toMap提供三个参数

keyMapper

valueMapper

mergeFunction: BinaryOperator

其中mergeFunction是解决当key相同时，取哪个value的问题，由返回值指定，当返回null时，从map中移除当前entry

package stream;

import java.util.*;

import java.util.function.Function;

import java.util.stream.Collectors;

import static java.util.stream.Collectors.toSet;

public class ToMap {

public static void main(String[] args) {

List duplicateList = Arrays.asList("a", "bb", "c", "d", "bb");

Map map = duplicateList

.stream()

.collect(

Collectors.toMap(

Function.identity(),

s -> {

return s.length() + " " + (new Random()).nextInt();

},

(preItem, nextItem) -> {

System.out.println("item" + preItem + " curItem: " + nextItem);

return nextItem;

}

)

);

System.out.println(map);

List givenList = Arrays.asList("a", "bb", "dd", "ccc");

Map> groupList = givenList

.stream()

.collect(Collectors.groupingBy(

String::length,

toSet()

));

System.out.println(groupList);

}

}