java8 list新特性

 1 import com.google.common.collect.Lists;
 2 
 3 import java.math.BigDecimal;
 4 import java.util.*;
 5 import java.util.function.Function;
 6 import java.util.function.Predicate;
 7 import java.util.stream.Collectors;
 8 import java.util.concurrent.ConcurrentHashMap;
 9 
10 import static java.util.stream.Collectors.collectingAndThen;
11 import static java.util.stream.Collectors.toCollection;
12 import static java.util.Comparator.comparingLong;;
13 
14 public class test {
15 
16     public void test(){
17         List<UserDomain> list = Lists.newArrayList();
18         //判断对象属性去重
19         long count = list.stream().filter(distinctByKey(UserDomain :: getUserName)).distinct().count();
20         if(count < list.size()){
21 
22         }
23 
24         //根据对象属性去重
25         list = list.stream().collect(
26                 collectingAndThen(toCollection(() -> new TreeSet<>(comparingLong(UserDomain::getId))), ArrayList::new)
27         );
28         //去重
29         list = list.stream().distinct().collect(Collectors.toList());
30 
31         //排序
32         list.sort(Comparator.comparing(UserDomain::getUserName));
33         list.sort(Comparator.comparing(UserDomain::getUserName).reversed());
34         list = list.stream().sorted(Comparator.comparing(UserDomain::getUserName, Comparator.nullsFirst(Comparator.naturalOrder()))).collect(Collectors.toList());
35         list = list.stream().sorted(Comparator.comparing(UserDomain::getUserName, Comparator.nullsFirst(Comparator.naturalOrder())).reversed()).collect(Collectors.toList());
36 
37         //过滤
38         list = list.stream().filter(a -> a.getUserName().equals("张三")).collect(Collectors.toList());
39 
40         //求和
41         BigDecimal totalMoney = list.stream().map(UserDomain::getUnitPrice).reduce(BigDecimal.ZERO, BigDecimal::add);
42 
43         //遍历(使用return可以达到continue的效果)
44         list.forEach(userList -> {
45             return;
46         });
47         //groupBy分组
48         Map<Long, List<UserDomain>> groupBy = list.stream().collect(Collectors.groupingBy(UserDomain::getId));
49 
50         //id为key，对象为value转Map
51         Map<Long,UserDomain> userMap = list.stream().collect(Collectors.toMap(UserDomain::getId, a -> a,(k1,k2)->k1));
52         //去重
53         userMap = list.stream().collect(Collectors.toMap(UserDomain::getId, user -> user,(k1,k2)->k1));
54         //JSONArray.toJSONString(userMap);
55         
56         //获取对象list中某个属性的值的lis
57         List<String> nameList = list.stream().map(UserDomain::getUserName).collect(Collectors.toList());
58         List<String> nameList1 = list.stream().map(e -> e.getUserName()).collect(Collectors.toList());
59         
60         
61     }
62 
63 
64     private static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
65         Map<Object, Boolean> seen = new ConcurrentHashMap<>();
66         return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
67     }
68 
69 }