Create CreateTopicDemo.java

add example

Signed-off-by: SimiHunjan <simi.hunjan.k@gmail.com>

Author: SimiHunjan

.github/examples/java/CreateTopicDemo.java

@@ -0,0 +1,70 @@
+import com.hedera.hashgraph.sdk.*;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.net.URI;
+import com.google.gson.Gson;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonArray;
+
+public class CreateTopicDemo {
+    public static void main(String[] args ) throws Exception {
+        // load your operator credentials
+        String operatorId = System.getenv("OPERATOR_ID");
+        String operatorKey = System.getenv("OPERATOR_KEY");
+
+        // initialize the client for testnet
+        Client client = Client.forTestnet()
+            .setOperator(AccountId.fromString(operatorId), PrivateKey.fromString(operatorKey));
+
+        // build & execute the topic creation transaction
+        TopicCreateTransaction transaction = new TopicCreateTransaction()
+            .setTopicMemo("My first HCS topic");
+
+        TransactionResponse txResponse = transaction.execute(client);
+        TransactionReceipt receipt = txResponse.getReceipt(client);
+        TopicId topicId = receipt.topicId;
+
+        System.out.println("\nTopic created: " + topicId);
+
+        // build & execute the message submission transaction
+        String message = "Hello, Hedera!";
+
+        TopicMessageSubmitTransaction messageTransaction = new TopicMessageSubmitTransaction()
+            .setTopicId(topicId)
+            .setMessage(message);
+
+        messageTransaction.execute(client);
+
+        System.out.println("\nMessage submitted: " + message);
+
+        // wait for Mirror Node to populate data
+        System.out.println("\nWaiting for Mirror Node to update...");
+        Thread.sleep(6000);
+
+        // query messages using Mirror Node
+        String mirrorNodeUrl = "https://testnet.mirrornode.hedera.com/api/v1/topics/" + topicId + "/messages";
+
+        HttpClient httpClient = HttpClient.newHttpClient( );
+        HttpRequest request = HttpRequest.newBuilder()
+            .uri(URI.create(mirrorNodeUrl))
+            .build();
+
+        HttpResponse<String> response = httpClient.send(request, HttpResponse.BodyHandlers.ofString( ));
+        Gson gson = new Gson();
+        JsonObject data = gson.fromJson(response.body(), JsonObject.class);
+
+        if (data.has("messages") && data.getAsJsonArray("messages").size() > 0) {
+            JsonArray messages = data.getAsJsonArray("messages");
+            JsonObject latestMessage = messages.get(messages.size() - 1).getAsJsonObject();
+            String encodedMessage = latestMessage.get("message").getAsString();
+            String messageContent = new String(java.util.Base64.getDecoder().decode(encodedMessage)).trim();
+            
+            System.out.println("\nLatest message: " + messageContent + "\n");
+        } else {
+            System.out.println("No messages found yet in Mirror Node");
+        }
+
+        client.close();
+    }
+}