如何根据ID合并两个对象列表? [英] How to merge two lists of objects based on IDs?

问题描述

我有两个列表.一个称为 oldList 并包含旧数据，第二个称为 updateList 并包含一组更新.

I have two lists. One is called oldList and contains old data, the 2nd one is called updateList and contains a set of updates.

我想使用这些规则创建一个新列表:

I want to create a new list with those rules:

• 从 oldList
中获取所有数据
• 使用唯一的 id
附加所有更新
• 如果 oldData 中已经存在更新项目的 id ，请替换

• take all data from the oldList
• append all updates with unique id
• if id of an updated item already exists in oldData, replace it

我想出了一个简单的代码:

I came up with a simple code:

Item.kt

data class Item (val id: String, val text: String)

Main.kt

fun main() {

    val oldList: List<Item> = listOf(
        Item("aaa1", "aaa2"),
        Item("bbb1", "bbb2"),
        Item("ddd1", "ddd2"))

    val updateList: List<Item> = listOf(
        Item("aaa1", "aaa3"),
        Item("ccc1", "ccc2"))

    val resultList = oldList.toMutableList()

    for (item in updateList) {
        val index = oldList.indexOfFirst { it.id == item.id }
        if (index < 0) {
            resultList.add(item)
        } else {
            resultList[index] = item
        }
    }

    println(resultList)
}

这很好用，但我可以想象它效率低下，并且可能还有一些不错的 Kotlin 习惯用法.有更好的解决方案吗?

This works fine but I can imagine it's inefficient and there might also be some nice Kotlin idiom. Is there a better solution?

推荐答案

如果原始顺序无关紧要，则可以合并列表，将新值放在第一位，以便在 distinctBy 中使用它们致电:

If the original order doesn't matter, you can combine the lists, putting the new values first to favor them in a distinctBy call:

val resultList = (updateList + oldList).distinctBy(Item::id)

如果顺序完全符合您的描述，则可以在合并它们之前将两个列表都转换为地图.当使用 + 组合到Maps时，第二个Map中的项目优先，但是保留第一个Map中的顺序.

If the order matters exactly as you described it, you can convert both lists to maps before combining them. When combining to Maps with +, the items in the second Map take precedence, but the order from the first Map is preserved.

val resultList = 
    (oldList.associateBy(Item::id) + updateList.associateBy(Item::id)).values.toList()

这篇关于如何根据ID合并两个对象列表?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！