BAEL-6622 (#15905)

* BAEL-7490 read write file in separate thread

* Change the to try resources

* Update the code to sync with article

* BAEL-6622 compare thenApply() and thenApplyAsync()

* BAEL-6622 change to unit test

* Tidy up the code

core-java-modules/core-java-concurrency-2/src/test/java/com/baeldung/concurrent/applyvsapplyasync/ThenApplyAndThenApplyAsyncUnitTest.java

@@ -0,0 +1,91 @@
+package com.baeldung.concurrent.applyvsapplyasync;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.CompletionException;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+public class ThenApplyAndThenApplyAsyncUnitTest {
+
+    @Test
+    public void givenCompletableFuture_whenUsingThenApply_thenResultIsAsExpected() {
+        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 5);
+        CompletableFuture<String> thenApplyResultFuture = future.thenApply(num -> "Result: " + num);
+
+        String thenApplyResult = thenApplyResultFuture.join();
+        assertEquals("Result: 5", thenApplyResult);
+    }
+
+    @Test
+    public void givenCompletableFuture_whenUsingThenApplyAsync_thenResultIsAsExpected() {
+        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 5);
+        CompletableFuture<String> thenApplyAsyncResultFuture = future.thenApplyAsync(num -> "Result: " + num);
+
+        String thenApplyAsyncResult = thenApplyAsyncResultFuture.join();
+        assertEquals("Result: 5", thenApplyAsyncResult);
+    }
+
+    @Test
+    public void givenCompletableFuture_whenUsingThenApply_thenExceptionIsPropagated() {
+        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 5);
+        CompletableFuture<String> resultFuture = future.thenApply(num -> "Result: " + num / 0);
+        assertThrows(CompletionException.class, () -> resultFuture.join());
+    }
+
+    @Test
+    public void givenCompletableFuture_whenUsingThenApply_thenExceptionIsHandledAsExpected() {
+        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 5);
+        CompletableFuture<String> resultFuture = future.thenApply(num -> "Result: " + num / 0);
+        try {
+            // Accessing the result
+            String result = resultFuture.join();
+            assertEquals("Result: 5", result);
+        } catch (CompletionException e) {
+            assertEquals("java.lang.ArithmeticException: / by zero", e.getMessage());
+            System.err.println("Exception caught: " + e.getMessage());
+        }
+    }
+
+    @Test
+    public void givenCompletableFuture_whenUsingThenApplyAsync_thenExceptionIsHandledAsExpected() {
+        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> 5);
+        CompletableFuture<String> thenApplyAsyncResultFuture = future.thenApplyAsync(num -> "Result: " + num / 0);
+
+        String result = thenApplyAsyncResultFuture.handle((res, error) -> {
+                if (error != null) {
+                    // Handle the error appropriately, e.g., return a default value
+                    return "Error occurred";
+                } else {
+                    return res;
+                }
+            })
+            .join(); // Now join() won't throw the exception
+        assertEquals("Error occurred", result);
+    }
+
+    @Test
+    public void givenCompletableFutureWithExecutor_whenUsingThenApplyAsync_thenThreadExecutesAsExpected() {
+        ExecutorService customExecutor = Executors.newFixedThreadPool(4);
+
+        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
+            try {
+                Thread.sleep(2000);
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+            }
+            return 5;
+        }, customExecutor);
+
+        CompletableFuture<String> resultFuture = future.thenApplyAsync(num -> "Result: " + num, customExecutor);
+
+        String result = resultFuture.join();
+        assertEquals("Result: 5", result);
+
+        customExecutor.shutdown();
+    }
+}