Importing external jar file in Android Studio---NoClassDefFoundError - jar

Question

I created a new project using Android Studio and copied a jar file into my application's libs. Then added it as a library, which made the IDE recognize it. Later, I added it to the build.gradle and it now compiles just fine. However, when I attempt to run the application on a device it crashes with NoClassDefFoundError.
Attached and in order:
The project tree on Android Studio. Notice the test-jar-to-import.jar inside HelloSheepProject/HelloSheep/libs/.
The contents of the MainActivity.java. It is attempting to create a new MyFile. Nothing else.
The build.gradle inside HelloSheepProject/HelloSheep/. I added the line compile files('libs/test-jar-to-import.jar').
The contents of MyFile.java. It was created in Eclipse and exported as a jar file.
The error when I trying to run it on a device.
Any ideas on what I am missing? I have tried a ./gradlew clean in HelloSheepProject but it didn't help.
package top.furrylamb.example.hellosheep;
import android.os.Bundle;
import android.app.Activity;
import top.furrylamb.example.MyFile;
public class MainActivity extends Activity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MyFile myFile = new MyFile("hi", "there");
}
}
buildscript {
repositories {
mavenCentral()
}
dependencies {
classpath 'com.android.tools.build:gradle:0.5.+'
}
}
apply plugin: 'android'
repositories {
mavenCentral()
}
dependencies {
compile 'com.android.support:support-v4:13.0.+'
//compile fileTree(dir: "libs", include: "*.jar")
compile files('libs/test-jar-to-import.jar')
}
android {
compileSdkVersion 17
buildToolsVersion "17.0.0"
defaultConfig {
minSdkVersion 14
targetSdkVersion 16
}
}
package top.furrylamb.example;
public class MyFile {
public final String left;
public final String right;
public MyFile(String left, String right) {
this.left = left;
this.right = right;
}
}
07-19 19:26:33.855 13656-13656/? E/AndroidRuntime: FATAL EXCEPTION: main
java.lang.NoClassDefFoundError: top.furrylamb.example.MyFile
at top.furrylamb.example.hellosheep.MainActivity.onCreate(MainActivity.java:15)
at android.app.Activity.performCreate(Activity.java:5206)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1083)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2064)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2125)
at android.app.ActivityThread.access$600(ActivityThread.java:140)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1227)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4898)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1006)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:773)
at dalvik.system.NativeStart.main(Native Method)

Answer 1

I found the solution to my problem. Since I've been lost on this for hours I'll keep the thread instead of deleting it.
The jar I was importing had been compiled/exported with Java 7 in mind. Once I enabled compliance with Java 6 it worked fine.
To sum it up, when adding an external jar file in Android Studio:
Copy jar to root/project/libs folder;
Right-click and add as library;
Add the jar to root/project/build.gradle (something like compile files('libs/test-jar-to-import.jar'));
Make sure the imported jar complies with Java 6 (7 will not do, for now).

Answer 2

In my case, I was getting the same NoClassDefFoundError error at runtime, even though it compiled ok. It had been working fine previously. But one day, I added a new package to my app, which resulted in the package name-space being changed. My jar had already been a part of the application's structure, so I was somewhat surprised at this new runtime error suddenly complaining about an unknown class in a jar library that was working previously.
The fix, in my case, was a simple Build->Clean Project.