Implement subscriptions import/export

- Import subscriptions from YouTube and SoundCloud (all services that the extractor support)
- Import/export a JSON representation of the subscriptions
- [Minor] Remove some javax annotations in favor of the one provided by the android support library

app/src/test/java/org/schabi/newpipe/subscription/services/ImportExportJsonHelperTest.java

@@ -0,0 +1,117 @@
+package org.schabi.newpipe.subscription.services;
+
+import org.junit.Test;
+import org.schabi.newpipe.extractor.subscription.SubscriptionExtractor;
+import org.schabi.newpipe.extractor.subscription.SubscriptionItem;
+import org.schabi.newpipe.subscription.ImportExportJsonHelper;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.InputStream;
+import java.util.Arrays;
+import java.util.List;
+
+import static org.junit.Assert.assertTrue;
+import static org.junit.Assert.fail;
+
+/**
+ * @see ImportExportJsonHelper
+ */
+public class ImportExportJsonHelperTest {
+    @Test
+    public void testEmptySource() throws Exception {
+        String emptySource = "{\"app_version\":\"0.11.6\",\"app_version_int\": 47,\"subscriptions\":[]}";
+
+        List<SubscriptionItem> items = ImportExportJsonHelper.readFrom(new ByteArrayInputStream(emptySource.getBytes("UTF-8")), null);
+        assertTrue(items.isEmpty());
+    }
+
+    @Test
+    public void testInvalidSource() throws Exception {
+        List<String> invalidList = Arrays.asList(
+                "{}",
+                "",
+                null,
+                "gibberish");
+
+        for (String invalidContent : invalidList) {
+            try {
+                if (invalidContent != null) {
+                    byte[] bytes = invalidContent.getBytes("UTF-8");
+                    ImportExportJsonHelper.readFrom((new ByteArrayInputStream(bytes)), null);
+                } else {
+                    ImportExportJsonHelper.readFrom(null, null);
+                }
+
+                fail("didn't throw exception");
+            } catch (Exception e) {
+                boolean isExpectedException = e instanceof SubscriptionExtractor.InvalidSourceException;
+                assertTrue("\"" + e.getClass().getSimpleName() + "\" is not the expected exception", isExpectedException);
+            }
+        }
+    }
+
+    @Test
+    public void ultimateTest() throws Exception {
+        // Read from file
+        final List<SubscriptionItem> itemsFromFile = readFromFile();
+
+        // Test writing to an output
+        final String jsonOut = testWriteTo(itemsFromFile);
+
+        // Read again
+        final List<SubscriptionItem> itemsSecondRead = readFromWriteTo(jsonOut);
+
+        // Check if both lists have the exact same items
+        if (itemsFromFile.size() != itemsSecondRead.size()) {
+            fail("The list of items were different from each other");
+        }
+
+        for (int i = 0; i < itemsFromFile.size(); i++) {
+            final SubscriptionItem item1 = itemsFromFile.get(i);
+            final SubscriptionItem item2 = itemsSecondRead.get(i);
+
+            final boolean equals = item1.getServiceId() == item2.getServiceId() &&
+                    item1.getUrl().equals(item2.getUrl()) &&
+                    item1.getName().equals(item2.getName());
+
+            if (!equals) {
+                fail("The list of items were different from each other");
+            }
+        }
+    }
+
+    private List<SubscriptionItem> readFromFile() throws Exception {
+        final InputStream inputStream = getClass().getClassLoader().getResourceAsStream("import_export_test.json");
+        final List<SubscriptionItem> itemsFromFile = ImportExportJsonHelper.readFrom(inputStream, null);
+
+        if (itemsFromFile == null || itemsFromFile.isEmpty()) {
+            fail("ImportExportJsonHelper.readFrom(input) returned a null or empty list");
+        }
+
+        return itemsFromFile;
+    }
+
+    private String testWriteTo(List<SubscriptionItem> itemsFromFile) throws Exception {
+        final ByteArrayOutputStream out = new ByteArrayOutputStream();
+        ImportExportJsonHelper.writeTo(itemsFromFile, out, null);
+        final String jsonOut = out.toString("UTF-8");
+
+        if (jsonOut.isEmpty()) {
+            fail("JSON returned by writeTo was empty");
+        }
+
+        return jsonOut;
+    }
+
+    private List<SubscriptionItem> readFromWriteTo(String jsonOut) throws Exception {
+        final ByteArrayInputStream inputStream = new ByteArrayInputStream(jsonOut.getBytes("UTF-8"));
+        final List<SubscriptionItem> secondReadItems = ImportExportJsonHelper.readFrom(inputStream, null);
+
+        if (secondReadItems == null || secondReadItems.isEmpty()) {
+            fail("second call to readFrom returned an empty list");
+        }
+
+        return secondReadItems;
+    }
+}