过滤与分组

filter

private fun filter() {
    val mList = mutableListOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 0)
    val resultList = mList.filter {
        it > 5
    }
    println(resultList)
}
//[6, 7, 8, 9]

groupBy :

分组。即根据条件把集合拆分为为一个Map<K,List>类型的集合

private fun groupBy() {
    val mList = arrayListOf(0, 1, 2, 3, 4, 5, 6)
    val groupByList = mList.groupBy {
        //定义key
        it % 2 == 0
    }
    println(groupByList)
    //这里返回两个元素的map
    //{true=[0, 2, 4, 6], false=[1, 3, 5]}
    groupByList.entries.forEach {
        println("${it.key}==========${it.value}")
    }
    //true==========[0, 2, 4, 6]
    //false==========[1, 3, 5]

}

private fun groupBy2() {
    val mList = arrayListOf(0, 1, 2, 3, 4, 5, 6)
    val groupByList = mList.groupBy {
        //定义key
        if (it % 2 == 0) {
            "偶数"
        } else {
            "奇数"
        }
    }
    println(groupByList)
    //这里返回两个元素的map
    //{偶数=[0, 2, 4, 6], 奇数=[1, 3, 5]}
    groupByList.entries.forEach {
        println("${it.key}==========${it.value}")
    }
    //偶数==========[0, 2, 4, 6]
    //奇数==========[1, 3, 5]

}