Convert AbstractContentMetadata to DefaultContentMetadata

As it turns out there isn't much gain by subclassing
AbstractContentMetadata. Moved abstract onChange method to a listener
interface.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188353673

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

@@ -15,6 +15,7 @@
  */
 package com.google.android.exoplayer2.upstream.cache;
 
+import android.support.annotation.Nullable;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.upstream.cache.Cache.CacheException;
 import com.google.android.exoplayer2.util.Assertions;
@@ -25,20 +26,45 @@ import java.util.Collections;
 import java.util.HashMap;
 import java.util.Map;
 
-/** Abstract implementation of {@link ContentMetadata}. Values are stored as byte arrays. */
-public abstract class AbstractContentMetadata implements ContentMetadata {
+/** Default implementation of {@link ContentMetadata}. Values are stored as byte arrays. */
+public final class DefaultContentMetadata implements ContentMetadata {
+
+  /** Listener for metadata change event. */
+  public interface ChangeListener {
+    /**
+     * Called when any metadata value is changed or removed.
+     *
+     * @param contentMetadata The reporting instance.
+     * @param metadataValues All metadata name, value pairs. It shouldn't be accessed out of this
+     *     method.
+     */
+    void onChange(DefaultContentMetadata contentMetadata, Map<String, byte[]> metadataValues)
+        throws CacheException;
+  }
 
   private final Map<String, byte[]> metadata;
+  @Nullable private ChangeListener changeListener;
 
-  protected AbstractContentMetadata() {
+  public DefaultContentMetadata() {
     this.metadata = new HashMap<>();
   }
 
-  /** @param metadata Initial name value pairs. */
-  protected AbstractContentMetadata(Map<String, byte[]> metadata) {
+  /**
+   * Constructs a {@link DefaultContentMetadata} using name, value pairs data which was passed to
+   * {@link ChangeListener#onChange(DefaultContentMetadata, Map)}.
+   *
+   * @param metadata Initial name, value pairs.
+   */
+  public DefaultContentMetadata(Map<String, byte[]> metadata) {
     this.metadata = new HashMap<>(metadata);
   }
 
+  /** Sets a {@link ChangeListener}. This method can be called once. */
+  public void setChangeListener(ChangeListener changeListener) {
+    Assertions.checkState(this.changeListener == null);
+    this.changeListener = Assertions.checkNotNull(changeListener);
+  }
+
   @Override
   public final Editor edit() {
     return new EditorImpl();
@@ -86,14 +112,6 @@ public abstract class AbstractContentMetadata implements ContentMetadata {
     }
   }
 
-  /**
-   * Called when any metadata value is changed or removed. {@code metadataValues} shouldn't be
-   * accessed out of this method.
-   *
-   * @param metadataValues All metadata name, value pairs.
-   */
-  protected abstract void onChange(Map<String, byte[]> metadataValues) throws CacheException;
-
   private void apply(ArrayList<String> removedValues, Map<String, byte[]> editedValues)
       throws CacheException {
     synchronized (metadata) {
@@ -101,7 +119,9 @@ public abstract class AbstractContentMetadata implements ContentMetadata {
         metadata.remove(removedValues.get(i));
       }
       metadata.putAll(editedValues);
-      onChange(Collections.unmodifiableMap(metadata));
+      if (changeListener != null) {
+        changeListener.onChange(this, Collections.unmodifiableMap(metadata));
+      }
     }
   }
 

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

@@ -20,6 +20,7 @@ import static com.google.common.truth.Truth.assertThat;
 import com.google.android.exoplayer2.C;
 import com.google.android.exoplayer2.upstream.cache.Cache.CacheException;
 import com.google.android.exoplayer2.upstream.cache.ContentMetadata.Editor;
+import com.google.android.exoplayer2.upstream.cache.DefaultContentMetadata.ChangeListener;
 import java.nio.charset.Charset;
 import java.util.ArrayList;
 import java.util.Collections;
@@ -30,14 +31,16 @@ import org.junit.Test;
 import org.junit.runner.RunWith;
 import org.robolectric.RobolectricTestRunner;
 
-/** Tests {@link AbstractContentMetadata}. */
+/** Tests {@link DefaultContentMetadata}. */
 @RunWith(RobolectricTestRunner.class)
-public class AbstractContentMetadataTest {
+public class DefaultContentMetadataTest {
 
-  private FakeAbstractContentMetadata contentMetadata;
+  private DefaultContentMetadata contentMetadata;
+  private FakeChangeListener changeListener;
 
   @Before
   public void setUp() throws Exception {
+    changeListener = new FakeChangeListener();
     contentMetadata = createAbstractContentMetadata();
   }
 
@@ -156,32 +159,31 @@ public class AbstractContentMetadataTest {
     editor.set("metadata name", "edited value");
     editor.remove("metadata name2");
     editor.commit();
-    assertThat(contentMetadata.remainingValues).containsExactly("metadata name", "metadata name3");
+    assertThat(changeListener.remainingValues).containsExactly("metadata name", "metadata name3");
   }
 
-  private FakeAbstractContentMetadata createAbstractContentMetadata(String... pairs) {
+  private DefaultContentMetadata createAbstractContentMetadata(String... pairs) {
     assertThat(pairs.length % 2).isEqualTo(0);
     HashMap<String, byte[]> map = new HashMap<>();
     for (int i = 0; i < pairs.length; i += 2) {
       map.put(pairs[i], getBytes(pairs[i + 1]));
     }
-    return new FakeAbstractContentMetadata(Collections.unmodifiableMap(map));
+    DefaultContentMetadata metadata = new DefaultContentMetadata(Collections.unmodifiableMap(map));
+    metadata.setChangeListener(changeListener);
+    return metadata;
   }
 
   private static byte[] getBytes(String value) {
     return value.getBytes(Charset.forName(C.UTF8_NAME));
   }
 
-  private static class FakeAbstractContentMetadata extends AbstractContentMetadata {
+  private static class FakeChangeListener implements ChangeListener {
 
     private ArrayList<String> remainingValues;
 
-    private FakeAbstractContentMetadata(Map<String, byte[]> metadataValues) {
-      super(metadataValues);
-    }
-
     @Override
-    protected void onChange(Map<String, byte[]> metadataValues) throws CacheException {
+    public void onChange(DefaultContentMetadata contentMetadata, Map<String, byte[]> metadataValues)
+        throws CacheException {
       remainingValues = new ArrayList<>(metadataValues.keySet());
     }
   }