Java8如何从一个Stream中过滤null值

第Java8如何从一个Stream中过滤null值目录从一个Stream中过滤null值Solution（解决）stream方法过滤条件的使用下面以List为例

从一个Stream中过滤null值

复习一个Stream包含null数据的例子.

Java8Examples.java

packagecom.mkyong.java8;

importjava.util.List;

importjava.util.stream.Collectors;

importjava.util.stream.Stream;

publicclassJava8Examples{

publicstaticvoidmain(String[]args){

StreamStringlanguage=Stream.of("java","python","node",null,"ruby",null,"php");

ListStringresult=language.collect(Collectors.toList());

result.forEach(System.out::println);

}

}

output

java

python

node

null//---NULL

ruby

null//---NULL

php

Solution（解决）

为了解决上面的问题，我们使用：Stream.filter(x-x!=null)

Java8Examples.java

packagecom.mkyong.java8;

importjava.util.List;

importjava.util.stream.Collectors;

importjava.util.stream.Stream;

publicclassJava8Examples{

publicstaticvoidmain(String[]args){

StreamStringlanguage=Stream.of("java","python","node",null,"ruby",null,"php");

//ListStringresult=language.collect(Collectors.toList());

ListStringresult=language.filter(x-x!=null).collect(Collectors.toList());

result.forEach(System.out::println);

}

}

output

java

python

node

ruby

php

另外，过滤器还可以用：Objects::nonNull

importjava.util.List;

ListStringresult=language.filter(Objects::nonNull).collect(Collectors.toList());

stream方法过滤条件的使用

@Data

@AllArgsConstructor

publicclassUser{

privateLongid;//id

privateIntegerage;//年龄

privateBytegentle;//性别

privateStringname;//名字

privateIntegerrank;//排名

Useruser0=newUser(1L,18,(byte)0,"张三",1);

Useruser1=newUser(2L,20,(byte)1,"李四",4);

Useruser2=newUser(3L,35,(byte)0,"王五",2);

Useruser3=newUser(4L,29,(byte)1,"赵六",3);

下面以List为例

实际上只要是Collection的子类，玩法都类似

1、生成stream

ListUserlist=Arrays.asList(user0,user1,user2,user3);

StreamUserstream=null;

stream=list.stream();//需要预判NPE

stream=Optional.of(list).orElseGet(Collections::emptyList).stream();//需要预判NPE

stream=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream();

stream=Optional.ofNullable(list).orElseGet(Collections::emptyList).parallelStream();//并行处理流

stream=Stream.of(user0,user1,user2,user3).parallel();//直接构造

stream=Stream.of(Arrays.asList(user0,user1),Arrays.asList(user2,user3)).flatMap(Collection::stream);//flatMap合并

2、stream操作

//过滤出性别为0的user

ListUseruserList=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().filter(user-(byte)0==user.getGentle()).collect(Collectors.toList());

//获取排名大于1的用户年龄set

SetIntegerageList=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().filter(user-1user.getRank()).map(User::getAge).collect(Collectors.toSet());

//合计性别为0的user的年龄

IntegertotalAge=Optional.ofNullable(userList).orElseGet(Collections::emptyList).stream().map(User::getAge).reduce(0,Integer::sum);

//按排名倒序排列

ListUsersortedUserList=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().sorted(Cparing(User::getRank,Comparator.reverseOrder())).collect(Collectors.toList());

//获取排名第2高的user

UserrankUser=Optional.ofNullable(sortedUserList).orElseGet(Collections::emptyList).stream().skip(1).findFirst().get();

//排名最高的user

UserhighestRankUser=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().max(Cparing(User::getRank)).get();

//是否存在排名大于1的user

booleanflag=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().anyMatch(user-user.getRank()

//是否所有user排名都大于1

booleanflag=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().allMatch(user-user.getRank()

//是否所有user排名都不大于5

booleanflag=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().noneMatch(user-user.getRank()

//按唯一id分组

MapLong,UseridUserMap=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.toMap(User::getId,Function.identity()));

//按唯一id,名字分组

MapLong,StringidNameMap=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.toMap(User::getId,User::getName));

//按年龄,名字分组，相同年龄的后出现的被覆盖

MapInteger,StringageNameMap=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.toMap(User::getAge,User::getName,(a,b)-a));

//按性别分组

MapByte,ListUsergentleUserMap=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.groupingBy(User::getGentle));

//按排名是否大于3分组

MapBoolean,ListUserpartitionUserMap=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.partitioningBy(user-user.getRank()3));

//按性别名字分组

MapByte,ListStringgentleNameMap=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.groupingBy(User::getGentle,Collectors.mapping(User::getName,Collectors.toList())));

//按性别年龄总和分组

MapByte,IntegergentleTotalAgeMap=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.groupingBy(User::getGentle,Collectors.reducing(0,User::getAge,Integer::sum)));

//迭代操作

Stream.iterate(0,i-i+1).limit(list.size()).forEach(i-{

System.out.println(list.get(i).getName());

//guavatable转换

TableLong,String,IntegeridNameRankTable=Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().map(user-ImmutableTable.