public static <S,T> Key<S,T> of(Class<S> sourceType, Class<T> targetType) {
            return new Key<S,T>(sourceType, targetType);
        }
Source and target conversion type pair.
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/DynamoDBTypeConverterFactory.java