[转载] java8 lambda表达式 List转为Map

参考链接： 使用Lambda表达式检查字符串在Java中是否仅包含字母

public static void main(String[] args) {

List<User> userList = new ArrayList<User>();

User user0 = new User("han1", "男1", 20);

User user1 = new User("han1", "男", 25);

User user2 = new User("han2", "男", 21);

User user3 = new User("han3", "男", 22);

User user4 = new User("han4", "男", 23);

User user5 = new User("han5", "男", 24);

userList.add(user0);

userList.add(user1);

userList.add(user2);

userList.add(user3);

userList.add(user4);

userList.add(user5);

/**

* list 转map

* 注意：要是key重复的话 会报错Duplicate key ....

* key name 都是han1

* 可以用 (k1,k2)->k1 来设置，如果有重复的key,则保留key1,舍弃key2

*/

// Arrays.stream(wechatPrintInfos.toArray()).forEach(e -> {

//OrderNolist.add(e.);

// });

// Map<Long, OrderAddress> addressListMap = addressList.stream().collect(Collectors.toMap(OrderAddress::getOrderNo, p -> p, (key1, key2) -> key1));

// Map<Integer, String> dictMap = Arrays.stream(addressList).filter(k -> null != k.getOrderNo()).collect(Collectors.toMap(e -> e.getDictKey().intValue(), DictionaryValueVo::getDictValue));

// orderAcceptList.stream().forEach(row->{

//orderAcceptService.updateByPrimaryKey(row);

// });

// List<Long> OrderNolist = new ArrayList<Long>();

// printService.queryPrintInfo(customerCode).stream().forEach(p ->

//OrderNolist.add(p.getOrderNo())

// );

System.out.println("userList="+JSONObject.toJSONString(userList));

Map<String,Object> compMap= userList.stream().collect(Collectors.toMap(User::getUserName, a -> a.getAge() + "===" + a.getSex(), (k1, k2) -> k1));

for(Object obj : compMap.keySet()) {

String key = (String) obj;//取到每一个key值

String value = (String) compMap.get(key);

System.out.println(key + "=" + value);

}

/**

* list中以某个属性分组，比如用name分组

*/

Map<String,List<User>> map= userList.stream().collect(Collectors.groupingBy(User::getUserName));

System.out.println(map);

}

下面是看的别人的Demo

package com.mc.day1.lambda;

import org.junit.Before;

import org.junit.Test;

import java.util.ArrayList;

import java.util.Arrays;

import java.util.List;

import java.util.Optional;

import java.util.stream.Stream;

/**

* =================规约与收集==================练习

*/

public class Lambda4 {

List<Transaction> transactions = null;

@Before

public void before(){

Trader raoul = new Trader("Raoul", "Cambridge");

Trader mario = new Trader("Mario", "Milan");

Trader alan = new Trader("Alan", "Cambridge");

Trader brian = new Trader("Brian", "Cambridge");

transactions = Arrays.asList(

new Transaction(brian, , 300),

new Transaction(raoul, , 1000),

new Transaction(raoul, , 400),

new Transaction(mario, , 710),

new Transaction(mario, , 700),

new Transaction(alan, , 950)

);

}

//1. 找出发生的所有交易， 并按交易额排序（从低到高）

@Test

public void test1(){

transactions.stream()

.filter((t) -> t.getYear() == )

.sorted((t1, t2) -> pare(t1.getValue(), t2.getValue()))

.forEach(System.out::println);

}

//2. 交易员都在哪些不同的城市工作过？

@Test

public void test2(){

transactions.stream()

.map((t) -> t.getTrader().getCity())

.distinct()

.forEach(System.out::println);

}

//3. 查找所有来自剑桥的交易员，并按姓名排序

@Test

public void test3(){

transactions.stream()

.filter((t) -> t.getTrader().getCity().equals("Cambridge"))

.map(Transaction::getTrader)

.sorted((t1, t2) -> t1.getName().compareTo(t2.getName()))

.distinct()

.forEach(System.out::println);

}

//4. 返回所有交易员的姓名字符串，按字母顺序排序

@Test

public void test4(){

transactions.stream()

.map((t) -> t.getTrader().getName())

.sorted()

.forEach(System.out::println);

System.out.println("-----------------------------------");

String str = transactions.stream()

.map((t) -> t.getTrader().getName())

.sorted()

.reduce("", String::concat);

System.out.println(str);

System.out.println("------------------------------------");

transactions.stream()

.map((t) -> t.getTrader().getName())

.flatMap(Lambda4::filterCharacter)

.sorted((s1, s2) -> pareToIgnoreCase(s2))

.forEach(System.out::print);

}

public static Stream<String> filterCharacter(String str){

List<String> list = new ArrayList<>();

for (Character ch : str.toCharArray()) {

list.add(ch.toString());

}

return list.stream();

}

//5. 有没有交易员是在米兰工作的？

@Test

public void test5(){

boolean bl = transactions.stream()

.anyMatch((t) -> t.getTrader().getCity().equals("Milan"));

System.out.println(bl);

}

//6. 打印生活在剑桥的交易员的所有交易额

@Test

public void test6(){

Optional<Integer> sum = transactions.stream()

.filter((e) -> e.getTrader().getCity().equals("Cambridge"))

.map(Transaction::getValue)

.reduce(Integer::sum);

System.out.println(sum.get());

}

//7. 所有交易中，最高的交易额是多少

@Test

public void test7(){

Optional<Integer> max = transactions.stream()

.map((t) -> t.getValue())

.max(Integer::compare);

System.out.println(max.get());

}

//8. 找到交易额最小的交易

@Test

public void test8(){

Optional<Transaction> op = transactions.stream()

.min((t1, t2) -> pare(t1.getValue(), t2.getValue()));

System.out.println(op.get());

}

}

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