Stream, return value of characters that didn't pass the filter

Question

I would like to count the characters that didn't pass the filter, and return this value too. How can i do that?

My exc: Within class LambdasStreamExercise, implement method dnaToWeight in order to determine the molecular weight of a DNA sequence string from its nucleotides. Implement these individual steps in your stream:

• Start with DNA.chars() to initiate a Stream. This will be one of integers.
• Filter out those that do not represent regular DNA (Challenge: and count them)
• Convert to a stream of Character objects from the given DNA sequence
• Convert the Characters into Nucleotide objects
• Convert the Nucleotide objects to their weights
• Sum the weights and return the result (And report the number of rejected nucleotides)

 public static double dnaToWeight(String DNA) {
        double dnaWeightTotal = DNA.chars()
                .filter(c -> c == 'A' || c == 'T' || c == 'G' || c == 'C')
                .mapToObj(c -> new Nucleotide((char) c))
                .mapToDouble(c -> (double) c.getWeight())
                .sum();
        return dnaWeightTotal;
    }

Answer 1

One clean way to do the job would be to make an intermediate list:

public static double dnaToWeight(String DNA) {
    List<Double> weights = DNA.chars()
            .filter(c -> c == 'A' || c == 'T' || c == 'G' || c == 'C')
            .mapToObj(c -> new Nucleotide((char) c).getWeight())
            .toList();
    int countNonRegular = DNA.length() - weights.size(); // to be used elsewhere
    return weights.stream().reduce(Double::sum).orElse(0.0);
}

Note: The method toList is new in Java 16.