java8新特性-stream流示例

￡神魔★判官ぃ 2023-07-18 04:52 47阅读 0赞

一、简介  
java.util.stream表示能应用在一组元素上一次执行的操作序列。stream操作分为中间操作和最终操作，最终操作返回一特定类型的计算结果，而中间操作返回stream本身，这样就可以将多个操作串起来。stream操作需要指定一个数据源，比如java.util.collection的子类、list或者set，map类型不支持。stream可以串行stream()执行或者并行parallelStream()执行。  
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二、Stream  
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三、示例  
1、foreach循环

@Test
        public void Foreach(){ 
            //你不鸟我，我也不鸟你
            List<String> list = Arrays.asList("you","don't","bird","me",",",
                    "i","don't","bird","you");
    
            //方式一：jdk1.8之前的循环遍历方式
            for(String item:list){ 
                System.out.println(item);
            }
    
            //方式二：使用stream流
            // void forEach(Consumer<? super T> action)
            list.stream().forEach(item -> System.out.println(item));
    
            //方式三：使用stream流+lambda表达式
            list.stream().forEach(System.out::println);
        }

2、filter过滤

public class User{ 
            private long id;
            private String name;
            private Integer age;
    
            public User(){ }
    
            public User(long id, String name, Integer age){ 
                this.id = id;
                this.name = name;
                this.age = age;
            }
    
            public Integer getAge(){ 
                return this.age;
            }
        }
    
        @Test
        public void filter(){ 
            List<User> users = Arrays.asList(
                    new User(1, "wanwei", 23),
                    new User(2, "zhangsan", 25),
                    new User(3, "lisi", 29)
            );
            users.stream().filter(user -> user.getAge()>24).forEach(user -> System.out.println(user.name));
        }

3、map映射

@Test
        public void map(){ 
            List<String> list = Arrays.asList("djfh", "sdf", "sdfg");
            list.stream().map(item -> item.toUpperCase()).forEach(System.out::println);
        }

4、flatmap

@Test
        public void flatmap(){ 
            List<Integer> a = Arrays.asList(1,2,3);
            List<Integer> b = Arrays.asList(4,5,6);
    
            List<List<Integer>> collect = Stream.of(a,b).collect(Collectors.toList());
            System.out.println(collect);
    
            //将多个集合中的元素合并为一个集合
            List<Integer> mergelist = Stream.of(a,b).flatMap(list -> list.stream()).collect(Collectors.toList());
            System.out.println(mergelist);
    
            //通过Builder方式来构建
            Stream<Object> stream = Stream.builder().add("hello").add("hi").add("HI").build();
            stream.forEach(System.out::println);
        }

5、sorted排序

@Test
        public void sorted(){ 
            List<String> list = Arrays.asList("c","b","e","a");
            list.stream().sorted((s1,s2) -> s1.compareTo(s2)).forEach(System.out::println);
        }

6、distinct去重

@Test
        public void distinct(){ 
            Stream<String> stream = Stream.of("hello","hello","hi","hi","i");
            stream.distinct().forEach(System.out::println);
        }

7、count总数量

@Test
        public void count(){ 
            Stream<String> stream = Stream.of("known","is","known","nok","is","nok");
            long count = stream.count();
            System.out.println(count);
        }

8、skip、limit

@Test
    public void skip(){ 
        List<String> list = Arrays.asList("a", "b", "c", "d", "e");
        // Stream<T> skip(long n)
        list.stream().skip(2).forEach(System.out::println);  // c、d、e
    }
    
    @Test
    public void limit(){ 
        List<String> list = Arrays.asList("a", "b", "c", "d", "e");
        list.stream().skip(2).limit(2).forEach(System.out::println);    // c、d
    }

9、concat

@Test
    public void concat(){ 
        List<String> list = Arrays.asList("a", "b");
        List<String> list2 = Arrays.asList("c", "d");
        Stream<String> concatStream = Stream.concat(list.stream(), list2.stream());
        concatStream.forEach(System.out::println);
    }

10、anyMatch、allMatch

@Test
    public void match(){ 
        // 你给我站住
        List<String> list = Arrays.asList("you", "give", "me", "stop");
        // boolean anyMatch(Predicate<? super T> predicate);
        // parallelStream可以并行计算，速度比stream更快
        boolean result = list.parallelStream().anyMatch(item -> item.equals("me"));
        System.out.println(result);
    }

11、reduce归纳

@Test
    public void reduce(){ 
        Stream<String> stream = Stream.of("you", "give", "me", "stop");
        // Optional<T> reduce(BinaryOperator<T> accumulator);
        Optional<String> optional = stream.reduce((before, after) -> before + "," + after);
        optional.ifPresent(System.out::println);    // you,give,me,stop
    }
    
    public static void main(String[] args) { 
        List<BigDecimal> list = Arrays.asList(
                new BigDecimal("11.11"),
                new BigDecimal("22.22"),
                new BigDecimal("33.33")
        );
        // 66.66
        BigDecimal sum = list.stream().reduce(BigDecimal.ZERO, BigDecimal::add);
        System.out.println(sum);
    }

12、findFirst、findAny

@Test
        public void findFirst(){ 
            Stream<String> stream = Stream.of("you", "give", "me", "stop");
            String value = stream.findFirst().get();
            System.out.println(value);
        }
    
        @Test
        public void findAny(){ 
            Stream<String> stream = Stream.of("you", "give", "me", "stop");
            String value2 = stream.findAny().get();
            System.out.println(value2);
        }

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