title: Include .java files as java source files in Android Studio to remove red circle with "J" on them
body: <p>Today after update android studio my source folder in project marks as below("J" inside a red circle mark). How can I fix it? I tried clear caches, re-import project, use <code>gradlew clean build</code>, but it doesn't help.
  <img src="https://i.stack.imgur.com/U3K85.png" alt="enter image description here"></p>
  
  <p>build.gradle</p>
  
  <pre><code>apply plugin: 'android'
  apply plugin: 'android-apt'
  
  android {
      compileSdkVersion 19
      buildToolsVersion "19.0.3"
  
  
      sourceSets {
          main {
              manifest.srcFile 'AndroidManifest.xml'
              java.srcDirs = ['src']
              resources.srcDirs = ['src']
              aidl.srcDirs = ['src']
              res.srcDirs = ['res']
              assets.srcDirs = ['assets']
          }
      }
  
      defaultConfig {
          minSdkVersion 14
          targetSdkVersion 19
          versionCode 1
          versionName "1.0"
      }
      buildTypes {
          release {
              runProguard false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
          }
      }
      lintOptions {
          abortOnError false
      }
  }
  
  def AAVersion = '3.0.1'
  dependencies {
     ......
  }
  
  apt {
      arguments {
       ....
      }
  }
  </code></pre>