Valid Supplier for collect method of stream

Question

I was just trying to create somewhat similar to Collectors.toList() of my own but it doesn't seem to work

import java.util.ArrayList;

public class ShipmentTracingDTO {

boolean destination = false;

public ShipmentTracingDTO(Boolean destination) {
    this.destination = destination;
}

public ShipmentTracingDTO() {
}

public static void main(String[] args) {
    ArrayList<ShipmentTracingDTO> tracings = new ArrayList<>();
    tracings.add(new ShipmentTracingDTO(true));
    tracings.add(new ShipmentTracingDTO(true));
    tracings.add(new ShipmentTracingDTO(false));
    tracings.add(new ShipmentTracingDTO(false));
    ArrayList<ShipmentTracingDTO> newTracings = new ArrayList<>();

// Error coming for ArrayList::new : The constructed object of type ArrayList is 
//incompatible with the descriptor's return type: R

    tracings.stream().collect(ArrayList::new, (left, right) -> left.add(right), (left, right) -> {
        left.addAll(right);
        return left;
    });
}

private boolean getDestination() {

    return destination;
}

}

My question is if ArrayList::new doesn't work here, what will work. I tried different variation but none seems to work

Answer 1

Just change it like so,

tracings.stream().collect(ArrayList::new, (left, right) -> left.add(right), (left, right) -> {
    left.addAll(right);
});

What you need is a BiConsumer NOT a BinaryOperator. What you are passing above is a BinaryOperator.

Here's an example BinaryOperator.

BinaryOperator<List<Integer>> s = (left, right) -> {
            left.addAll(right);
            return left;
};

As a good engineering practice, always prefer method references to lambdas. So here's the enhanced version using method references in place of lambdas.

tracings.stream().collect(ArrayList::new, List::add, List::addAll);