UPDATE:
Since the version 22.1.0, the class ActionBarActivity is deprecated, so instead use AppCompatActivity. For more details see here
Using ActionBarActivity:
In Eclipse:
1 - Make sure library project(appcompat_v7) is open & is proper referenced (added as library) in your application project.
2 - Delete android-support-v4.jar from your project's libs folder(if jar is present).
3 - Appcompat_v7 must have android-support-v4.jar & android-support-v7-appcompat.jar inside it's libs folder. (If jars are not present copy them from /sdk/extras/android/support/v7/appcompat/libs folder of your installed android sdk location)
4- Check whether ActionBarActivity is properly imported.
import android.support.v7.app.ActionBarActivity;
In Android Studio
Just add compile dependencies to app's build.gradle
dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
compile 'com.android.support:appcompat-v7:22.1.1'
}