rangeSet.add(range);

      TreeRangeSet<Integer> complement = TreeRangeSet.create();
      complement.add(Range.<Integer>all());
      complement.remove(range);

      assertEquals(complement, rangeSet.complement());
      assertThat(rangeSet.complement().asRanges())
          .containsExactlyElementsIn(complement.asRanges())
          .inOrder();
    }
  }

      rangeSet.add(range);

      TreeRangeSet<Integer> complement = TreeRangeSet.create();
      complement.add(Range.<Integer>all());
      complement.remove(range);

      assertEquals(complement, rangeSet.complement());
      assertThat(rangeSet.complement().asRanges())
          .containsExactlyElementsIn(complement.asRanges())
          .inOrder();
    }
  }

      }

      ImmutableRangeSet<Integer> built = builder.build();
      assertEquals(mutable, built);
      assertEquals(ImmutableRangeSet.copyOf(mutable), built);
      assertEquals(mutable.complement(), built.complement());

      for (int i = 0; i <= 11; i++) {
        assertEquals(mutable.contains(i), built.contains(i));
      }

      SerializableTester.reserializeAndAssert(built);

    } else {
      rangesByLowerBound.put(range.lowerBound, range);
    }
  }

  @LazyInit @CheckForNull private transient RangeSet<C> complement;

  @Override
  public RangeSet<C> complement() {
    RangeSet<C> result = complement;
    return (result == null) ? complement = new Complement() : result;
  }

  @VisibleForTesting
  static final class RangesByUpperBound<C extends Comparable<?>>

@GwtIncompatible // TreeRangeSet
public abstract class AbstractRangeSetTest extends TestCase {
  public static void testInvariants(RangeSet<?> rangeSet) {
    testInvariantsInternal(rangeSet);
    testInvariantsInternal(rangeSet.complement());
  }

  private static <C extends Comparable<?>> void testInvariantsInternal(RangeSet<C> rangeSet) {
    assertEquals(rangeSet.asRanges().isEmpty(), rangeSet.isEmpty());

      }

      ImmutableRangeSet<Integer> built = builder.build();
      assertEquals(mutable, built);
      assertEquals(ImmutableRangeSet.copyOf(mutable), built);
      assertEquals(mutable.complement(), built.complement());

      for (int i = 0; i <= 11; i++) {
        assertEquals(mutable.contains(i), built.contains(i));
      }

      SerializableTester.reserializeAndAssert(built);

    }
  }

  @Override
  public ImmutableRangeSet<C> complement() {
    ImmutableRangeSet<C> result = complement;
    if (result != null) {
      return result;
    } else if (ranges.isEmpty()) {
      return complement = all();
    } else if (ranges.size() == 1 && ranges.get(0).equals(Range.all())) {
      return complement = of();
    } else {

    }
  }

  @Override
  public ImmutableRangeSet<C> complement() {
    ImmutableRangeSet<C> result = complement;
    if (result != null) {
      return result;
    } else if (ranges.isEmpty()) {
      return complement = all();
    } else if (ranges.size() == 1 && ranges.get(0).equals(Range.all())) {
      return complement = of();
    } else {

Although the [Tutorial - User Guide](../tutorial/index.md){.internal-link target=_blank} and this **Advanced User Guide** are written as a guided tutorial (like a book) and should be enough for you to **learn FastAPI**, you might want to complement it with additional courses.

Or it might be the case that you just prefer to take other courses because they adapt better to your learning style.

@GwtIncompatible // TreeRangeSet
public abstract class AbstractRangeSetTest extends TestCase {
  public static void testInvariants(RangeSet<?> rangeSet) {
    testInvariantsInternal(rangeSet);
    testInvariantsInternal(rangeSet.complement());
  }

  private static <C extends Comparable<?>> void testInvariantsInternal(RangeSet<C> rangeSet) {
    assertEquals(rangeSet.asRanges().isEmpty(), rangeSet.isEmpty());