Java高级-stream流

1.介绍

stream流可以非常方便与精简的形式遍历集合，实现过滤、排序等功能

2.将List转成Set

stream.collect(Collectors.toSet());

public class Test01 {public static void main(String[] args) {ArrayList<UserEntity> userEntities = new ArrayList<>();userEntities.add(new UserEntity("张三",16));userEntities.add(new UserEntity("李四", 51));userEntities.add(new UserEntity("王五", 73));userEntities.add(new UserEntity("赵六", 12));userEntities.add(new UserEntity("赵六", 12));// 创建stream有两种方式// 1.串行流// 2.并行流// 并行流 比 串行流 效率要高Stream<UserEntity> stream = userEntities.stream();// 转换成set集合Set<UserEntity> userEntitySet = stream.collect(Collectors.toSet());userEntitySet.forEach(userEntity -> {System.out.println(userEntity);});}
}

3.将List转成Map

需要指定谁作为key，谁作为value
stream.collect(Collectors.toMap());

public class Test02 {public static void main(String[] args) {ArrayList<UserEntity> userEntities = new ArrayList<>();userEntities.add(new UserEntity("张三",16));userEntities.add(new UserEntity("李四", 51));userEntities.add(new UserEntity("王五", 73));userEntities.add(new UserEntity("赵六", 12));// list集合并没有key// 指定userName作为key，user对象作为valueStream<UserEntity> userEntityStream = userEntities.stream();/*** new Function<UserEntity(List集合中的类型), String(Map中的Key)>()*/Map<String, UserEntity> collect = userEntityStream.collect(Collectors.toMap(new Function<UserEntity, String>() {@Overridepublic String apply(UserEntity userEntity) {// 设置key的值return userEntity.getUserName();}}, new Function<UserEntity, UserEntity>() {@Overridepublic UserEntity apply(UserEntity userEntity) {// 设置value的值return userEntity;}}));collect.forEach(new BiConsumer() {@Overridepublic void accept(Object key, Object value) {System.out.println(key + ": " + value);}});}
}

4.计算求和reduce

public class Test03 {public static void main(String[] args) {Stream<Integer> integerStream = Stream.of(10, 89, 204, 56);Optional<Integer> reduce = integerStream.reduce(new BinaryOperator<Integer>() {@Overridepublic Integer apply(Integer integer, Integer integer2) {return integer + integer2;}});System.out.println(reduce.get());System.out.println("================================================");// 计算每个user的年龄的总数ArrayList<UserEntity> userEntities = new ArrayList<>();userEntities.add(new UserEntity("张三",16));userEntities.add(new UserEntity("李四", 51));userEntities.add(new UserEntity("王五", 73));userEntities.add(new UserEntity("赵六", 12));userEntities.add(new UserEntity("赵六", 12));Stream<UserEntity> userEntityStream = userEntities.stream();Optional<UserEntity> sum = userEntityStream.reduce(new BinaryOperator<UserEntity>() {@Overridepublic UserEntity apply(UserEntity u1, UserEntity u2) {return new UserEntity("sum", u1.getAge() + u2.getAge());}});System.out.println(sum.get().getAge());}
}

5.查找最大值max和最小值min

public class Test04 {public static void main(String[] args) {ArrayList<UserEntity> userEntities = new ArrayList<>();userEntities.add(new UserEntity("张三",16));userEntities.add(new UserEntity("李四", 51));userEntities.add(new UserEntity("王五", 73));userEntities.add(new UserEntity("赵六", 12));userEntities.add(new UserEntity("赵六", 12));// 最大值Stream<UserEntity> userEntityStream1 = userEntities.stream();Optional<UserEntity> max = userEntityStream1.max((o1, o2) -> o1.getAge() - o2.getAge());System.out.println("max = " + max.get());// 最小值Stream<UserEntity> userEntityStream2 = userEntities.stream();Optional<UserEntity> min = userEntityStream2.min((o1, o2) -> o1.getAge() - o2.getAge());System.out.println("min = " + min.get());}
}

6.Match匹配

anyMatch：判断条件里，任意一个元素成功，返回true
AllMatch：判断条件里，所有的都是成功，返回true
noneMatch跟allMatch相反，判断条件里的元素，所有的都不是，返回true

public class Test05 {public static void main(String[] args) {ArrayList<UserEntity> userEntities = new ArrayList<>();userEntities.add(new UserEntity("zhangsan",16));userEntities.add(new UserEntity("lisi", 51));userEntities.add(new UserEntity("wangwu", 73));userEntities.add(new UserEntity("zhaoliu", 12));userEntities.add(new UserEntity("zhaoliu", 12));Stream<UserEntity> userEntityStream = userEntities.stream();boolean b = userEntityStream.noneMatch(new Predicate<UserEntity>() {@Overridepublic boolean test(UserEntity userEntity) {return "zhangsan".equals(userEntity.getUserName());}});System.out.println("b = " + b);}
}

7.过滤器 filter

public class Test06 {public static void main(String[] args) {ArrayList<UserEntity> userEntities = new ArrayList<>();userEntities.add(new UserEntity("zhangsan",26));userEntities.add(new UserEntity("lisi", 51));userEntities.add(new UserEntity("wangwu", 73));userEntities.add(new UserEntity("zhaoliu", 12));userEntities.add(new UserEntity("zhaoliu", 63));Stream<UserEntity> userEntityStream = userEntities.stream();userEntityStream.filter(new Predicate<UserEntity>() {@Overridepublic boolean test(UserEntity userEntity) {return "zhaoliu".equals(userEntity.getUserName()) && userEntity.getAge() > 2;}}).forEach(userEntity -> System.out.println(userEntity));}
}

8.分页limit 跳过skip

public class Test07 {public static void main(String[] args) {ArrayList<UserEntity> userEntities = new ArrayList<>();userEntities.add(new UserEntity("zhangsan",26));userEntities.add(new UserEntity("lisi", 51));userEntities.add(new UserEntity("wangwu", 73));userEntities.add(new UserEntity("zhaoliu", 12));userEntities.add(new UserEntity("zhaoliu", 63));// 只查询前5条Stream<UserEntity> userEntityStream1 = userEntities.stream();userEntityStream1.limit(3).forEach(userEntity -> System.out.println(userEntity));System.out.println("====================");// 查询 第2条-4条Stream<UserEntity> userEntityStream2 = userEntities.stream();userEntityStream2.skip(2).limit(2).forEach(userEntity -> System.out.println(userEntity));}
}

9.数据排序 sorted

public class Test08 {public static void main(String[] args) {ArrayList<UserEntity> userEntities = new ArrayList<>();userEntities.add(new UserEntity("zhangsan",26));userEntities.add(new UserEntity("lisi", 51));userEntities.add(new UserEntity("wangwu", 73));userEntities.add(new UserEntity("zhaoliu", 12));userEntities.add(new UserEntity("zhaoliu", 63));Stream<UserEntity> userEntityStream = userEntities.stream();userEntityStream.sorted(new Comparator<UserEntity>() {@Overridepublic int compare(UserEntity o1, UserEntity o2) {return o2.getAge() - o1.getAge();}}).forEach(userEntity -> System.out.println(userEntity));}
}