title: Room cannot find implementation
body: <p>I have a problem with testing a Room database: 
  when I run the test, I get this exception: </p>
  
  <pre><code>java.lang.RuntimeException: cannot find implementation for database.TicketDatabase. TicketDatabase_Impl does not exist
  at android.arch.persistence.room.Room.getGeneratedImplementation(Room.java:92)
  at android.arch.persistence.room.RoomDatabase$Builder.build(RoomDatabase.java:454)
  at com.sw.ing.gestionescontrini.DatabaseTest.setDatabase(DatabaseTest.java:36)
  at java.lang.reflect.Method.invoke(Native Method)
  at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
  at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
  at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
  at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
  at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
  at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runners.Suite.runChild(Suite.java:128)
  at org.junit.runners.Suite.runChild(Suite.java:27)
  at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
  at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
  at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
  at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
  at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
  at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
  at org.junit.runner.JUnitCore.run(JUnitCore.java:115)
  at android.support.test.internal.runner.TestExecutor.execute(TestExecutor.java:59)
  at android.support.test.runner.AndroidJUnitRunner.onStart(AndroidJUnitRunner.java:262)
  at android.app.Instrumentation$InstrumentationThread.run(Instrumentation.java:1886)
  </code></pre>
  
  <p>Class DatabaseTest: </p>
  
  <pre><code>public class DatabaseTest {
  
      TicketDatabase database;
      DAO ticketDAO;
  
  
      @Before
      public void setDatabase() throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
          Context context = InstrumentationRegistry.getTargetContext();
          database = Room.inMemoryDatabaseBuilder(context, TicketDatabase.class).build();
  
          Method method = TicketDatabase.class.getDeclaredMethod("ticketDao()");
          method.setAccessible(true);
          ticketDAO = (DAO) method.invoke(database, null);
      }
  }
  </code></pre>
  
  <p>gradle file: </p>
  
  <pre><code>apply plugin: 'com.android.application'
  
  android {
      compileSdkVersion 26
      buildToolsVersion '26.0.2'
      defaultConfig {
          applicationId "com.sw.ing.gestionescontrini"
          minSdkVersion 15
          targetSdkVersion 26
          versionCode 1
          versionName "1.0"
          testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
      }
      defaultConfig {
          multiDexEnabled true
      }
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
          }
      }
  }
  
  dependencies {
      compile fileTree(dir: 'libs', include: ['*.jar'])
      androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
          exclude group: 'com.android.support', module: 'support-annotations'
      })
      compile 'com.android.support:appcompat-v7:26.+'
      compile 'com.android.support.constraint:constraint-layout:1.0.2'
      testCompile 'junit:junit:4.12'
  
      annotationProcessor 'android.arch.lifecycle:compiler:1.0.0'
      compile 'android.arch.persistence.room:rxjava2:1.0.0-rc1'
      testCompile'android.arch.persistence.room:testing:1.0.0-rc1'
  
  }
  </code></pre>
  
  <p>I don't really know what this implementation should be, I searched for a solution but all I found doesn't work for me. For instance, many found a solution adding kapt "android.arch.persistence.room..." but gradle doesn't recognize "kapt".</p>