Give FieldNamingStrategy the ability to return multiple String names

gson/src/main/java/com/google/gson/FieldNamingStrategy.java

@@ -16,7 +16,10 @@
 
 package com.google.gson;
 
+import com.google.gson.annotations.SerializedName;
 import java.lang.reflect.Field;
+import java.util.Collections;
+import java.util.List;
 
 /**
  * A mechanism for providing custom field naming in Gson. This allows the client code to translate
@@ -37,4 +40,15 @@
    * @since 1.3
    */
   public String translateName(Field f);
+
+  /**
+   * Translates the field name into its JSON field alternative names representation. used for
+   * deserialization only. This is similar to {@link SerializedName#alternate()}.
+   *
+   * @param f the field object that we are translating
+   * @return the list of possible translated field names.
+   */
+  default List<String> translateToAlternateNames(Field f) {
+    return Collections.emptyList();
+  }
 }

gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java

@@ -86,8 +86,12 @@ private boolean includeField(Field f, boolean serialize) {
   private List<String> getFieldNames(Field f) {
     SerializedName annotation = f.getAnnotation(SerializedName.class);
     if (annotation == null) {
-      String name = fieldNamingPolicy.translateName(f);
-      return Collections.singletonList(name);
+      String fieldName = fieldNamingPolicy.translateName(f);
+      List<String> alternates = fieldNamingPolicy.translateToAlternateNames(f);
+      List<String> fieldNames = new ArrayList<>(alternates.size() + 1);
+      fieldNames.add(fieldName);
+      fieldNames.addAll(alternates);
+      return fieldNames;
     }
 
     String serializedName = annotation.value();

gson/src/test/java/com/google/gson/functional/NamingPolicyTest.java

@@ -26,6 +26,7 @@
 import com.google.gson.common.TestTypes.ClassWithSerializedNameFields;
 import com.google.gson.common.TestTypes.StringWrapper;
 import java.lang.reflect.Field;
+import java.util.List;
 import java.util.Locale;
 import org.junit.Before;
 import org.junit.Test;
@@ -237,6 +238,55 @@ public void testAtSignInSerializedName() {
     assertThat(new Gson().toJson(new AtName())).isEqualTo("{\"@foo\":\"bar\"}");
   }
 
+  @Test
+  public void testGsonWithAlternateNamesDeserialiation() {
+    Gson gson =
+        builder
+            .setFieldNamingStrategy(
+                new FieldNamingStrategy() {
+
+                  @Override
+                  public String translateName(Field f) {
+                    return "badname";
+                  }
+
+                  @Override
+                  public List<String> translateToAlternateNames(Field f) {
+                    return List.of("SomeConstantStringInstanceField");
+                  }
+                })
+            .create();
+    String target = "{\"SomeConstantStringInstanceField\":\"someValue\"}";
+    StringWrapper deserializedObject = gson.fromJson(target, StringWrapper.class);
+    assertThat(deserializedObject.someConstantStringInstanceField).isEqualTo("someValue");
+  }
+
+  @Test
+  public void testGsonWithAlternateNamesSerialization() {
+    Gson gson =
+        builder
+            .setFieldNamingStrategy(
+                new FieldNamingStrategy() {
+
+                  @Override
+                  public String translateName(Field f) {
+                    return "some-constant-string-instance-field";
+                  }
+
+                  @Override
+                  public List<String> translateToAlternateNames(Field f) {
+                    return List.of("SomeConstantStringInstanceField");
+                  }
+                })
+            .create();
+    StringWrapper target = new StringWrapper("blah");
+    assertThat(gson.toJson(target))
+        .isEqualTo(
+            "{\"some-constant-string-instance-field\":\""
+                + target.someConstantStringInstanceField
+                + "\"}");
+  }
+
   static final class AtName {
     @SerializedName("@foo")
     String f = "bar";