@Test
  public void inc_withAmount() {
    counter.inc(10);
    assertEquals(10, counter.get());

    counter.inc(0);
    assertEquals(10, counter.get());
  }
 @Test
 public void test_toString() {
   String s = counter.toString();
   assertEquals("Counter{value=0}", s);
 }
 @Test
 public void inc() {
   counter.inc();
   assertEquals(1, counter.get());
 }