java8 lambda表达式 List转为Map

public static void main(String[] args) {
    List userList = new ArrayList();
    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 addressListMap = addressList.stream().collect(Collectors.toMap(OrderAddress::getOrderNo, p -> p, (key1, key2) -> key1));
//		Map 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 OrderNolist = new ArrayList();
//		printService.queryPrintInfo(customerCode).stream().forEach(p ->
//			OrderNolist.add(p.getOrderNo())
//		);

    System.out.println("userList="+JSONObject.toJSONString(userList));
    Map 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> 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 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, 2011, 300),
	                new Transaction(raoul, 2012, 1000),
	                new Transaction(raoul, 2011, 400),
	                new Transaction(mario, 2012, 710),
	                new Transaction(mario, 2012, 700),
	                new Transaction(alan, 2012, 950)
	        );
	    }
	    
	    //1. 找出2011年发生的所有交易， 并按交易额排序（从低到高）
	    @Test
	    public void test1(){
	        transactions.stream()
	                    .filter((t) -> t.getYear() == 2011)
	                    .sorted((t1, t2) -> Integer.compare(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) -> s1.compareToIgnoreCase(s2))
	                    .forEach(System.out::print);
	    }
	    
	    public static Stream filterCharacter(String str){
	        List 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 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 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 op = transactions.stream()
	                    .min((t1, t2) -> Integer.compare(t1.getValue(), t2.getValue()));
	        
	        System.out.println(op.get());
	    }
	
	}