diff --git a/libraries/muxer/src/main/java/androidx/media3/muxer/DefaultMp4Writer.java b/libraries/muxer/src/main/java/androidx/media3/muxer/DefaultMp4Writer.java
index be88416aab..c365a75200 100644
--- a/libraries/muxer/src/main/java/androidx/media3/muxer/DefaultMp4Writer.java
+++ b/libraries/muxer/src/main/java/androidx/media3/muxer/DefaultMp4Writer.java
@@ -30,16 +30,24 @@ import com.google.common.collect.Range;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
+import java.util.ArrayList;
 import java.util.Collections;
+import java.util.List;
 import java.util.concurrent.atomic.AtomicBoolean;
 
 /**
  * The default implementation of {@link Mp4Writer} which writes all the samples in a single mdat
  * box.
  */
-/* package */ final class DefaultMp4Writer extends Mp4Writer {
+/* package */ final class DefaultMp4Writer implements Mp4Writer {
   private static final long INTERLEAVE_DURATION_US = 1_000_000L;
 
+  private final FileOutputStream outputStream;
+  private final FileChannel output;
+  private final Mp4MoovStructure moovGenerator;
+  private final AnnexBToAvccConverter annexBToAvccConverter;
+  private final List<Track> tracks;
   private final AtomicBoolean hasWrittenSamples;
 
   private long mdatStart;
@@ -62,7 +70,11 @@ import java.util.concurrent.atomic.AtomicBoolean;
       FileOutputStream outputStream,
       Mp4MoovStructure moovGenerator,
       AnnexBToAvccConverter annexBToAvccConverter) {
-    super(outputStream, moovGenerator, annexBToAvccConverter);
+    this.outputStream = outputStream;
+    this.output = outputStream.getChannel();
+    this.moovGenerator = moovGenerator;
+    this.annexBToAvccConverter = annexBToAvccConverter;
+    tracks = new ArrayList<>();
     hasWrittenSamples = new AtomicBoolean(false);
     lastMoovWritten = Range.closed(0L, 0L);
   }
diff --git a/libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Writer.java b/libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Writer.java
index 6ccf326108..9e33d576d6 100644
--- a/libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Writer.java
+++ b/libraries/muxer/src/main/java/androidx/media3/muxer/FragmentedMp4Writer.java
@@ -35,6 +35,7 @@ import com.google.common.collect.ImmutableList;
 import java.io.FileOutputStream;
 import java.io.IOException;
 import java.nio.ByteBuffer;
+import java.nio.channels.FileChannel;
 import java.util.ArrayList;
 import java.util.List;
 import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
@@ -43,7 +44,7 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
  * An {@link Mp4Writer} implementation which writes samples into multiple fragments as per the
  * fragmented MP4 (ISO/IEC 14496-12) standard.
  */
-/* package */ final class FragmentedMp4Writer extends Mp4Writer {
+/* package */ final class FragmentedMp4Writer implements Mp4Writer {
   /** Provides a limited set of sample metadata. */
   public static class SampleMetadata {
     public final long durationVu;
@@ -57,6 +58,11 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
     }
   }
 
+  private final FileOutputStream outputStream;
+  private final FileChannel output;
+  private final Mp4MoovStructure moovGenerator;
+  private final AnnexBToAvccConverter annexBToAvccConverter;
+  private final List<Track> tracks;
   private final int fragmentDurationUs;
 
   private @MonotonicNonNull Track videoTrack;
@@ -70,7 +76,11 @@ import org.checkerframework.checker.nullness.qual.MonotonicNonNull;
       Mp4MoovStructure moovGenerator,
       AnnexBToAvccConverter annexBToAvccConverter,
       int fragmentDurationUs) {
-    super(outputStream, moovGenerator, annexBToAvccConverter);
+    this.outputStream = outputStream;
+    this.output = outputStream.getChannel();
+    this.moovGenerator = moovGenerator;
+    this.annexBToAvccConverter = annexBToAvccConverter;
+    tracks = new ArrayList<>();
     this.fragmentDurationUs = fragmentDurationUs;
     minInputPresentationTimeUs = Long.MAX_VALUE;
     currentFragmentSequenceNumber = 1;
diff --git a/libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Writer.java b/libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Writer.java
index 63a1db2e16..e20eef2c7a 100644
--- a/libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Writer.java
+++ b/libraries/muxer/src/main/java/androidx/media3/muxer/Mp4Writer.java
@@ -24,52 +24,24 @@ import androidx.media3.common.Format;
 import androidx.media3.common.MimeTypes;
 import androidx.media3.muxer.Mp4Muxer.TrackToken;
 import com.google.common.collect.ImmutableList;
-import java.io.FileOutputStream;
 import java.io.IOException;
 import java.nio.ByteBuffer;
-import java.nio.channels.FileChannel;
 import java.util.ArrayDeque;
 import java.util.ArrayList;
 import java.util.Deque;
 import java.util.List;
 
 /** Writes MP4 data to the disk. */
-/* package */ abstract class Mp4Writer {
-  protected final FileOutputStream outputStream;
-  protected final FileChannel output;
-  protected final Mp4MoovStructure moovGenerator;
-  protected final AnnexBToAvccConverter annexBToAvccConverter;
-  protected final List<Track> tracks;
+/* package */ interface Mp4Writer {
 
-  /**
-   * Creates an instance.
-   *
-   * @param outputStream The {@link FileOutputStream} to write the data to.
-   * @param moovGenerator An {@link Mp4MoovStructure} instance to generate the moov box.
-   * @param annexBToAvccConverter The {@link AnnexBToAvccConverter} to be used to convert H.264 and
-   *     H.265 NAL units from the Annex-B format (using start codes to delineate NAL units) to the
-   *     AVCC format (which uses length prefixes).
-   */
-  public Mp4Writer(
-      FileOutputStream outputStream,
-      Mp4MoovStructure moovGenerator,
-      AnnexBToAvccConverter annexBToAvccConverter) {
-    this.outputStream = outputStream;
-    this.output = outputStream.getChannel();
-    this.moovGenerator = moovGenerator;
-    this.annexBToAvccConverter = annexBToAvccConverter;
-    tracks = new ArrayList<>();
-  }
+  TrackToken addTrack(int sortKey, Format format);
 
-  public abstract TrackToken addTrack(int sortKey, Format format);
+  void writeSampleData(Mp4Muxer.TrackToken token, ByteBuffer byteBuffer, BufferInfo bufferInfo)
+      throws IOException;
 
-  public abstract void writeSampleData(
-      Mp4Muxer.TrackToken token, ByteBuffer byteBuffer, BufferInfo bufferInfo) throws IOException;
+  void close() throws IOException;
 
-  public abstract void close() throws IOException;
-
-  protected static class Track
-      implements Mp4Muxer.TrackToken, Mp4MoovStructure.TrackMetadataProvider {
+  class Track implements Mp4Muxer.TrackToken, Mp4MoovStructure.TrackMetadataProvider {
     public final Format format;
     public final int sortKey;
     public final List<BufferInfo> writtenSamples;