Add Gemini overview and Vertex AI Gemini API snippets (#686)

Code snippets are from these existing pages:
https://developer.android.com/ai/gemini
https://developer.android.com/ai/vertex-ai-firebase

Co-authored-by: Kat Kuan <kkuan2011@users.noreply.github.com>

Author: kkuan2011

misc/src/main/java/com/example/snippets/ai/GeminiOverview.kt

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.snippets.ai
+
+import com.google.firebase.Firebase
+import com.google.firebase.ai.ai
+import com.google.firebase.ai.type.GenerativeBackend
+
+class GeminiOverview {
+
+    suspend fun generateContent() {
+        // [START android_gemini_ai_models_overview]
+        // For Vertex AI, use `backend = GenerativeBackend.vertexAI()`
+        val model = Firebase.ai(backend = GenerativeBackend.googleAI())
+            .generativeModel("gemini-2.5-flash")
+
+        val response = model.generateContent("Write a story about a magic backpack")
+        val output = response.text
+        // [END android_gemini_ai_models_overview]
+    }
+}

misc/src/main/java/com/example/snippets/ai/GeminiOverviewJava.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.snippets.ai;
+
+import com.google.common.util.concurrent.FutureCallback;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.google.firebase.ai.FirebaseAI;
+import com.google.firebase.ai.GenerativeModel;
+import com.google.firebase.ai.java.GenerativeModelFutures;
+import com.google.firebase.ai.type.Content;
+import com.google.firebase.ai.type.GenerateContentResponse;
+import com.google.firebase.ai.type.GenerativeBackend;
+import java.util.concurrent.Executor;
+
+class GeminiOverviewJava {
+
+    void generateContent(Executor executor) {
+        // [START android_gemini_ai_models_overview_java]
+        // For Vertex AI, use `backend = GenerativeBackend.vertexAI()`
+        GenerativeModel firebaseAI = FirebaseAI.getInstance(GenerativeBackend.googleAI())
+                .generativeModel("gemini-2.5-flash");
+
+        // Use the GenerativeModelFutures Java compatibility layer which offers
+        // support for ListenableFuture and Publisher APIs
+        GenerativeModelFutures model = GenerativeModelFutures.from(firebaseAI);
+
+        Content prompt = new Content.Builder()
+            .addText("Write a story about a magic backpack.")
+            .build();
+
+        ListenableFuture<GenerateContentResponse> response = model.generateContent(prompt);
+        Futures.addCallback(response, new FutureCallback<GenerateContentResponse>() {
+            @Override
+            public void onSuccess(GenerateContentResponse result) {
+                String resultText = result.getText();
+                // ...
+            }
+
+            @Override
+            public void onFailure(Throwable t) {
+                t.printStackTrace();
+            }
+        }, executor);
+        // [END android_gemini_ai_models_overview_java]
+    }
+}

misc/src/main/java/com/example/snippets/ai/VertexAiGeminiApi.kt

@@ -0,0 +1,41 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.snippets.ai
+
+import com.google.firebase.Firebase
+import com.google.firebase.ai.ai
+import com.google.firebase.ai.type.GenerativeBackend
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.launch
+
+class VertexAiGeminiApi {
+
+    // [START android_snippets_vertex_ai_gemini_api_model]
+    val model = Firebase.ai(backend = GenerativeBackend.vertexAI())
+        .generativeModel("gemini-2.5-flash")
+    // [END android_snippets_vertex_ai_gemini_api_model]
+
+    fun generateText(scope: CoroutineScope) {
+        // [START android_snippets_vertex_ai_generate_content]
+        // Note: generateContent() is a suspend function, which integrates well
+        // with existing Kotlin code.
+        scope.launch {
+            val response = model.generateContent("Write a story about a magic backpack.")
+        }
+        // [END android_snippets_vertex_ai_generate_content]
+    }
+}

misc/src/main/java/com/example/snippets/ai/VertexAiGeminiApiJava.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.snippets.ai;
+
+import com.google.common.util.concurrent.FutureCallback;
+import com.google.common.util.concurrent.Futures;
+import com.google.common.util.concurrent.ListenableFuture;
+import com.google.firebase.ai.FirebaseAI;
+import com.google.firebase.ai.GenerativeModel;
+import com.google.firebase.ai.java.GenerativeModelFutures;
+import com.google.firebase.ai.type.Content;
+import com.google.firebase.ai.type.GenerateContentResponse;
+import com.google.firebase.ai.type.GenerativeBackend;
+
+import java.util.concurrent.Executor;
+
+public class VertexAiGeminiApiJava {
+
+    // [START android_snippets_vertex_ai_gemini_api_model_java]
+    GenerativeModel firebaseAI = FirebaseAI.getInstance(GenerativeBackend.vertexAI())
+            .generativeModel("gemini-2.5-flash");
+
+    GenerativeModelFutures model = GenerativeModelFutures.from(firebaseAI);
+    // [END android_snippets_vertex_ai_gemini_api_model_java]
+
+    void generateText(Executor executor) {
+        // [START android_snippets_vertex_ai_generate_content_java]
+        Content prompt = new Content.Builder()
+                .addText("Write a story about a magic backpack.")
+                .build();
+
+        ListenableFuture<GenerateContentResponse> response = model.generateContent(prompt);
+        Futures.addCallback(response, new FutureCallback<GenerateContentResponse>() {
+            @Override
+            public void onSuccess(GenerateContentResponse result) {
+                String resultText = result.getText();
+                // ...
+            }
+
+            @Override
+            public void onFailure(Throwable t) {
+                t.printStackTrace();
+            }
+        }, executor);
+        // [END android_snippets_vertex_ai_generate_content_java]
+    }
+}