Scratch

Scratch.java

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.function.Consumer;
import java.util.function.Function;

class Scratch {
    public static void main(String[] args) {
        String[] letters = {"a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"};
        ExecutorService executorService = Executors.newFixedThreadPool(4);
        doIt(letters, new Processor(executorService), String::toUpperCase);
        executorService.shutdown();
    }

    public static void doIt(String[] letters, Processor processor, Function<String, String> function) {
        for (final String letter : letters) {
            processor.process(letter, function, System.out::println);
        }
    }

    static class Processor {
        final ExecutorService executorService;

        Processor(ExecutorService executorService) {
            this.executorService = executorService;
        }

        public Future<?> process(String letter, Function<String, String> function, Consumer<String> handler) {
            return executorService.submit(() -> handler.accept(function.apply(letter)));
        }
    }
}