Document @Json and @JsonQualifier in the README.

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

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+/*
+ * Copyright (C) 2016 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.FromJson;
+import com.squareup.moshi.JsonAdapter;
+import com.squareup.moshi.JsonQualifier;
+import com.squareup.moshi.Moshi;
+import com.squareup.moshi.ToJson;
+import java.lang.annotation.Retention;
+
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+public final class CustomQualifier {
+  public void run() throws Exception {
+    String json = ""
+        + "{\n"
+        + "  \"color\": \"#ff0000\",\n"
+        + "  \"height\": 768,\n"
+        + "  \"width\": 1024\n"
+        + "}\n";
+
+    Moshi moshi = new Moshi.Builder()
+        .add(new ColorAdapter())
+        .build();
+    JsonAdapter<Rectangle> jsonAdapter = moshi.adapter(Rectangle.class);
+
+    Rectangle rectangle = jsonAdapter.fromJson(json);
+    System.out.println(rectangle);
+  }
+
+  public static void main(String[] args) throws Exception {
+    new CustomQualifier().run();
+  }
+
+  static class Rectangle {
+    int width;
+    int height;
+    @HexColor int color;
+
+    @Override public String toString() {
+      return String.format("%dx%d #%06x", width, height, color);
+    }
+  }
+
+  @Retention(RUNTIME)
+  @JsonQualifier
+  public @interface HexColor {
+  }
+
+  static class ColorAdapter {
+    @ToJson String toJson(@HexColor int rgb) {
+      return String.format("#%06x", rgb);
+    }
+
+    @FromJson @HexColor int fromJson(String rgb) {
+      return Integer.parseInt(rgb.substring(1), 16);
+    }
+  }
+}