I want to make gson able to return an EnumMap
object. I use the following code
package sandbox;
import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;
import java.util.EnumMap;
import java.util.Map;
/**
*
* @author yccheok
*/
public class Sandbox {
public static void main(String[] args) throws InterruptedException {
testGson();
}
public static enum Country {
Malaysia,
UnitedStates
}
public static void testGson() {
Map<Country, String> enumMap = new EnumMap<Country, String>(Country.class);
enumMap.put(Country.Malaysia, "RM");
enumMap.put(Country.UnitedStates, "USD");
Gson gson = new Gson();
String string = gson.toJson(enumMap);
System.out.println("toJSon : " + string);
enumMap = gson.fromJson(string, new TypeToken<EnumMap<Country, String>>(){}.getType());
System.out.println("fromJSon : " + enumMap);
System.out.println("fromJSon : " + enumMap.getClass());
}
}
However, I'm getting the following
toJSon : {"Malaysia":"RM","UnitedStates":"USD"}
fromJSon : {Malaysia=RM, UnitedStates=USD}
fromJSon : class java.util.LinkedHashMap
even though I had used new TypeToken<EnumMap<Country, String>>(){}.getType()
to specific I want EnumMap
instead of LinkedHashMap
How can I make gson to return EnumMap
?
Even with a type token, Gson can only deserialize data into classes that have a default constructor. And EnumMap
doesn't have one (it needs to be instantiated with the type of enum that its elements will match). The easiest way around this problem is to define and use an InstanceCreator
:
This interface is implemented to create instances of a class that does not define a no-args constructor. If you can modify the class, you should instead add a private, or public no-args constructor. However, that is not possible for library classes, such as JDK classes, or a third-party library that you do not have source-code of. In such cases, you should define an instance creator for the class. Implementations of this interface should be registered with GsonBuilder.registerTypeAdapter(Type, Object) method before Gson will be able to use them.
Heres some example code:
InstanceCreator:
class EnumMapInstanceCreator<K extends Enum<K>, V> implements
InstanceCreator<EnumMap<K, V>> {
private final Class<K> enumClazz;
public EnumMapInstanceCreator(final Class<K> enumClazz) {
super();
this.enumClazz = enumClazz;
}
@Override
public EnumMap<K, V> createInstance(final Type type) {
return new EnumMap<K, V>(enumClazz);
}
}
Test code:
final Gson gson = new GsonBuilder().registerTypeAdapter(
new TypeToken<EnumMap<Country, String>>() {
}.getType(),
new EnumMapInstanceCreator<Country, String>(Country.class))
.create();
final Map<Country, String> enumMap = new EnumMap<Country, String>(
Country.class);
enumMap.put(Country.Malaysia, "RM");
enumMap.put(Country.UnitedStates, "USD");
String string = gson.toJson(enumMap);
System.out.println("toJSon : " + string);
final Map<Country, String> reverseEnumMap = gson.fromJson(string,
new TypeToken<EnumMap<Country, String>>() {
}.getType());
System.out.println("fromJSon (Class): " + reverseEnumMap.getClass());
System.out.println("fromJSon : " + reverseEnumMap);
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