Add example for custom qualifier with an element.

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

@@ -0,0 +1,140 @@
+/*
+ * 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.Json;
+import com.squareup.moshi.JsonAdapter;
+import com.squareup.moshi.JsonQualifier;
+import com.squareup.moshi.JsonReader;
+import com.squareup.moshi.JsonWriter;
+import com.squareup.moshi.Moshi;
+import com.squareup.moshi.Types;
+import java.io.IOException;
+import java.lang.annotation.Annotation;
+import java.lang.annotation.Retention;
+import java.lang.reflect.Type;
+import java.util.Collections;
+import java.util.LinkedHashSet;
+import java.util.Set;
+import javax.annotation.Nullable;
+
+import static java.lang.annotation.RetentionPolicy.RUNTIME;
+
+final class FallbackEnum {
+  @Retention(RUNTIME)
+  @JsonQualifier
+  public @interface Fallback {
+    /**
+     * The enum name.
+     */
+    String value();
+  }
+
+  public static final class FallbackEnumJsonAdapter<T extends Enum<T>> extends JsonAdapter<T> {
+    public static final Factory FACTORY = new Factory() {
+      @Nullable @Override @SuppressWarnings("unchecked")
+      public JsonAdapter<?> create(Type type, Set<? extends Annotation> annotations, Moshi moshi) {
+        Class<?> rawType = Types.getRawType(type);
+        if (!rawType.isEnum()) return null;
+        if (annotations.isEmpty()) {
+          return null;
+        }
+        Class<Enum> enumType = (Class<Enum>) rawType;
+        Set<? extends Annotation> delegateAnnotations = null;
+        Enum<?> fallback = null;
+        for (Annotation annotation : annotations) {
+          if (annotation instanceof Fallback) {
+            delegateAnnotations = new LinkedHashSet<>(annotations);
+            delegateAnnotations.remove(annotation);
+            delegateAnnotations = Collections.unmodifiableSet(delegateAnnotations);
+            fallback = Enum.valueOf(enumType, ((Fallback) annotation).value());
+            break;
+          }
+        }
+        if (delegateAnnotations == null) {
+          return null;
+        }
+        return new FallbackEnumJsonAdapter<>(enumType, fallback);
+      }
+    };
+
+    final Class<T> enumType;
+    final String[] nameStrings;
+    final T[] constants;
+    final JsonReader.Options options;
+    final T defaultValue;
+
+    FallbackEnumJsonAdapter(Class<T> enumType, T defaultValue) {
+      this.enumType = enumType;
+      this.defaultValue = defaultValue;
+      try {
+        constants = enumType.getEnumConstants();
+        nameStrings = new String[constants.length];
+        for (int i = 0; i < constants.length; i++) {
+          T constant = constants[i];
+          Json annotation = enumType.getField(constant.name()).getAnnotation(Json.class);
+          String name = annotation != null ? annotation.name() : constant.name();
+          nameStrings[i] = name;
+        }
+        options = JsonReader.Options.of(nameStrings);
+      } catch (NoSuchFieldException e) {
+        throw new AssertionError(e);
+      }
+    }
+
+    @Override public T fromJson(JsonReader reader) throws IOException {
+      int index = reader.selectString(options);
+      if (index != -1) return constants[index];
+      reader.nextString();
+      return defaultValue;
+    }
+
+    @Override public void toJson(JsonWriter writer, T value) throws IOException {
+      writer.value(nameStrings[value.ordinal()]);
+    }
+
+    @Override public String toString() {
+      return "JsonAdapter(" + enumType.getName() + ").defaultValue( " + defaultValue + ")";
+    }
+  }
+
+  static final class Example {
+    enum Transportation {
+      WALKING, BIKING, TRAINS, PLANES
+    }
+
+    @Fallback("WALKING") final Transportation transportation;
+
+    Example(Transportation transportation) {
+      this.transportation = transportation;
+    }
+
+    @Override public String toString() {
+      return transportation.toString();
+    }
+  }
+
+  public static void main(String[] args) throws Exception {
+    Moshi moshi = new Moshi.Builder()
+        .add(FallbackEnumJsonAdapter.FACTORY)
+        .build();
+    JsonAdapter<Example> adapter = moshi.adapter(Example.class);
+    System.out.println(adapter.fromJson("{\"transportation\":\"CARS\"}"));
+  }
+
+  private FallbackEnum() {
+  }
+}