This is Java. You have to import classes and refer to them by package.
import android.os.Bundle;
import android.app.Activity;
import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.highgui.Highgui;
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Mat img1 = Highgui.imread("mnt/sdcard/xx/IMG-20130123-WA0004.jpg");
Mat img2 = Highgui.imread("mnt/sdcard/xx/IMG-20130124-WA0000.jpg");
double l2_norm = Core.norm(img1, img2);
}
}