & Restart -> Invalidate & Restart

Update

download Android SDK Build-tools 24.0.1

Answer 2

Sometimes, It gets these errors, most of the time we use sync Project if it didn't hover mouse to

Build >Clean Project >Rebuild Project 

If this also, didn't fixed the problem then you might have done something wrong in gradle. You can update gradle if none of these works..

And One more thing I just saw in your question API 14 is not supported by AppCompactActivity so, you must extend Activity for that. AppCompactActivity supports from API 15. For more information see this

Answer 3

Try re-downloading the Android Support Repository from the Sdk manager.

Also update the Android Sdk, Platform-tools, Build Tools, Google Support Library to the Latest version.

Answer 4

Problem solved!
this Messages window in this picture click to see
says Could not download support-compat.aar (com.android.support-compat:24.2.0): No cached version available for offline mode
So I checked the first project which is OK, compare the app\src\build.gradle files in the two projects. the OK one's dependencies is like this:

dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
testCompile 'junit:junit:4.12'
compile 'com.android.support:appcompat-v7:24.1.1'
}

while the error one's dependencies is like this:

dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
testCompile 'junit:junit:4.12'
compile 'com.android.support:appcompat-v7:24.2.0'
}

So I modified the error one's dependencies, then Build->Clean Project, Build->Rebuild Project , it's OK now!

The basic reason is that my network is not available to google's server, many things cannot be downloaded.

Answer 5

If your Android Studio showing that cannot resolve symbol AppCompatActivity or cannot resolve symbol R then you can do one simple thing which is SYNC GRADLE. Click Here for the purpose of demonstration, you can watch it.

Answer 6

Simple Solution Here:

If your Android Studio showing that cannot resolve symbol AppCompatActivity or cannot resolve symbol R then you can do one simple thing which is SYNC GRADLE.
"Tools" -> "Android" -> "Sync Project with Gradle Files"
Click Here for the purpose of demonstration, you can watch it.