How to convert following Java code to Java 8

Question

I am trying to convert the following Java code to Java 8. I have written the following code to calculate the average temperatures.

public static double calculateTemps(String country, Map<String, List<Temperatures>> tempMap) {
    double temp = 0.0f;
    int count = 0;

    if (country.equalsIgnoreCase("India")) {
        for (Map.Entry<String, List<Temperatures>> m : tempMap.entrySet()) {
            for (Temperatures t : m.getValue()) {
                temp = temp + t.getTemp();
                count++;
            }
        }
    }
    System.out.println(count);
    return temp / count;
}

The above code is working fine. Now, I am trying to convert it to Java 8. I have written the following code, but I am getting compile-time errors

public static double calculateTemps(String country, Map<String, List<Temperatures>> tempMap) {
    double temp = 0.0f;
    int count = 0;

    tempMap.entrySet().stream().filter(temps -> temps.getKey()
                    .equalsIgnoreCase("India"))
            .forEach(temps -> {
                temp = temp + temps.getValue();
            }).collect(Collectors.toSet());
}

I thought map is better suited here, but after going through a few questions on Stack Overflow I thought for-each is better suited here. Not sure. Could anyone please enlighten me?

Answer 1

Your code has multiple problems:

1. You're trying to add Temperatures objects to temp instead of Temperatures.getTemp()
2. You're trying to modify variable temp inside a lambda, but temp must be effectively final
3. You're calling collect on the void method forEach , which is not possible.

You can make use of features of DoubleStream to calculate the average:

return tempMap.entrySet().stream()
        .filter(temps -> temps.getKey().equalsIgnoreCase("India"))
        .flatMap(temps -> temps.getValue().stream())
        .mapToDouble(temp -> temp.getTemp())
        .average().orElse(0.0);

As an aside, the filter condition is different from the condition used in the original method (which checked against the country parameter), but I have preserved it from your original attempt. Check carefully if it is really what you need.

Answer 2

You can try this

public static double calculateTemps(String country, Map<String, List<Temperatures>> tempMap) {
    return tempMap.entrySet()
                  .stream()
                  .filter(temps -> temps.getKey().equalsIgnoreCase("India"))
                  .flatMap(entry -> entry.getValue().stream())
                  .mapToDouble(Temperatures::getTemp)
                  .average()
                  .orElse(0.0);
}

Answer 3

you can reference like this:

    public static double calculateTemps(String country, Map<String, List<Temperatures>> tempMap) {
        double temp = 0.0f;
        int count = 0;

        if (country.equalsIgnoreCase("India")) {

            double result = tempMap.entrySet().stream()
                    .flatMap(entry -> entry.getValue().stream())
                    .mapToDouble(Temperatures::getTemp)
                    .average().orElse(0.0);
            System.out.println(result);
        }

        return 0;
    }