Lambda.java

package nextstep.fp;

import java.util.List;

public class Lambda {
    public static void printAllOld(List<Integer> numbers) {
        System.out.println("printAllOld");

        for (int number : numbers) {
            System.out.println(number);
        }
    }

    public static void printAllLambda(List<Integer> numbers) {
        System.out.println("printAllLambda");

        numbers.forEach(System.out::println);
    }

    public static void runThread() {
        new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("Hello from thread");
            }
        }).start();
    }

    public static int sumAll(List<Integer> numbers) {
        return sumBy(numbers, (number -> true));
    }

    public static int sumAllEven(List<Integer> numbers) {
        return sumBy(numbers, (number -> number % 2 == 0));
    }

    public static int sumAllOverThree(List<Integer> numbers) {
        return sumBy(numbers, (number -> number > 3));
    }

    private static int sumBy(List<Integer> numbers, SumCondition condition) {
        return numbers.stream()
                .filter(condition::match)
                .reduce(Integer::sum)
                .orElse(0);
    }

    @FunctionalInterface
    public interface SumCondition {
        boolean match(int number);
    }
}