Writing a serializer for Gson using Java 8

Question

I have a own class called MyDate and want to write a serializer of it for Gson. This code works:

GsonBuilder builder = new GsonBuilder();
builder.registerTypeAdapter(MyDate.class, new JsonSerializer<MyDate>() {
    @Override
    public JsonElement serialize(MyDate date, Type typeOfSrc, JsonSerializationContext context) {
        return new JsonPrimitive(date.toString());
    }
});

However I want to use the power of Java 8 and therefore tried

builder.registerTypeAdapter(MyDate.class, (date, typeOfSrc, context) ->new JsonPrimitive(date.toString()));

But here eclipse tells me

The target type of this expression must be a functional interface

What's wrong about that Java 8 code?

Answer 1

In order to replace an anonymous class with a lambda, the parameter must be a Single Method Interface (SMI).

This is an interface with a single abstract method.

GsonBuilder.registerTypeAdaper takes an Object as the second argument.

You need to first assign your lambda then pass in the method:

final JsonSerializer<MyDate> serializer = (date, typeOfSrc, context) -> new JsonPrimitive(date.toString());

builder.registerTypeAdapter(MyDate.class, serializer);

This way you tell compiler which SMI you would like to implement.