How does a method with multiple parameters get passed to a Function that takes a List?

serah :

I am not able to understand how the function gets passed via a lambda into this method

public class OrderUtil {
 public static <I, O> List<O> runInBatches(List<I> inputList, 
 Function<List<I>, List<O>> functionToRunInBatches) {
    return Lists.partition(inputList, BATCH_CHUNK_SIZE).stream()
            .flatMap(batch -> functionToRunInBatches.apply(batch).stream())
            .collect(toList());
    }
}

I see the below code, I am not able to understand how the lambda function below translates to functionToRunInBatches above? orderDao.getOrderForDates(...) takes three parameters (orders, startdate, enddate) but my function takes a list and returns a list. How does this call work fine?

I have read the tutorials and documentation on Function.

Would it be possible for someone to break down how the lambda gets mapped to the Function above? I am unable to visualise how this ends up working.

 private List<Order> getOrderForDates(List<Long> orderNumbers, 
                                           tring startDate, String endDate){
    return OrderUtil.runInBatches(orderNumbers,
            orderBatch -> orderDAO.getOrderForDates(orderBatch, startDate, endDate));
}

Max Vollmer :

The lambda is turned into a new Function object by the compiler. It overrides the apply method with the code given in the lambda expression.

So this:

private List<Order> getOrderForDates(List<Long> orderNumbers, String startDate, String endDate){
    return OrderUtil.runInBatches(orderNumbers, orderBatch -> orderDAO.getOrderForDates(orderBatch, startDate, endDate));
}

is equivalent to this:

private List<Order> getOrderForDates(List<Long> orderNumbers, String startDate, String endDate){
    return OrderUtil.runInBatches(orderNumbers, new Function<List<Long>, List<Order>>() {
        @Override
        public List<Order> apply(List<Long> orderBatch) {
            return orderDAO.getOrderForDates(orderBatch, startDate, endDate);
        }
    });
}

Your runInBatches method then simply calls apply on that Function object.