src/main/java/org/fest/assertions/GenericAssert.java
public final @NotNull S isIn(@NotNull Collection<?> values) {
    checkNotNull(values);
    if (isActualIn(values)) {
      return myself();
    }
    failIfCustomMessageIsSet();
    throw failure(unexpectedNotIn(actual, values));
  }
public final @NotNull S isIn(@NotNull Collection<?> values) {
    checkNotNull(values);
    if (isActualIn(values)) {
      return myself();
    }
    failIfCustomMessageIsSet();
    throw failure(unexpectedNotIn(actual, values));
  }
Verifies that the actual value is in the given collection.

@param values the given collection to search the actual value in. must not be null.
@return this assertion object.
@throws AssertionError       if the actual value is not in the given collection.
@throws NullPointerException if the given collection is null.