Added executor framework

src/main/java/com/hithomelabs/dsa/concurrency/ExecutorFramework/ExecutorInterfaces.java

@@ -0,0 +1,78 @@
+package com.hithomelabs.dsa.concurrency.ExecutorFramework;
+
+import java.util.concurrent.*;
+
+public class ExecutorInterfaces {
+
+    public static void main(String[] args) {
+
+        // * * An executor, is literally a function interface with a single method, replaces new Thread(runnable).start()
+        // * * An implementation of executor, Executors is a helper class having more factory methods
+
+        Executor executor = Executors.newFixedThreadPool(2);
+        executor.execute(() -> System.out.println("Hello World"));
+
+        // * * an executor does not shut down on it's own
+        // * * ExecutorService provides more high level control
+
+        // * *  Casting the Executor reference, to leverage the more extensive API
+        ExecutorService executorService = (ExecutorService) executor;
+
+        // * * Executor Service provides a submit method:
+
+        Callable<String> task = () -> {
+            System.out.println("will return hello after 2 seconds");
+            Thread.sleep(2000);
+            return "Hello";
+        };
+
+        Future<String> futureHello = executorService.submit(task);
+        try {
+            System.out.println(futureHello.get());
+        } catch (InterruptedException | ExecutionException e) {
+            throw new RuntimeException(e);
+        }
+
+        // * * Shuts down once already submitted tasks are complete
+        executorService.shutdown();
+
+        // * *  exploring different scenarios of shutdown.
+        // * * Attempting to submit task after shutDown Initiated
+
+        System.out.println("Scenario 2:");
+        executorService = Executors.newFixedThreadPool(2);
+
+        executorService.submit(task);
+        executorService.submit(task);
+        // * * Third task should be in the queue but must be completed
+        executorService.submit(task);
+
+        executorService.shutdown();
+
+        // * * Uncomment to try awaitTermination
+//        try {
+//            if(executorService.awaitTermination(3, TimeUnit.SECONDS)){
+//                System.out.println("Terminated in time");
+//            }
+//            else{
+//                System.out.println("Waited 3 seconds");
+//            }
+//        } catch (InterruptedException e) {
+//            System.out.println("Can't take this long");
+//            throw new RuntimeException(e);
+//        }
+
+        // * * Uncomment to try shutdown now
+//        List<Runnable> couldNotComplete = executorService.shutdownNow();
+//        couldNotComplete.stream().forEach(System.out::println);
+
+        try {
+            // * * Task gets rejected
+            executorService.submit(task);
+        } catch (Exception e) {
+            System.out.println("Task gets rejected: reason: ");
+            System.out.println(e.getMessage());
+        }
+    }
+
+}