RxJava2. How make Observable from VoidMethod. And how test Rx-Exceptions with JUnit.

RxFromAction.java

import io.reactivex.Completable;
import io.reactivex.disposables.Disposable;
import io.reactivex.functions.Action;
import io.reactivex.observers.DisposableCompletableObserver;
import io.reactivex.observers.TestObserver;
import org.junit.Test;

import java.util.Observable;

public class RxFromAction {

    /**
     * Just function.
     * @throws Exception
     */
    void f() throws Exception {
        throw new Exception("was false");
    }

    /**
     * {@link Observable} from function.
     * @return
     */
    Completable of () {
        return Completable.fromAction(this::f);
    }

    /**
     * Check for 'onError' has been catched.
     * @param args
     */
    public static void main(String[] args) {

        Disposable d = new RxFromAction().of().subscribeWith(new DisposableCompletableObserver() {

            @Override
            public void onComplete() {
                System.out.println("onComplete");
            }

            @Override
            public void onError(Throwable throwable) {
                System.out.println("onError: " + throwable.getLocalizedMessage());
            }
        });

        d.dispose();

    }

    /**
     * How to test for error!
     */
    @Test
    public void test() {

        TestObserver to = new TestObserver();
        of().subscribe(to);

        to.assertError(Exception.class);

    }

}