Functional programming example in java8: replace template method pattern with function composition.

TemplateMethodFunctional.java

import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.stream.Collectors;

/**
 * Functional replacement for Template Method in java 8
 * Created by ton on 25/10/16.
 */
public class TemplateMethodFunctional {

    /**
     * Take a conversion function from Double to String, and a consumer of a list of Strings,
     * and combine them to create a new consumer that accepts a list of Doubles.
     * @param numToLetter a Function from Double to String.
     * @param printGradeReport a Consumer for a List of Strings.
     * @return a Consumer for a List of Double.
     */
    private static Consumer<List<Double>> makeGradeReporter( Function<Double, String> numToLetter,
            Consumer<List<String>> printGradeReport) {

        return lst -> printGradeReport.accept(
                lst.stream().map(numToLetter)
                        .collect(Collectors.toList())
        );
    }

    /**
     * Convert a double from 0.0 to 5.0 to a grade in the range 'A' - 'F'.
     * @param grade grade as a Double
     * @return the corresponding letter
     */
    private static String fullGradeConverter(Double grade) {
        if (grade <= 5.0 && grade > 4.0)
            return "A";
        else if (grade <= 4.0 && grade > 3.0)
            return "B";
        else if (grade <= 3.0 && grade > 2.0)
            return "C";
        else if (grade <= 2.0 && grade > 1.0)
            return "D";
        else if (grade <= 1.0 && grade > 0.0)
            return "E";
        else if (grade == 0.0)
            return "F";
        else
            return "N/A";
    }

    /**
     * Consume a List of Strings by printing each one on a new line.
     * @param letters
     */
    private static void simplePrint(List<String> letters) {
        letters.stream().forEach(System.out::println);
    }

    public static void main(String[] args) {
        List<Double> grades = Arrays.asList(2.0, 2.5, 5.0, 1.0);
        Consumer<List<Double>> example = makeGradeReporter(
                TemplateMethodFunctional::fullGradeConverter,
                TemplateMethodFunctional::simplePrint);
        
        example.accept(grades);
    }
}