How to convert following Java code to Java 8

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

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.