https://www.baeldung.com/java-runnable-callable http://tutorials.jenkov.com/java-util-concurrent/java-callable.html https://www.geeksforgeeks.org/callable-future-java/ https://www.baeldung.com/java-future https://www.baeldung.com/java-completablefuture https://www.baeldung.com/java-9-completablefuture https://www.baeldung.com/java-executor-wait-for-threads https://stackify.com/java-thread-pools/