BAEL-6814 Convert List<CompletableFuture> to CompletableFuture<List> (#15028)

* Code for BAEL-6814 Convert List<CompletableFuture> to CompletableFuture<List>

* Refactor some comments

core-java-modules/core-java-concurrency-basic-3/src/main/java/com/baeldung/concurrent/completablefuturelist/Application.java

@@ -0,0 +1,100 @@
+package com.baeldung.concurrent.completablefuturelist;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.stream.Collectors;
+
+public class Application {
+
+    ScheduledExecutorService asyncOperationEmulation;
+
+    Application initialize() {
+        asyncOperationEmulation = Executors.newScheduledThreadPool(10);
+        return this;
+    }
+
+    CompletableFuture<String> asyncOperation(String operationId) {
+        CompletableFuture<String> cf = new CompletableFuture<>();
+        asyncOperationEmulation.submit(() -> {
+            // The following lines simulate an exception happening on the 567th operation
+            //  if (operationId.endsWith("567")) {
+            //      cf.completeExceptionally(new Exception("Error on operation " + operationId));
+            //      return;
+            //  }
+            try {
+                Thread.sleep(100);
+                cf.complete(operationId);
+            } catch (InterruptedException e) {
+                System.err.println("Thread interrupted error");
+                cf.completeExceptionally(e);
+            }
+        });
+        return cf;
+    }
+
+    void startNaive() {
+        List<CompletableFuture<String>> futures = new ArrayList<>();
+        for (int i = 1; i <= 1000; i++) {
+            String operationId = "Naive-Operation-" + i;
+            futures.add(asyncOperation(operationId));
+        }
+
+        CompletableFuture<List<String>> aggregate = CompletableFuture.completedFuture(new ArrayList<>());
+        for (CompletableFuture<String> future : futures) {
+            aggregate = aggregate.thenCompose(list -> {
+                try {
+                    list.add(future.get());
+                    return CompletableFuture.completedFuture(list);
+                } catch (Exception e) {
+                    final CompletableFuture<List<String>> excFuture = new CompletableFuture<>();
+                    excFuture.completeExceptionally(e);
+                    return excFuture;
+                }
+            });
+        }
+
+        try {
+            final List<String> results = aggregate.join();
+            System.out.println("Printing first 10 results");
+            for (int i = 0; i < 10; i++) {
+                System.out.println("Finished " + results.get(i));
+            }
+        } finally {
+            close();
+        }
+    }
+
+    void start() {
+        List<CompletableFuture<String>> futures = new ArrayList<>();
+        for (int i = 1; i <= 1000; i++) {
+            String operationId = "Operation-" + i;
+            futures.add(asyncOperation(operationId));
+        }
+        CompletableFuture<?>[] futuresArray = futures.toArray(new CompletableFuture<?>[0]);
+        CompletableFuture<List<String>> listFuture = CompletableFuture.allOf(futuresArray).thenApply(v -> futures.stream().map(CompletableFuture::join).collect(Collectors.toList()));
+        try {
+            final List<String> results = listFuture.join();
+            System.out.println("Printing first 10 results");
+            for (int i = 0; i < 10; i++) {
+                System.out.println("Finished " + results.get(i));
+            }
+        } finally {
+            close();
+        }
+    }
+
+    void close() {
+        asyncOperationEmulation.shutdownNow();
+    }
+
+    public static void main(String[] args) {
+        new Application().initialize()
+        // Switch between .startNaive() and .start() to test both implementations
+        //  .startNaive();
+          .start();
+    }
+
+}