# Join multiple channels (/en/realtime-media/rtc/build/join-and-manage-channels/join-multiple-channels/android)

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Agora RTC SDK enables you to simultaneously join multiple channels. This capability allows you to receive and publish audio and video streams across multiple channels concurrently.

      
## Understand the tech

RTC SDK's multi-channel functionality is based on two key components:

* `RtcConnection`

  The `RtcConnection` object identifies a connection. It contains the following information:

  * Channel name
  * User ID of the local user

  You create multiple `RtcConnection` objects, each with a different channel name and user ID. Each `RtcConnection` instance can independently publish multiple audio streams and a single video stream.

* `RtcEngineEx`

  The class contains methods tailored for interacting with a designated `RtcConnection` object.

  To join multiple channels, you call `joinChannelEx` method in the `RtcEngineEx` class multiple times, using a different `RtcConnection` instance each time.

When joining multiple channels:

* Ensure that the user ID for each `RtcConnection` object is unique and nonzero.

* Configure publishing and subscribing options for the `RtcConnection` object in `joinChannelEx`.

* Pass the `IRtcEngineEventHandler` object to the `eventHandler` parameter when calling the `joinChannelEx` method to receive multiple channel-related event notifications.

## Prerequisites

Ensure that you have implemented the [SDK quickstart](/en/realtime-media/rtc/get-started-sdk) in your project.

## Implementation

This section explains how to join a second channel as a host after you have already joined the first channel.

1. Declare variables for `RtcEngineEx` and `RtcConnection` objects.

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       ```java
       private RtcEngineEx engine;
       private RtcConnection rtcConnection2 = new RtcConnection();
       ```
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       ```kotlin
       private lateinit var engine: RtcEngineEx
       private val rtcConnection2 = RtcConnection()
       ```
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2. Initialize the engine instance.

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       ```java
       engine = (RtcEngineEx) RtcEngine.create(config);
       ```
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       ```kotlin
       engine = RtcEngine.create(config) as RtcEngineEx
       ```
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3. Join the channel using a random user ID.

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       ```java
       private boolean joinSecondChannel() {
          ChannelMediaOptions option = new ChannelMediaOptions();
          mediaOptions.autoSubscribeAudio = true;
          mediaOptions.autoSubscribeVideo = true;
          rtcConnection2.channelId = "channel-2";
          rtcConnection2.localUid = new Random().nextInt(512)+512;
          int ret = engine.joinChannelEx("your token", rtcConnection2, mediaOptions,
            iRtcEngineEventHandler2);
          return (ret == 0);
        }
       ```
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       ```kotlin
       private fun joinSecondChannel(): Boolean {
         val mediaOptions = ChannelMediaOptions().apply {
           autoSubscribeAudio = true
           autoSubscribeVideo = true
         }
         rtcConnection2.channelId = "channel-2"
         rtcConnection2.localUid = (Random().nextInt(512) + 512)
         val ret = engine.joinChannelEx("your token", rtcConnection2, mediaOptions, iRtcEngineEventHandler2)
         return ret == 0
       }
       ```
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4. Listen for events in `rtcConnection2` and set up the remote video in the `onUserJoined` callback.

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       ```java
       private final IRtcEngineEventHandler iRtcEngineEventHandler2 = new IRtcEngineEventHandler() {
         @Override
         public void onJoinChannelSuccess(String channel, int uid, int elapsed) {
           Log.i(TAG, String.format("channel2 onJoinChannelSuccess channel %s uid %d", channel2, uid));
           showLongToast(String.format("onJoinChannelSuccess channel %s uid %d", channel2, uid));
         }
         @Override
         public void onUserJoined(int uid, int elapsed) {
           Log.i(TAG, "channel2 onUserJoined->" + uid);
           showLongToast(String.format("user %d joined!", uid));
           Context context = getContext();
           if (context == null) {
             return;
           }
           handler.post(() ->
           {
             // Display the remote video stream
             SurfaceView surfaceView = null;
             if (fl_remote2.getChildCount() > 0) {
               fl_remote2.removeAllViews();
             }
             // Create the rendering view through RtcEngine
             surfaceView = new SurfaceView(context);
             surfaceView.setZOrderMediaOverlay(true);
             // Add the view to the remote container
             fl_remote2.addView(surfaceView, new FrameLayout.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT));

             // Set the remote view
             engine.setupRemoteVideoEx(new VideoCanvas(surfaceView, RENDER_MODE_FIT, uid), rtcConnection2);
           });
         }
       };
       ```
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       ```kotlin
       private val iRtcEngineEventHandler2 = object : IRtcEngineEventHandler() {
         override fun onJoinChannelSuccess(channel: String?, uid: Int, elapsed: Int) {
           Log.i(TAG, String.format("channel2 onJoinChannelSuccess channel %s uid %d", channel2, uid))
           showLongToast(String.format("onJoinChannelSuccess channel %s uid %d", channel2, uid))
         }

         override fun onUserJoined(uid: Int, elapsed: Int) {
           Log.i(TAG, "channel2 onUserJoined->$uid")
           showLongToast("user $uid joined!")

           val context = context
           if (context == null) {
             return
           }

           handler.post {
             // Display the remote video stream
             var surfaceView: SurfaceView? = null
             if (fl_remote2.childCount > 0) {
               fl_remote2.removeAllViews()
             }
             // Create the rendering view through RtcEngine
             surfaceView = SurfaceView(context)
             surfaceView?.zOrderMediaOverlay = true
             // Add the view to the remote container
             fl_remote2.addView(surfaceView, FrameLayout.LayoutParams(
               ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT))

             // Set the remote view
             engine.setupRemoteVideoEx(VideoCanvas(surfaceView, RENDER_MODE_FIT, uid), rtcConnection2)
           }
         }
       }
       ```
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## Reference

This section contains content that completes the information on this page, or points you to documentation that explains other aspects to this product.

### Sample project

Agora provides the [JoinMultipleChannels](https://github.com/AgoraIO/API-Examples/blob/main/Android/APIExample/app/src/main/java/io/agora/api/example/examples/advanced/JoinMultipleChannel.java) open-source sample project for your reference. Download the project or view the source code for a more detailed example.

### API reference

* [`RtcEngineEx`](https://api-ref.agora.io/en/video-sdk/android/4.x/API/class_irtcengineex.html#class_irtcengineex)
* [`RtcConnection`](https://api-ref.agora.io/en/video-sdk/android/4.x/API/class_rtcconnection.html)
* [`joinChannelEx`](https://api-ref.agora.io/en/video-sdk/android/4.x/API/class_irtcengineex.html#api_irtcengineex_joinchannelex)
* [`setupRemoteVideoEx`](https://api-ref.agora.io/en/video-sdk/android/4.x/API/class_irtcengineex.html#api_irtcengineex_setupremotevideoex)