foreword

After graduating and entering the job, I started to get in touch with Stream. I found it very convenient. I recorded the method of Stream as comprehensively as possible.

1. forEach traversal

        forEach: This method receives a Consumer interface function, and hands each stream element to the function for processing.
        forEach method: used to traverse the data in the stream.
        Note: It is a final method. After traversal, other methods in the Stream stream cannot be called

public class Stream_ForEach {
    public static void main(String[] args) {
    //获取一个Stream流
    Stream<String>stream= Stream.of("张三","李四","王五","赵六");
        //使用Stream流的方法forEach对stream流中的数据遍历
        stream.forEach((String name)->{
            System.out.println(name);
        });
    }
}

2. filter filter

        filter: used to filter the data in the Stream stream
        filter(Predicate<? super T>predicate)
        The parameter Predicate of the filter method is a functional interface, and the abstract method         boolean test (T t)
        in the lambda expression Predicate can be used

public class Stream_filter {
    public static void main(String[] args) {
        //创建一个Stream流
        Stream<String> stream = Stream.of("张三", "李四", "王五", "赵六1", "刘老七");

        //单条件过滤
        Stream<String> stream1 = stream.filter((String name) -> {
            return name.startsWith("刘");
        });

        //多条件过滤
        List<String> stream2 = stream.filter((String name) -> {
            if(name.length()>=3 && name.equals("刘老七")){
                return true;
            }
                return false;
        }).collect(Collectors.toList);

        //遍历stream1
        stream1.forEach((name)-> System.out.println(name));
        
        //输出stream2
        System.out.println(stream2)
        
    }
}

3. distinct deduplication

public class Stream_distinct {
    public static void main(String[] args) {
        //创建一个Stream流
        Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
        //去重
        List<String> stream1 = stream().distinct().collect(Collectors.toList());
        //输出stream1
        System.out.println(stream1)
    }
}

4. limit interception

        limit: used to intercept the elements in the stream
        limit can intercept the stream, only the first n
        limit (long maxSize);
        the parameter is a long type, if the current length of the set is greater than the parameter, it will be intercepted, otherwise no operation will be performed.
        limit is a Delay method, you can continue to use the Stream method

public class Stream_limit {
    public static void main(String[] args) {
        //创建一个Stream流
        Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
        //去重
        List<String> list = stream().distinct().collect(Collectors.toList());
        //截取去重后的前2个元素
        list = list.stream().limit(2).collect(Collectors.toList();
        //输出stream1
        System.out.println(list)
    }
}

5. skip skip

        skip method: used to skip elements
        skip(long n)
        If the current length of the stream is greater than n, skip the first n, otherwise an empty stream with a length of 0 will be obtained

public class Stream_skip {
    public static void main(String[] args) {
        //创建一个Stream流
        Stream<String> stream = Stream.of("张三", "张三","张三","李四", "王五", "赵六1", "刘老七");
        //去重
        List<String> list = stream().distinct().collect(Collectors.toList());
        //跳过去重后的前2个元素
        list = list.stream().skip(2).collect(Collectors.toList());
        //输出stream1
        System.out.println(list)
    }
}

6. sort sorted

public class Stream_sorted {
    public static void main(String[] args) {
        List<Test> list = new ArrayList<>();
        list.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        list.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        list.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));

        //根据年龄从大到小排序
        list = list.stream()
                   .sorted(Comparator.comparing(Test::getAge).reversed())
                   .collect(Collectors.toList());
        list.forEach(System.out::println);


    }
}

7. Maximum value max, min

public class Stream_max_min {
    public static void main(String[] args) {
        List<Test> list = new ArrayList<>();
        list.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        list.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        list.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        
        //获取年龄最大的人
        Test maxPeople = list.stream().max(Comparator.companing(Test::getAge)).get();
      
        //获取年龄最小的人
        Test minPeople = list.stream().min(Comparator.companing(Test::getAge)).get();

    }
}

8. Statistics reduce

public class Stream_reduce {
    public static void main(String[] args) {
        List<Test> testList = new ArrayList<Test>();
        testList.add(new Test("小明",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        testList.add(new Test("小红",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        testList.add(new Test("小兰",22,new BigDecimal("3200"),new BigDecimal("1.3")));

        //统计年龄总和
        int totalAge =testList.stream().mapToInt(Test::getAge).sum();

        //统计工资总和
        BigDecimal totalSalary = testList.stream().map(Test::getSalary)

        //统计工资乘以各自系数的总和(向上保留两位)
        BigDecimal totalRatioSalary = testList.stream()
                                              .map(s->s.getSalary()
                                              .multiply(s.getRatio())
                                              .setScale(2,BigDecimal.ROUND_HALF_UP))
                                              .reduce(BigDecimal.ZERO,BigDecimal::add);
    }
}

9. Convert List structure to Map structure

public class Stream_List_Map {
    public static void main(String[] args) {
        List<Test> testList = new ArrayList<Test>();
        testList.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        testList.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        testList.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //根据姓名转map,map的key为name
        Map<String, Test> nameMap= testList.stream().collect(Collectors.toMap(Test::getName, Test -> Test);
        System.out.println(map);
    }
}

10. Convert List<Object> to List<String>


public class Stream_object_string {
    public static void main(String[] args) {
        List<Test> testList = new ArrayList<Test>();
        testList.add(new Test("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        testList.add(new Test("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        testList.add(new Test("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //获取姓名集合
        List<String> nameList = testList.stream().map(Test::getName()).collect(Collectors.toList());
        System.out.println("value:"+nameList);
    }
}

11. List<Object> object to List<Object>

public class Stream_object_object {
    public static void main(String[] args) {
        List<People> peopleList = new ArrayList<People>();
        peopleList.add(new People("张三",23,new BigDecimal("3000"),new BigDecimal("1.1")));
        peopleList.add(new People("李四",24,new BigDecimal("2800"),new BigDecimal("1.2")));
        peopleList.add(new People("王五",22,new BigDecimal("3200"),new BigDecimal("1.3")));
        //对象转对象
        List<Student> studentList = peopleList.stream().map(s->{
            return new Student(s.getName(),s.getAge());
        }).collect(Collectors.toList());
        System.out.println("value:"+studentList);
    }
}

Summarize

slightly

A complete collection of common methods of Stream