要实现Android中使用ExoPlayer 2平滑切换音乐并实现重叠，你可以使用以下代码示例：

首先，在build.gradle文件中添加ExoPlayer的依赖：

implementation 'com.google.android.exoplayer:exoplayer:2.X.X'

然后，你可以创建一个ExoPlayerHelper类，该类封装了ExoPlayer的常用操作和事件监听器。

import android.content.Context;
import android.net.Uri;
import android.os.Handler;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import com.google.android.exoplayer2.DefaultLoadControl;
import com.google.android.exoplayer2.ExoPlayer;
import com.google.android.exoplayer2.ExoPlayerFactory;
import com.google.android.exoplayer2.LoadControl;
import com.google.android.exoplayer2.SimpleExoPlayer;
import com.google.android.exoplayer2.extractor.DefaultExtractorsFactory;
import com.google.android.exoplayer2.source.ExtractorMediaSource;
import com.google.android.exoplayer2.source.LoopingMediaSource;
import com.google.android.exoplayer2.source.MediaSource;
import com.google.android.exoplayer2.source.TrackGroupArray;
import com.google.android.exoplayer2.trackselection.AdaptiveTrackSelection;
import com.google.android.exoplayer2.trackselection.DefaultTrackSelector;
import com.google.android.exoplayer2.trackselection.TrackSelectionArray;
import com.google.android.exoplayer2.trackselection.TrackSelector;
import com.google.android.exoplayer2.ui.PlayerView;
import com.google.android.exoplayer2.upstream.BandwidthMeter;
import com.google.android.exoplayer2.upstream.DefaultBandwidthMeter;
import com.google.android.exoplayer2.upstream.DefaultDataSourceFactory;
import com.google.android.exoplayer2.upstream.DataSource;
import com.google.android.exoplayer2.util.Util;

public class ExoPlayerHelper {

    private Context mContext;
    private SimpleExoPlayer mExoPlayer;
    private PlayerView mPlayerView;
    private MediaSource mMediaSource;
    private boolean mIsPlaying = false;

    public ExoPlayerHelper(Context context, PlayerView playerView) {
        mContext = context;
        mPlayerView = playerView;
        initializePlayer();
    }

    private void initializePlayer() {
        BandwidthMeter bandwidthMeter = new DefaultBandwidthMeter();
        TrackSelection.Factory videoTrackSelectionFactory = new AdaptiveTrackSelection.Factory(bandwidthMeter);
        TrackSelector trackSelector = new DefaultTrackSelector(videoTrackSelectionFactory);
        LoadControl loadControl = new DefaultLoadControl();
        mExoPlayer = ExoPlayerFactory.newSimpleInstance(mContext, trackSelector, loadControl);
        mPlayerView.setPlayer(mExoPlayer);
    }

    public void play(String url) {
        Uri uri = Uri.parse(url);
        DataSource.Factory dataSourceFactory = new DefaultDataSourceFactory(mContext,
                Util.getUserAgent(mContext, mContext.getApplicationInfo().name));
        mMediaSource = new ExtractorMediaSource.Factory(dataSourceFactory)
                .setExtractorsFactory(new DefaultExtractorsFactory())
                .createMediaSource(uri);
        mExoPlayer.prepare(mMediaSource);
        mExoPlayer.setPlayWhenReady(true);
        mIsPlaying = true;
    }

    public void stop() {
        if (mExoPlayer != null) {
            mExoPlayer.stop();
            mExoPlayer.release();
            mExoPlayer = null;
            mPlayerView.setPlayer(null);
            mIsPlaying = false;
        }
    }

    public boolean isPlaying() {
        return mIsPlaying;
    }

    public void setVolume(float volume) {
        if (mExoPlayer != null) {
            mExoPlayer.setVolume(volume);
        }
    }

    public void setLooping(boolean looping) {
        if (mMediaSource != null) {
            mMediaSource = new LoopingMediaSource(mMediaSource);
        }
    }

    public void setOnCompletionListener(final OnCompletionListener listener) {
        if (mExoPlayer != null) {
            mExoPlayer.addListener(new ExoPlayer.EventListener() {
                @Override
                public void onTimelineChanged(Timeline timeline, @Nullable Object manifest, int reason) {}

                @Override
                public void onTracksChanged(TrackGroupArray trackGroups, TrackSelectionArray trackSelections) {}

                @Override
                public void onLoadingChanged(boolean isLoading) {}

                @Override
                public void onPlayerStateChanged(boolean playWhenReady, int playbackState) {
                    if (playbackState == ExoPlayer.STATE_ENDED) {
                        listener.onCompletion();
                    }
                }

                @Override
                public void onPlayerError(ExoPlaybackException error) {}

                @Override
                public void onPositionDiscontinuity(int reason) {}
            });
        }
    }

    public interface OnCompletionListener {
        void on