added the code (#15272)

Co-authored-by: Amit Kumatr <amit.kumar@fyndna.com>

core-java-modules/core-java-concurrency-advanced-5/src/main/java/com/baeldung/callback/completablefuture/CompletableFutureCallbackExample.java

@@ -0,0 +1,30 @@
+package com.baeldung.callback.completablefuture;
+
+import java.util.concurrent.CompletableFuture;
+
+public class CompletableFutureCallbackExample {
+    public static void main(String[] args) {
+        CompletableFuture<String> completableFuture = new CompletableFuture<>();
+        Runnable runnable = downloadFile(completableFuture);
+        completableFuture.whenComplete((res, error) -> {
+            if (error != null) {
+                // handle the exception scenario
+            } else if (res != null) {
+                // send data to DB
+            }
+        });
+        new Thread(runnable).start();
+    }
+
+    private static Runnable downloadFile(CompletableFuture<String> completableFuture) {
+        return () -> {
+            try {
+                //logic to download file
+                Thread.sleep(5000);
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+            completableFuture.complete("pic.jpg");
+        };
+    }
+}

core-java-modules/core-java-concurrency-advanced-5/src/main/java/com/baeldung/callback/listenablefuture/ListenableFutureCallbackExample.java

@@ -0,0 +1,42 @@
+package com.baeldung.callback.listenablefuture;
+
+import java.util.concurrent.Callable;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import com.google.common.util.concurrent.FutureCallback;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.google.common.util.concurrent.ListeningExecutorService;
+import com.google.common.util.concurrent.MoreExecutors;
+
+
+public class ListenableFutureCallbackExample {
+    public static void main(String[] args) {
+
+        ExecutorService executorService = Executors.newFixedThreadPool(1);
+        ListeningExecutorService pool = MoreExecutors.listeningDecorator(executorService);
+        ListenableFuture<String> listenableFuture = pool.submit(downloadFile());
+
+        Futures.addCallback(listenableFuture, new FutureCallback<String>() {
+            @Override
+            public void onSuccess(String result) {
+                // code to push fileName to DB
+            }
+
+            @Override
+            public void onFailure(Throwable throwable) {
+                // code to take appropriate action when there is an error
+            }
+        }, executorService);
+    }
+
+    private static Callable<String> downloadFile() {
+        return () -> {
+            // Mimicking the downloading of a file by adding a sleep call
+            Thread.sleep(5000);
+            return "pic.jpg";
+        };
+    }
+
+}