Generic classes with type parameters parsing/serializing

[zoltish]

Aloha!

For a big project of mine my data is stored as such:

abstract class A <T1, T2> {
    T1 first;
    T2 second;
    ...other simple fields...
}

class B<This,That>{
}

Since the scope for the data is so wide there are N number of classes that all depend on A like this. In GSON I use TypeAdapters to include the class name of each subclass in the json so that it can be read back properly, Id like to see something similar in LoganSquare.

I cannot get it to work the way LoganSquare is built at the moment (If its supported at all?) as it always tries to create instances of A directly.

Thanks!

[eirnym]

👍

[mannodermaus]

LoganSquare, at this point, is able to handle generics for the kind of scenario you describe. Just make sure to annotate both the abstract base class and its sub-classes properly and you're good to go:

@JsonObject
public abstract class A<T1, T2> {

    @JsonField
    T1 first;

    @JsonField
    T2 second;
}

@JsonObject
public class B extends A<String, String> {

    @JsonField
    int somethingElse;
}

With this setup, you can serialize and parse like this:

B model = new B("Item A", "Item B", 100);
String json = LoganSquare.serialize(model);
System.out.println(json);    // prints "{"somethingElse":100,"first":"Item A","second":"Item B"}"

If you need to include type information for each class as well (even though you don't need to with LoganSquare), this works by adding a Class @JsonField to the abstract base class with a custom type converter:

@JsonObject
public abstract class A<T1, T2> {

    // (the rest from above...)

    // Use "Class" without any type variable for the field; assign it e.g. in the constructor ("this.type = getClass();")
    @JsonField(typeConverter = ClassConverter.class)
    Class type;
}

public final class ClassConverter implements TypeConverter<Class> {

    @Override
    public Class parse(JsonParser jsonParser) throws IOException {
        try {
            return Class.forName(jsonParser.getValueAsString());
        } catch (ClassNotFoundException e) {
            throw new IOException(e);
        }
    }

    @Override
    public void serialize(Class object, String fieldName, boolean writeFieldNameForObject, JsonGenerator jsonGenerator) throws IOException {
        jsonGenerator.writeFieldName(fieldName);
        jsonGenerator.writeString(object.getCanonicalName());
    }
}

With this setup, the code snippet above prints {"somethingElse":100,"first":"Item A","second":"Item B","type":"your.full.package.name.B"}.