Cherry-Pick: Add a generic java8-like Optional class

For use with RxJava2 where null was to be returned (RxJava2 requires non-null).
Cherry-picked from PR #2954

core/src/main/java/de/danoeh/antennapod/core/util/Optional.java

@@ -0,0 +1,213 @@
+/*
+ * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+package de.danoeh.antennapod.core.util;
+
+import java.util.NoSuchElementException;
+import java.util.Objects;
+
+// AntennaPod's stripped-down version of Java/Android platform's java.util.Optional
+// so that it can be used on lower API level (API level 14)
+
+// Android-changed: removed ValueBased paragraph.
+/**
+ * A container object which may or may not contain a non-null value.
+ * If a value is present, {@code isPresent()} will return {@code true} and
+ * {@code get()} will return the value.
+ *
+ * <p>Additional methods that depend on the presence or absence of a contained
+ * value are provided, such as {@link #orElse(java.lang.Object) orElse()}
+ * (return a default value if value not present) and
+ * {@link #ifPresent(java.util.function.Consumer) ifPresent()} (execute a block
+ * of code if the value is present).
+ *
+ * @since 1.8
+ */
+public final class Optional<T> {
+    /**
+     * Common instance for {@code empty()}.
+     */
+    private static final Optional<?> EMPTY = new Optional<>();
+
+    /**
+     * If non-null, the value; if null, indicates no value is present
+     */
+    private final T value;
+
+    /**
+     * Constructs an empty instance.
+     *
+     * @implNote Generally only one empty instance, {@link Optional#EMPTY},
+     * should exist per VM.
+     */
+    private Optional() {
+        this.value = null;
+    }
+
+    /**
+     * Returns an empty {@code Optional} instance.  No value is present for this
+     * Optional.
+     *
+     * @apiNote Though it may be tempting to do so, avoid testing if an object
+     * is empty by comparing with {@code ==} against instances returned by
+     * {@code Option.empty()}. There is no guarantee that it is a singleton.
+     * Instead, use {@link #isPresent()}.
+     *
+     * @param <T> Type of the non-existent value
+     * @return an empty {@code Optional}
+     */
+    public static<T> Optional<T> empty() {
+        @SuppressWarnings("unchecked")
+        Optional<T> t = (Optional<T>) EMPTY;
+        return t;
+    }
+
+    /**
+     * Constructs an instance with the value present.
+     *
+     * @param value the non-null value to be present
+     * @throws NullPointerException if value is null
+     */
+    private Optional(T value) {
+        this.value = Objects.requireNonNull(value);
+    }
+
+    /**
+     * Returns an {@code Optional} with the specified present non-null value.
+     *
+     * @param <T> the class of the value
+     * @param value the value to be present, which must be non-null
+     * @return an {@code Optional} with the value present
+     * @throws NullPointerException if value is null
+     */
+    public static <T> Optional<T> of(T value) {
+        return new Optional<>(value);
+    }
+
+    /**
+     * Returns an {@code Optional} describing the specified value, if non-null,
+     * otherwise returns an empty {@code Optional}.
+     *
+     * @param <T> the class of the value
+     * @param value the possibly-null value to describe
+     * @return an {@code Optional} with a present value if the specified value
+     * is non-null, otherwise an empty {@code Optional}
+     */
+    public static <T> Optional<T> ofNullable(T value) {
+        return value == null ? empty() : of(value);
+    }
+
+    /**
+     * If a value is present in this {@code Optional}, returns the value,
+     * otherwise throws {@code NoSuchElementException}.
+     *
+     * @return the non-null value held by this {@code Optional}
+     * @throws NoSuchElementException if there is no value present
+     *
+     * @see Optional#isPresent()
+     */
+    public T get() {
+        if (value == null) {
+            throw new NoSuchElementException("No value present");
+        }
+        return value;
+    }
+
+    /**
+     * Return {@code true} if there is a value present, otherwise {@code false}.
+     *
+     * @return {@code true} if there is a value present, otherwise {@code false}
+     */
+    public boolean isPresent() {
+        return value != null;
+    }
+
+
+    /**
+     * Return the value if present, otherwise return {@code other}.
+     *
+     * @param other the value to be returned if there is no value present, may
+     * be null
+     * @return the value, if present, otherwise {@code other}
+     */
+    public T orElse(T other) {
+        return value != null ? value : other;
+    }
+
+    /**
+     * Indicates whether some other object is "equal to" this Optional. The
+     * other object is considered equal if:
+     * <ul>
+     * <li>it is also an {@code Optional} and;
+     * <li>both instances have no value present or;
+     * <li>the present values are "equal to" each other via {@code equals()}.
+     * </ul>
+     *
+     * @param obj an object to be tested for equality
+     * @return {code true} if the other object is "equal to" this object
+     * otherwise {@code false}
+     */
+    @Override
+    public boolean equals(Object obj) {
+        if (this == obj) {
+            return true;
+        }
+
+        if (!(obj instanceof Optional)) {
+            return false;
+        }
+
+        Optional<?> other = (Optional<?>) obj;
+        return (value == other.value) || (value != null && value.equals(other.value));
+    }
+
+    /**
+     * Returns the hash code value of the present value, if any, or 0 (zero) if
+     * no value is present.
+     *
+     * @return hash code value of the present value or 0 if no value is present
+     */
+    @Override
+    public int hashCode() {
+        return value != null ? value.hashCode() : 0;
+    }
+
+    /**
+     * Returns a non-empty string representation of this Optional suitable for
+     * debugging. The exact presentation format is unspecified and may vary
+     * between implementations and versions.
+     *
+     * @implSpec If a value is present the result must include its string
+     * representation in the result. Empty and present Optionals must be
+     * unambiguously differentiable.
+     *
+     * @return the string representation of this instance
+     */
+    @Override
+    public String toString() {
+        return value != null
+                ? String.format("Optional[%s]", value)
+                : "Optional.empty";
+    }
+}

core/src/main/java/de/danoeh/antennapod/core/util/playback/PlaybackController.java

@@ -12,7 +12,6 @@ import android.media.MediaPlayer;
 import android.os.Build;
 import android.os.IBinder;
 import android.support.annotation.NonNull;
-import android.support.annotation.Nullable;
 import android.support.v4.content.ContextCompat;
 import android.text.TextUtils;
 import android.util.Log;
@@ -37,6 +36,7 @@ import de.danoeh.antennapod.core.service.playback.PlaybackServiceMediaPlayer;
 import de.danoeh.antennapod.core.service.playback.PlayerStatus;
 import de.danoeh.antennapod.core.storage.DBTasks;
 import de.danoeh.antennapod.core.util.Converter;
+import de.danoeh.antennapod.core.util.Optional;
 import de.danoeh.antennapod.core.util.playback.Playable.PlayableUtils;
 import io.reactivex.Maybe;
 import io.reactivex.MaybeOnSubscribe;
@@ -187,13 +187,13 @@ public abstract class PlaybackController {
         serviceBinder = Observable.fromCallable(this::getPlayLastPlayedMediaIntent)
                 .subscribeOn(Schedulers.io())
                 .observeOn(AndroidSchedulers.mainThread())
-                .subscribe(intent -> {
+                .subscribe(optionalIntent -> {
                     boolean bound = false;
                     if (!PlaybackService.started) {
-                        if (intent != null) {
+                        if (optionalIntent.isPresent()) {
                             Log.d(TAG, "Calling start service");
-                            ContextCompat.startForegroundService(activity, intent);
-                            bound = activity.bindService(intent, mConnection, 0);
+                            ContextCompat.startForegroundService(activity, optionalIntent.get());
+                            bound = activity.bindService(optionalIntent.get(), mConnection, 0);
                         } else {
                             status = PlayerStatus.STOPPED;
                             setupGUI();
@@ -212,12 +212,13 @@ public abstract class PlaybackController {
      * Returns an intent that starts the PlaybackService and plays the last
      * played media or null if no last played media could be found.
      */
-    @Nullable private Intent getPlayLastPlayedMediaIntent() {
+    @NonNull
+    private Optional<Intent> getPlayLastPlayedMediaIntent() {
         Log.d(TAG, "Trying to restore last played media");
         Playable media = PlayableUtils.createInstanceFromPreferences(activity);
         if (media == null) {
             Log.d(TAG, "No last played media found");
-            return null;
+            return Optional.empty();
         }
 
         boolean fileExists = media.localFileAvailable();
@@ -226,10 +227,10 @@ public abstract class PlaybackController {
             DBTasks.notifyMissingFeedMediaFile(activity, (FeedMedia) media);
         }
 
-        return new PlaybackServiceStarter(activity, media)
+        return Optional.of(new PlaybackServiceStarter(activity, media)
                 .startWhenPrepared(false)
                 .shouldStream(lastIsStream || !fileExists)
-                .getIntent();
+                .getIntent());
     }