}
    ImmutableMap<ClassWithTerribleHashCode, Integer> map = builder.buildKeepingLast();
    assertThat(map).containsExactlyEntriesIn(expected).inOrder();
    assertThat(map).isInstanceOf(JdkBackedImmutableMap.class);
  }

  @GwtIncompatible // Pattern, Matcher
  public void testBuilder_keepingLast_thenOrThrow() {
    ImmutableMap.Builder<String, Integer> builder =
        new Builder<String, Integer>()

          Entry<K, V> onlyEntry = requireNonNull(entries[0]);
          return of(onlyEntry.getKey(), onlyEntry.getValue());
        default:
          entriesUsed = true;
          return JdkBackedImmutableMap.create(size, entries, /* throwIfDuplicateKeys= */ true);
      }
    }

    /**
     * Scans the first {@code size} elements of {@code entries} looking for duplicate keys. If