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How to realize this method only with Streams?

    /**
     * Get all cities near current city in radius.
     *
     * @param cityName - city
     * @param radius   - radius in kilometers for search
     * @throws IllegalArgumentException if city with cityName city doesn't exist.
     */
    public List<String> getCitiesNear(String cityName, int radius) {

        List<String> nearCities = new ArrayList<>();

        List<Integer> distances = new ArrayList<>();
        for (int i = 0; i < this.citiesNames().size(); i++) {
            distances.add(this.getDistance(cityName, this.citiesNames().get(i)));

        }

        for (int i = 0; i < distances.size(); i++) {
            if (distances.get(i) <= radius) {
                if (!this.citiesNames().get(i).equals(cityName)) {
                    nearCities.add(this.citiesNames().get(i));
                }
            }
        }

        return nearCities;
    }

Method citiesNames() returns List<“String”>. The names of cities.

Method “int getDistance(String srcCityName, String destCityName)” returns distance between srcCityName and srcCityName.

PS: It is forbidden to use loops, iterators inside this class. Only streams and methods that accept predicates can be used. You cannot declare other fields in the class.

This is my homework)

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Answer

if (citiesNames().contains(cityName) {
    return citiesNames().stream()
                        .filter(city -> getDistance(cityName, city) <= radius)
                        .collect(Collectors.toList());
}
else {
    throw new IllegalArgumentException(cityName + " not found.");
}

filter method returns a stream that contains only the near cities.
collect method creates a List containing all the elements in the filtered stream.

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