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

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Agora Video 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 [#understand-the-tech]

    Video 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 [#prerequisites]

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

    ## Implementation [#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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      <CodeBlockTab value="java">
        ```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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    1. 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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    </CodeBlockTabs>

    1. 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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    1. 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 [#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 [#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 [#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)