Fix nullness warnings in testutil package

PiperOrigin-RevId: 327190676

library/common/src/main/java/com/google/android/exoplayer2/util/Util.java

@@ -405,6 +405,21 @@ public final class Util {
     return concatenation;
   }
 
+  /**
+   * Copies the contents of {@code list} into {@code array}.
+   *
+   * <p>{@code list.size()} must be the same as {@code array.length} to ensure the contents can be
+   * copied into {@code array} without leaving any nulls at the end.
+   *
+   * @param list The list to copy items from.
+   * @param array The array to copy items to.
+   */
+  @SuppressWarnings("nullness:toArray.nullable.elements.not.newarray")
+  public static <T> void nullSafeListToArray(List<T> list, T[] array) {
+    Assertions.checkState(list.size() == array.length);
+    list.toArray(array);
+  }
+
   /**
    * Creates a {@link Handler} on the current {@link Looper} thread.
    *

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeAdaptiveMediaPeriod.java

@@ -31,6 +31,7 @@ import com.google.android.exoplayer2.upstream.DefaultLoadErrorHandlingPolicy;
 import com.google.android.exoplayer2.upstream.TransferListener;
 import com.google.android.exoplayer2.util.Assertions;
 import com.google.android.exoplayer2.util.MimeTypes;
+import com.google.android.exoplayer2.util.Util;
 import java.util.ArrayList;
 import java.util.List;
 import org.checkerframework.checker.nullness.compatqual.NullableType;
@@ -48,7 +49,7 @@ public class FakeAdaptiveMediaPeriod extends FakeMediaPeriod
   @Nullable private final TransferListener transferListener;
   private final long durationUs;
 
-  @MonotonicNonNull private Callback callback;
+  private @MonotonicNonNull Callback callback;
   private ChunkSampleStream<FakeChunkSource>[] sampleStreams;
   private SequenceableLoader sequenceableLoader;
 
@@ -99,7 +100,7 @@ public class FakeAdaptiveMediaPeriod extends FakeMediaPeriod
       }
     }
     sampleStreams = newSampleStreamArray(validStreams.size());
-    validStreams.toArray(sampleStreams);
+    Util.nullSafeListToArray(validStreams, sampleStreams);
     this.sequenceableLoader = new CompositeSequenceableLoader(sampleStreams);
     return returnPositionUs;
   }

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeDataSource.java

@@ -217,8 +217,7 @@ public class FakeDataSource extends BaseDataSource {
    * this method.
    */
   public final DataSpec[] getAndClearOpenedDataSpecs() {
-    DataSpec[] dataSpecs = new DataSpec[openedDataSpecs.size()];
+    DataSpec[] dataSpecs = openedDataSpecs.toArray(new DataSpec[0]);
-    openedDataSpecs.toArray(dataSpecs);
     openedDataSpecs.clear();
     return dataSpecs;
   }