Make Cache.getCachedSpans return empty set rather than null

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=181161289

library/core/src/androidTest/java/com/google/android/exoplayer2/upstream/cache/CachedRegionTrackerTest.java

@@ -15,12 +15,17 @@
  */
 package com.google.android.exoplayer2.upstream.cache;
 
+import static org.mockito.Mockito.any;
+import static org.mockito.Mockito.anyString;
+import static org.mockito.Mockito.when;
+
 import android.test.InstrumentationTestCase;
 import com.google.android.exoplayer2.extractor.ChunkIndex;
 import com.google.android.exoplayer2.testutil.MockitoUtil;
 import com.google.android.exoplayer2.util.Util;
 import java.io.File;
 import java.io.IOException;
+import java.util.TreeSet;
 import org.mockito.Mock;
 
 /**
@@ -48,6 +53,8 @@ public final class CachedRegionTrackerTest extends InstrumentationTestCase {
   protected void setUp() throws Exception {
     super.setUp();
     MockitoUtil.setUpMockito(this);
+    when(cache.addListener(anyString(), any(Cache.Listener.class)))
+        .thenReturn(new TreeSet<CacheSpan>());
     tracker = new CachedRegionTracker(cache, CACHE_KEY, CHUNK_INDEX);
     cacheDir = Util.createTempDirectory(getInstrumentation().getContext(), "ExoPlayerTest");
     index = new CachedContentIndex(cacheDir);

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/Cache.java

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.upstream.cache;
 
+import android.support.annotation.NonNull;
 import android.support.annotation.Nullable;
 import java.io.File;
 import java.io.IOException;
@@ -80,15 +81,16 @@ public interface Cache {
 
   /**
    * Registers a listener to listen for changes to a given key.
-   * <p>
-   * No guarantees are made about the thread or threads on which the listener is called, but it is
-   * guaranteed that listener methods will be called in a serial fashion (i.e. one at a time) and in
-   * the same order as events occurred.
+   *
+   * <p>No guarantees are made about the thread or threads on which the listener is called, but it
+   * is guaranteed that listener methods will be called in a serial fashion (i.e. one at a time) and
+   * in the same order as events occurred.
    *
    * @param key The key to listen to.
    * @param listener The listener to add.
    * @return The current spans for the key.
    */
+  @NonNull
   NavigableSet<CacheSpan> addListener(String key, Listener listener);
 
   /**
@@ -103,9 +105,10 @@ public interface Cache {
    * Returns the cached spans for a given cache key.
    *
    * @param key The key for which spans should be returned.
-   * @return The spans for the key. May be null if there are no such spans.
+   * @return The spans for the key.
    */
-  @Nullable NavigableSet<CacheSpan> getCachedSpans(String key);
+  @NonNull
+  NavigableSet<CacheSpan> getCachedSpans(String key);
 
   /**
    * Returns all keys in the cache.

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheUtil.java

@@ -253,9 +253,6 @@ public final class CacheUtil {
   /** Removes all of the data in the {@code cache} pointed by the {@code key}. */
   public static void remove(Cache cache, String key) {
     NavigableSet<CacheSpan> cachedSpans = cache.getCachedSpans(key);
-    if (cachedSpans == null) {
-      return;
-    }
     for (CacheSpan cachedSpan : cachedSpans) {
       try {
         cache.removeSpan(cachedSpan);

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/CachedRegionTracker.java

@@ -50,14 +50,12 @@ public final class CachedRegionTracker implements Cache.Listener {
 
     synchronized (this) {
       NavigableSet<CacheSpan> cacheSpans = cache.addListener(cacheKey, this);
-      if (cacheSpans != null) {
-        // Merge the spans into regions. mergeSpan is more efficient when merging from high to low,
-        // which is why a descending iterator is used here.
-        Iterator<CacheSpan> spanIterator = cacheSpans.descendingIterator();
-        while (spanIterator.hasNext()) {
-          CacheSpan span = spanIterator.next();
-          mergeSpan(span);
-        }
+      // Merge the spans into regions. mergeSpan is more efficient when merging from high to low,
+      // which is why a descending iterator is used here.
+      Iterator<CacheSpan> spanIterator = cacheSpans.descendingIterator();
+      while (spanIterator.hasNext()) {
+        CacheSpan span = spanIterator.next();
+        mergeSpan(span);
       }
     }
   }

library/core/src/main/java/com/google/android/exoplayer2/upstream/cache/SimpleCache.java

@@ -134,7 +134,8 @@ public final class SimpleCache implements Cache {
   @Override
   public synchronized NavigableSet<CacheSpan> getCachedSpans(String key) {
     CachedContent cachedContent = index.get(key);
-    return cachedContent == null || cachedContent.isEmpty() ? null
+    return cachedContent == null || cachedContent.isEmpty()
+        ? new TreeSet<CacheSpan>()
         : new TreeSet<CacheSpan>(cachedContent.getSpans());
   }
 

library/core/src/test/java/com/google/android/exoplayer2/upstream/cache/CacheAsserts.java

@@ -101,7 +101,8 @@ import java.util.ArrayList;
   public static void assertDataNotCached(Cache cache, String... uriStrings) {
     for (String uriString : uriStrings) {
       assertWithMessage("There is cached data for '" + uriString + "'")
-          .that(cache.getCachedSpans(CacheUtil.generateKey(Uri.parse(uriString)))).isNull();
+          .that(cache.getCachedSpans(CacheUtil.generateKey(Uri.parse(uriString))))
+          .isEmpty();
     }
   }
 

library/core/src/test/java/com/google/android/exoplayer2/upstream/cache/SimpleCacheTest.java

@@ -77,7 +77,7 @@ public class SimpleCacheTest {
 
     assertThat(simpleCache.getKeys()).isEmpty();
     NavigableSet<CacheSpan> cachedSpans = simpleCache.getCachedSpans(KEY_1);
-    assertThat(cachedSpans == null || cachedSpans.isEmpty()).isTrue();
+    assertThat(cachedSpans.isEmpty()).isTrue();
     assertThat(simpleCache.getCacheSpace()).isEqualTo(0);
     assertThat(cacheDir.listFiles()).hasLength(0);
 
@@ -283,7 +283,7 @@ public class SimpleCacheTest {
 
     // Although store() has failed, it should remove the first span and add the new one.
     NavigableSet<CacheSpan> cachedSpans = simpleCache.getCachedSpans(KEY_1);
-    assertThat(cachedSpans).isNotNull();
+    assertThat(cachedSpans).isNotEmpty();
     assertThat(cachedSpans).hasSize(1);
     assertThat(cachedSpans.pollFirst().position).isEqualTo(15);
   }

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

@@ -16,6 +16,7 @@
 package com.google.android.exoplayer2.testutil;
 
 import static junit.framework.Assert.assertEquals;
+import static junit.framework.Assert.assertTrue;
 
 import android.net.Uri;
 import android.test.MoreAsserts;
@@ -29,7 +30,6 @@ import com.google.android.exoplayer2.upstream.cache.CacheUtil;
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.util.ArrayList;
-import junit.framework.Assert;
 
 /**
  * Assertion methods for {@link Cache}.
@@ -105,8 +105,9 @@ public final class CacheAsserts {
   /** Asserts that there is no cache content for the given {@code uriStrings}. */
   public static void assertDataNotCached(Cache cache, String... uriStrings) {
     for (String uriString : uriStrings) {
-      Assert.assertNull("There is cached data for '" + uriString + "',",
-          cache.getCachedSpans(CacheUtil.generateKey(Uri.parse(uriString))));
+      assertTrue(
+          "There is cached data for '" + uriString + "',",
+          cache.getCachedSpans(CacheUtil.generateKey(Uri.parse(uriString))).isEmpty());
     }
   }