diff --git a/extensions/okhttp/src/main/java/com/google/android/exoplayer2/ext/okhttp/OkHttpDataSource.java b/extensions/okhttp/src/main/java/com/google/android/exoplayer2/ext/okhttp/OkHttpDataSource.java
index e182e802fc..60aa19e364 100644
--- a/extensions/okhttp/src/main/java/com/google/android/exoplayer2/ext/okhttp/OkHttpDataSource.java
+++ b/extensions/okhttp/src/main/java/com/google/android/exoplayer2/ext/okhttp/OkHttpDataSource.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.ext.okhttp;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.net.Uri;
 import android.support.annotation.Nullable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/audio/AudioTrackPositionTracker.java b/library/core/src/main/java/com/google/android/exoplayer2/audio/AudioTrackPositionTracker.java
index 34cc88e794..0095001299 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/audio/AudioTrackPositionTracker.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/audio/AudioTrackPositionTracker.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.audio;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.media.AudioTimestamp;
 import android.media.AudioTrack;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/emsg/EventMessage.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/emsg/EventMessage.java
index a4a3b36e43..7d70d9de1c 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/emsg/EventMessage.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/emsg/EventMessage.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.emsg;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.os.Parcelable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ApicFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ApicFrame.java
index 7f5b8dca12..53976da0d1 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ApicFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ApicFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.os.Parcelable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/BinaryFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/BinaryFrame.java
index 449165fe07..c48829ae54 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/BinaryFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/BinaryFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.os.Parcelable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ChapterFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ChapterFrame.java
index a5f99de0fe..7ffb6d028c 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ChapterFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ChapterFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.support.annotation.Nullable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ChapterTocFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ChapterTocFrame.java
index 5797f7a237..c4a7c06e49 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ChapterTocFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/ChapterTocFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.support.annotation.Nullable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/CommentFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/CommentFrame.java
index ceba88f83f..5666e48939 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/CommentFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/CommentFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.os.Parcelable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/GeobFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/GeobFrame.java
index 2c4befcc12..990d8f2e48 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/GeobFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/GeobFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.os.Parcelable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/InternalFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/InternalFrame.java
index 5a8e2d64d5..c191676ce2 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/InternalFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/InternalFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.support.annotation.Nullable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/PrivFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/PrivFrame.java
index 7b8504229f..a10ce229d9 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/PrivFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/PrivFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.os.Parcelable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/TextInformationFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/TextInformationFrame.java
index b7f83d5269..62175ee90c 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/TextInformationFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/TextInformationFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.os.Parcelable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/UrlLinkFrame.java b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/UrlLinkFrame.java
index c3c5d9b16d..4b35131bea 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/UrlLinkFrame.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/metadata/id3/UrlLinkFrame.java
@@ -15,7 +15,7 @@
  */
 package com.google.android.exoplayer2.metadata.id3;
 
-import static org.checkerframework.checker.nullness.NullnessUtil.castNonNull;
+import static com.google.android.exoplayer2.util.Util.castNonNull;
 
 import android.os.Parcel;
 import android.os.Parcelable;
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/util/Assertions.java b/library/core/src/main/java/com/google/android/exoplayer2/util/Assertions.java
index 53c196a14f..c6ad5dfe52 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/util/Assertions.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/util/Assertions.java
@@ -104,6 +104,7 @@ public final class Assertions {
    * @return The non-null reference that was validated.
    * @throws NullPointerException If {@code reference} is null.
    */
+  @SuppressWarnings({"contracts.postcondition.not.satisfied", "return.type.incompatible"})
   @EnsuresNonNull({"#1"})
   public static <T> T checkNotNull(@Nullable T reference) {
     if (ExoPlayerLibraryInfo.ASSERTIONS_ENABLED && reference == null) {
@@ -122,6 +123,7 @@ public final class Assertions {
    * @return The non-null reference that was validated.
    * @throws NullPointerException If {@code reference} is null.
    */
+  @SuppressWarnings({"contracts.postcondition.not.satisfied", "return.type.incompatible"})
   @EnsuresNonNull({"#1"})
   public static <T> T checkNotNull(@Nullable T reference, Object errorMessage) {
     if (ExoPlayerLibraryInfo.ASSERTIONS_ENABLED && reference == null) {
@@ -137,6 +139,7 @@ public final class Assertions {
    * @return The non-null, non-empty string that was validated.
    * @throws IllegalArgumentException If {@code string} is null or 0-length.
    */
+  @SuppressWarnings({"contracts.postcondition.not.satisfied", "return.type.incompatible"})
   @EnsuresNonNull({"#1"})
   public static String checkNotEmpty(@Nullable String string) {
     if (ExoPlayerLibraryInfo.ASSERTIONS_ENABLED && TextUtils.isEmpty(string)) {
@@ -154,6 +157,7 @@ public final class Assertions {
    * @return The non-null, non-empty string that was validated.
    * @throws IllegalArgumentException If {@code string} is null or 0-length.
    */
+  @SuppressWarnings({"contracts.postcondition.not.satisfied", "return.type.incompatible"})
   @EnsuresNonNull({"#1"})
   public static String checkNotEmpty(@Nullable String string, Object errorMessage) {
     if (ExoPlayerLibraryInfo.ASSERTIONS_ENABLED && TextUtils.isEmpty(string)) {
diff --git a/library/core/src/main/java/com/google/android/exoplayer2/util/Util.java b/library/core/src/main/java/com/google/android/exoplayer2/util/Util.java
index 5a492fe987..2099645690 100644
--- a/library/core/src/main/java/com/google/android/exoplayer2/util/Util.java
+++ b/library/core/src/main/java/com/google/android/exoplayer2/util/Util.java
@@ -69,6 +69,7 @@ import java.util.concurrent.ThreadFactory;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
 import org.checkerframework.checker.initialization.qual.UnknownInitialization;
+import org.checkerframework.checker.nullness.qual.EnsuresNonNull;
 import org.checkerframework.checker.nullness.qual.PolyNull;
 
 /**
@@ -232,6 +233,17 @@ public final class Util {
     list.subList(fromIndex, toIndex).clear();
   }
 
+  /**
+   * Casts a nullable variable to a non-null variable without runtime null check.
+   *
+   * <p>Use {@link Assertions#checkNotNull(Object)} to throw if the value is null.
+   */
+  @SuppressWarnings({"contracts.postcondition.not.satisfied", "return.type.incompatible"})
+  @EnsuresNonNull("#1")
+  public static <T> T castNonNull(@Nullable T value) {
+    return value;
+  }
+
   /**
    * Copies and optionally truncates an array. Prevents null array elements created by {@link
    * Arrays#copyOf(Object[], int)} by ensuring the new length does not exceed the current length.
@@ -627,10 +639,10 @@ public final class Util {
   /**
    * Returns the index of the largest element in {@code list} that is less than (or optionally equal
    * to) a specified {@code value}.
-   * <p>
-   * The search is performed using a binary search algorithm, so the list must be sorted. If the
-   * list contains multiple elements equal to {@code value} and {@code inclusive} is true, the
-   * index of the first one will be returned.
+   *
+   * <p>The search is performed using a binary search algorithm, so the list must be sorted. If the
+   * list contains multiple elements equal to {@code value} and {@code inclusive} is true, the index
+   * of the first one will be returned.
    *
    * @param <T> The type of values being searched.
    * @param list The list to search.
@@ -643,8 +655,11 @@ public final class Util {
    * @return The index of the largest element in {@code list} that is less than (or optionally equal
    *     to) {@code value}.
    */
-  public static <T> int binarySearchFloor(List<? extends Comparable<? super T>> list, T value,
-      boolean inclusive, boolean stayInBounds) {
+  public static <T extends Comparable<? super T>> int binarySearchFloor(
+      List<? extends Comparable<? super T>> list,
+      T value,
+      boolean inclusive,
+      boolean stayInBounds) {
     int index = Collections.binarySearch(list, value);
     if (index < 0) {
       index = -(index + 2);
@@ -693,10 +708,10 @@ public final class Util {
   /**
    * Returns the index of the smallest element in {@code list} that is greater than (or optionally
    * equal to) a specified value.
-   * <p>
-   * The search is performed using a binary search algorithm, so the list must be sorted. If the
-   * list contains multiple elements equal to {@code value} and {@code inclusive} is true, the
-   * index of the last one will be returned.
+   *
+   * <p>The search is performed using a binary search algorithm, so the list must be sorted. If the
+   * list contains multiple elements equal to {@code value} and {@code inclusive} is true, the index
+   * of the last one will be returned.
    *
    * @param <T> The type of values being searched.
    * @param list The list to search.
@@ -705,13 +720,16 @@ public final class Util {
    *     index. If false then the returned index corresponds to the smallest element strictly
    *     greater than the value.
    * @param stayInBounds If true, then {@code (list.size() - 1)} will be returned in the case that
-   *     the value is greater than the largest element in the list. If false then
-   *     {@code list.size()} will be returned.
+   *     the value is greater than the largest element in the list. If false then {@code
+   *     list.size()} will be returned.
    * @return The index of the smallest element in {@code list} that is greater than (or optionally
    *     equal to) {@code value}.
    */
-  public static <T> int binarySearchCeil(List<? extends Comparable<? super T>> list, T value,
-      boolean inclusive, boolean stayInBounds) {
+  public static <T extends Comparable<? super T>> int binarySearchCeil(
+      List<? extends Comparable<? super T>> list,
+      T value,
+      boolean inclusive,
+      boolean stayInBounds) {
     int index = Collections.binarySearch(list, value);
     if (index < 0) {
       index = ~index;
@@ -970,7 +988,7 @@ public final class Util {
    * @param list A list of integers.
    * @return The list in array form, or null if the input list was null.
    */
-  public static int[] toArray(List<Integer> list) {
+  public static int @PolyNull [] toArray(@PolyNull List<Integer> list) {
     if (list == null) {
       return null;
     }
@@ -1053,15 +1071,15 @@ public final class Util {
   }
 
   /**
-   * Returns a copy of {@code codecs} without the codecs whose track type doesn't match
-   * {@code trackType}.
+   * Returns a copy of {@code codecs} without the codecs whose track type doesn't match {@code
+   * trackType}.
    *
    * @param codecs A codec sequence string, as defined in RFC 6381.
    * @param trackType One of {@link C}{@code .TRACK_TYPE_*}.
-   * @return A copy of {@code codecs} without the codecs whose track type doesn't match
-   *     {@code trackType}.
+   * @return A copy of {@code codecs} without the codecs whose track type doesn't match {@code
+   *     trackType}.
    */
-  public static String getCodecsOfType(String codecs, int trackType) {
+  public static @Nullable String getCodecsOfType(String codecs, int trackType) {
     String[] codecArray = splitCodecs(codecs);
     if (codecArray.length == 0) {
       return null;
@@ -1251,7 +1269,7 @@ public final class Util {
    *     "clearkey"}.
    * @return The derived {@link UUID}, or {@code null} if one could not be derived.
    */
-  public static UUID getDrmUuid(String drmScheme) {
+  public static @Nullable UUID getDrmUuid(String drmScheme) {
     switch (Util.toLowerInvariant(drmScheme)) {
       case "widevine":
         return C.WIDEVINE_UUID;
@@ -1427,7 +1445,7 @@ public final class Util {
    * @return The original value of the file name before it was escaped, or null if the escaped
    *     fileName seems invalid.
    */
-  public static String unescapeFileName(String fileName) {
+  public static @Nullable String unescapeFileName(String fileName) {
     int length = fileName.length();
     int percentCharacterCount = 0;
     for (int i = 0; i < length; i++) {
@@ -1473,8 +1491,9 @@ public final class Util {
 
   /** Recursively deletes a directory and its content. */
   public static void recursiveDelete(File fileOrDirectory) {
-    if (fileOrDirectory.isDirectory()) {
-      for (File child : fileOrDirectory.listFiles()) {
+    File[] directoryFiles = fileOrDirectory.listFiles();
+    if (directoryFiles != null) {
+      for (File child : directoryFiles) {
         recursiveDelete(child);
       }
     }