I am trying to write a method that takes in a list of numeric values - eg List<Integer>, List<Float>, List<Double> etc - and give me the average.

public double getAverage(List<? extends Number> stats) {
    double sum = 0.00;
    if(!stats.isEmpty()) {
        // sum = stats.stream()
        //            .reduce(0, (a, b) -> a   b);
        // return sum / stats.size();
    }
}

These are the errors I get:

Operator ' ' cannot be applied to 'capture<? extends java.lang.Number>', 'capture<? extends java.lang.Number>'

CodePudding user response：

Try this:

public double getAverage(List<? extends Number> stats) {
    return stats.stream()
      .mapToDouble(Number::doubleValue)
      .summaryStatistics() // small overhead of unnecessary arithmetic
      .getAverage(); // returns 0 when empty
}

or

public double getAverage(List<? extends Number> stats) {
    return stats.stream()
      .mapToDouble(Number::doubleValue)
      .average()
      .orElse(0); // explicit value for empty (but no other value makes sense)
}

depending on which style you prefer.

CodePudding user response：

OptionalDouble average()

Where, OptionalDouble is a container object which may or may not contain a double value.

  public class Test {

    public static OptionalDouble getAverage(List<? extends Number> stats) {
        return stats.
                stream().
                 mapToDouble(d -> d.doubleValue()).
                    average();
            
    }
    
    public static void main(String[] args) throws IOException {

        List<Integer> list = Arrays.asList(1, 4, 5, 67, 3);
        if(getAverage(list).isPresent());
        {
            Double average = getAverage(list).getAsDouble();
            System.out.println(average);
        }
        
    }
}

or

Using Goolge Guava

 Double averge = Stats.meanOf(list);

      

it gets syntactically simplified