package com.iluwatar.callback;

import org.junit.Test;

import com.iluwatar.callback.App;

/**
 * 
 * Application test
 *
 */
public class AppTest {

	@Test
	public void test() {
		String[] args = {};
		App.main(args);
	}
}

package com.iluwatar.callback;

import org.junit.Test;

import static org.junit.Assert.assertEquals;

/**
 * Add a field as a counter. Every time the callback method is called increment this
 * field. Unit test checks that the field is being incremented.
 *
 * Could be done with mock objects as well where the call method call is verified.
 */
public class AppTest {

    private Integer callingCount = 0;

	@Test
	public void test() {
        Callback callback = new Callback() {
            @Override
            public void call() {
                callingCount++;
            }
        };

        Task task = new SimpleTask();

        assertEquals("Initial calling count of 0", new Integer(0), callingCount);

        task.executeWith(callback);

        assertEquals("Callback called once", new Integer(1), callingCount);

        task.executeWith(callback);

        assertEquals("Callback called twice", new Integer(2), callingCount);

	}
}

Add unit test to show that the callback method is called.