在 toMap 中处理重复项时访问收集的对象

Question

在下面的示例中，我想打印出被丢弃的物品的 ID。有没有办法不仅获取值，还获取对象本身？

import java.util.*;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        // Example list with potential duplicates
        List<Item> items = Arrays.asList(
                new Item(1, "Item A"),
                new Item(2, "Item B"),
                new Item(1, "Item C"), // Duplicate key
                new Item(3, "Item D")
        );

        // Building the Map
        Map<Integer, String> itemMap = items.stream()
            .collect(Collectors.toMap(
                Item::getId,                 // Key mapper
                Item::getName,               // Value mapper
                (existing, incoming) -> {    // Merge function
                    // Print info about the duplicate
                    System.out.println("Duplicate key found. Existing value: " + existing + ", Incoming value: " + incoming);
                    // HOW CAN I PRINT THE INCOMING ELEMENTS ID HERE? <<<<<<<<<<<<
                    return incoming; // Keep the incoming value
                }
            ));

        // Print the resulting map
        System.out.println("Resulting Map: " + itemMap);
    }
}

class Item {
    private final int id;
    private final String name;

    public Item(int id, String name) {
        this.id = id;
        this.name = name;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "Item{id=" + id + ", name='" + name + "'}";
    }
}

Answer 1

您可以使用

Map<Integer, String> itemMap = items.stream()
    .collect(Collectors.groupingBy(
        Item::getId,                                         // Key mapper
        Collectors.collectingAndThen(
            Collectors.reducing((existing, incoming) -> {    // Merge function
            // Print info about the duplicate
            System.out.println("Duplicate key " + existing.getId()
                + " found. Existing value: " + existing.getName()
                + ", Incoming value: " + incoming.getName());
            return incoming; // Keep the incoming value
        }), o -> o.get().getName()                           // Value mapper
    )));

Duplicate key 1 found. Existing value: Item A, Incoming value: Item C
Resulting Map: {1=Item C, 2=Item B, 3=Item D}

但您也可以考虑传统循环：

Map<Integer, String> itemMap = new HashMap<>();
for(Item i: items) {
    String old = itemMap.put(i.getId(), i.getName());
    if(old != null) {
        System.out.println("Duplicate key " + i.getId()
          + " found. Existing value: " + old + ", Incoming value: " + i.getName());
    }
}