有效的流的收集方法供应商

Question

我只是想尝试创建一些类似于Collectors.toList()我自己的东西,但似乎没有用

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;
}

}

我的问题是,如果ArrayList::new这里不起作用,将起作用.我尝试了不同的变化,但似乎都没有

Answer 1

只需改变它,

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

你需要的是一个BiConsumer不是BinaryOperator.你上面传递的是一个BinaryOperator.

这是一个例子BinaryOperator.

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

作为一种优秀的工程实践,总是更喜欢lambdas的方法引用.所以这里是使用方法引用代替lambdas的增强版本.

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

Answer 2

你似乎在寻找:

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

这与lambda表示相同:

tracings.stream()
        .collect(ArrayList::new, 
                 (left, right) -> left.add(right), 
                 (left, right) -> left.addAll(right)); // notice no 'return'

原因:Stream.collect期望a BiConsumer作为参数,其具有返回类型的accept方法void.