ByteStrings example. (#419)

Closes: https://github.com/square/moshi/issues/31

examples/src/main/java/com/squareup/moshi/recipes/ByteStrings.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright (C) 2018 Square, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.squareup.moshi.recipes;
+
+import com.squareup.moshi.JsonAdapter;
+import com.squareup.moshi.JsonReader;
+import com.squareup.moshi.JsonWriter;
+import com.squareup.moshi.Moshi;
+import java.io.IOException;
+import okio.ByteString;
+
+public final class ByteStrings {
+  public void run() throws Exception {
+    String json = "\"TW9zaGksIE9saXZlLCBXaGl0ZSBDaGluPw\"";
+
+    Moshi moshi = new Moshi.Builder()
+        .add(ByteString.class, new Base64ByteStringAdapter())
+        .build();
+    JsonAdapter<ByteString> jsonAdapter = moshi.adapter(ByteString.class);
+
+    ByteString byteString = jsonAdapter.fromJson(json);
+    System.out.println(byteString);
+  }
+
+  /**
+   * Formats byte strings using <a href="http://www.ietf.org/rfc/rfc2045.txt">Base64</a>. No line
+   * breaks or whitespace is included in the encoded form.
+   */
+  public final class Base64ByteStringAdapter extends JsonAdapter<ByteString> {
+    @Override public ByteString fromJson(JsonReader reader) throws IOException {
+      String base64 = reader.nextString();
+      return ByteString.decodeBase64(base64);
+    }
+
+    @Override public void toJson(JsonWriter writer, ByteString value) throws IOException {
+      String string = value.base64();
+      writer.value(string);
+    }
+  }
+
+  public static void main(String[] args) throws Exception {
+    new ByteStrings().run();
+  }
+}