add Cf05 demo:展平组合(thenCompose)

src/main/java/space/anyi/Cf05_ThenCompose.java

@@ -0,0 +1,32 @@
+package space.anyi;
+
+import java.util.concurrent.CompletableFuture;
+
+public class Cf05_ThenCompose {
+    public static void main(String[] args) throws Exception {
+        CompletableFuture<String> userFuture = CompletableFuture.supplyAsync(() -> {
+            System.out.println("1. 查询用户");
+            return "用户:李雷";
+        });
+
+        // 如果使用 thenApply，返回的是嵌套的 CompletableFuture<CompletableFuture<String>>，
+        // 需要两层 join 才能取出结果，层级一多就变成"回调地狱"
+        CompletableFuture<CompletableFuture<String>> nested =
+                userFuture.thenApply(Cf05_ThenCompose::getOrder);
+        System.out.println("thenApply 拼出嵌套: " + nested.getClass().getSimpleName());
+        System.out.println("  两层 join 取结果: " + nested.join().join());
+
+        // thenCompose 接受一个返回 CompletableFuture 的函数，
+        // 并将内层 Future 自动展平，保持链式调用只有一个层级
+        CompletableFuture<String> flat = userFuture.thenCompose(Cf05_ThenCompose::getOrder);
+        System.out.println("thenCompose 展平结果: " + flat.join());
+    }
+
+    // 模拟: 拿到用户后再异步查询他的订单
+    private static CompletableFuture<String> getOrder(String user) {
+        return CompletableFuture.supplyAsync(() -> {
+            System.out.println("2. 异步查询 \"" + user + "\" 的订单");
+            return "订单 O-10008";
+        });
+    }
+}