当使用Java Lambda(如Map <key1，Map <key2，List&lt; Obj&gt;&gt;)时如何在group-2中使用group-1结果key1. [英] How to use group-1 result key1 in group-2 when use Java Lambda like Map&lt;key1, Map&lt;key2, List&lt;Obj&gt;&gt;&gt;

问题描述

如何使用Java Lambda在group-2中使用group-1结果.

How to use group-1 result in group-2 by Java Lambda.

这个问题是我解决的问题的更新: 如何隐蔽列表到地图< T，< ； K，List< Person>>使用Java Lambda吗?

This question is update from my a solved problem: How to covert List to Map<T, <K, List<Person>>> use java lambda?

您可以看到最后的代码. 函数key2要使用函数key1的结果，该怎么做?

you can see the last code. the function key2 want to use function key1's result, how to do it?

class Person{
   int age;
   int cityCode;
   String name;
}

method:

// Map<age, Map<cityCode, List<Person>>>
public Map<Integer, Map<Integer, List<Person>>> covertListToMap(List<Person> list){

      // TODO: How to make List<Person> to Map<age, Map<cityCode, List<Person>>>

}

Function<Person, Integer> key1 = (Person p) -> {
            // do many sth then get a key1Result:
            int key1Result = p.getAge() * new Random(10).nextInt();
            return key1Result;
        };


Function<Person, Integer> key2 = (Person p) -> {
            //  Question: how to get and use Function key1 key1Result:

            int key1Result = 0;

            int result = p.getCityCode() + key1Result;
            return result;
        };

Map<Integer, Map<Integer, List<Person>>> collect = list.stream().collect(groupingBy(key1, groupingBy(key2)));

我写了一个错误的示例来显示我的意思:

I write a wrong sample to show my meaning:

Map<Integer, Map<Integer, List<Person>>> collect = list.stream().collect(groupingBy((Person p) -> {
            int key1Result = p.getAge() * new Random(10).nextInt();
            return key1Result;  // perhaps, now, key1Result is 10
        }, groupingBy((Person p) -> {

            // now, I want to get the key1Result which is 10, How to do it?  
            int key1Result = 0;

            int result = p.getCityCode() + key1Result;
            return result;
        })));

推荐答案

您可以使用函数apply(T t).根据 javadoc :

You can use the function apply(T t). According to the javadoc:

将此函数应用于给定参数.

Applies this function to the given argument.

对于您而言，这意味着:

In your case, it would mean:

int key1Result = key1.apply(p)

编辑:在您修改问题时，我添加了一个答案，如果我理解您想做的事，那应该是正确的:

Edit: As you modified your question, I add an answer, should be right if I understood what you wanted to do:

AtomicInteger atomicInteger = new AtomicInteger(0);
Map<Integer, Map<Integer, List<Person>>> collect = personList.stream().collect(Collectors
        .groupingBy(p -> {
                int age = p.getAge() * new Random(10).nextInt();
                atomicInteger.set(age);
                return age;
            }, Collectors.groupingBy(p2 -> p2.getCityCode() + atomicInteger.get())));

如果您想了解有关AtomicInteger的更多信息，请参见

If you want to read more about AtomicInteger, see atomic. Basically it is a wrapper that will keep its variable between threads.

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