Java8如何从一个Stream中过滤null值

package com.mkyong.java8; 

import java.util.List;

import java.util.stream.Collectors;

import java.util.stream.Stream;

public class Java8Examples { 

    public static void main(String[] args) { 

        Stream<String> language = Stream.of( "java" , "python" , "node" , null , "ruby" , null , "php" ); 

        List<String> result = language.collect(Collectors.toList()); 

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

    }

}

package com.mkyong.java8; 

import java.util.List;

import java.util.stream.Collectors;

import java.util.stream.Stream;

public class Java8Examples { 

    public static void main(String[] args) { 

        Stream<String> language = Stream.of( "java" , "python" , "node" , null , "ruby" , null , "php" ); 

        //List<String> result = language.collect(Collectors.toList()); 

        List<String> result = language.filter(x -> x!= null ).collect(Collectors.toList()); 

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

    }

}

import java.util.List;

List<String> result = language.filter(Objects::nonNull).collect(Collectors.toList());

@Data

@AllArgsConstructor    

public class User {

     private Long id;      // id

     private Integer age;  // 年龄

     private Byte gentle;  // 性别

     private String name;  // 名字

     private Integer rank; // 排名

}

User user0 = new User(1L, 18 , ( byte ) 0 , "张三" , 1 );

User user1 = new User(2L, 20 , ( byte ) 1 , "李四" , 4 );

User user2 = new User(3L, 35 , ( byte ) 0 , "王五" , 2 );

User user3 = new User(4L, 29 , ( byte ) 1 , "赵六" , 3 );

List<User> list = Arrays.asList(user0, user1, user2, user3);

Stream<User> stream = 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

// 过滤出性别为0的user

List<User> userList = 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

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

// 合计性别为0的user的年龄

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

// 按排名倒序排列

List<User> sortedUserList = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().sorted(Comparator测试数据paring(User::getRank, Comparator.reverseOrder())).collect(Collectors.toList());

// 获取排名第2高的user

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

// 排名最高的user

User highestRankUser = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().max(Comparator测试数据paring(User::getRank)).get();

// 是否存在排名大于1的user

boolean flag = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().anyMatch(user -> user.getRank() > 1 );

// 是否所有user排名都大于1

boolean flag = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().allMatch(user -> user.getRank() > 1 );

// 是否所有user排名都不大于5

boolean flag = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().noneMatch(user -> user.getRank() > 5 );

// 按唯一id分组

Map<Long, User> idUserMap = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.toMap(User::getId, Function.identity()));

// 按唯一id,名字分组

Map<Long, String> idNameMap = 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

Map<Integer, String> ageNameMap = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.toMap(User::getAge, User::getName, (a, b) -> a));

// 按性别分组

Map<Byte, List<User>> gentleUserMap = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.groupingBy(User::getGentle));

// 按排名是否大于3分组

Map<Boolean, List<User>> partitionUserMap = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().collect(Collectors.partitioningBy(user -> user.getRank() > 3 ));

// 按性别名字分组

Map<Byte, List<String>> gentleNameMap = 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

Map<Byte, Integer> gentleTotalAgeMap = 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());

});

// guava table转换

Table<Long, String, Integer> idNameRankTable = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream().map(user -> ImmutableTable.of(user.getId(), user.getName(), user.getRank())).collect(HashBasedTable::create, HashBasedTable::putAll, HashBasedTable::putAll);

// stream只能被terminal一次，下面是错误示范

Stream<User> stream = Optional.ofNullable(list).orElseGet(Collections::emptyList).stream();

stream.collect(Collectors.toMap(User::getId, Function.identity()));

stream.collect(Collectors.toMap(User::getId, User::getName)); // java.lang.IllegalStateException: stream has already been operated upon or closed

// ssc-common的com.meicloud.mcu测试数据mon.util.StreamUtil简单封装了一些流操作，欢迎试用

// 参考资料：https://HdhCmsTestibm测试数据/developerworks/cn/java/j-lo-java8streamapi/index.html