Java在线运行

版本:

所属目录
点击了解高性能代码运行API
运行结果
教程手册
代码仓库
极速运行
终端运行
图形+终端

                        
以下是用户最新保存的代码
sudoku solver (not completed) 发布于:2024-03-16 23:48 java第一次作业 发布于:2024-03-15 00:22 获取个十百 发布于:2024-03-14 23:38 MD5编码规则 与时间格式化 发布于:2024-03-13 14:18 传递继承的示例 发布于:2024-03-12 18:10 斐波那契数列 发布于:2024-03-12 17:36 DS 代码示例保存。 发布于:2024-03-06 08:47 测试功能运行 发布于:2024-03-04 17:18 写出一个完整的声明复数类的iava程序输入2个括号字符串如 (1,2) (34) 发布于:2024-03-03 20:16 这是数据科学与算法 发布于:2024-02-28 02:04 健康和健康很快就会健康和健康 发布于:2024-02-26 16:45 人员信息匹配 发布于:2024-02-24 18:00 构造方法使用 发布于:2024-02-24 16:25 方法的重载 发布于:2024-02-22 12:50 迷宫老鼠问题 发布于:2024-02-21 16:33 类与对象的使用 发布于:2024-02-21 15:32 撒地方大师傅 发布于:2024-02-19 11:16 我的测试代码 发布于:2024-02-19 10:19 java AES 加密 发布于:2024-02-18 16:47 插入元素排序 发布于:2024-02-18 12:42 打印杨辉三角形 发布于:2024-02-18 10:46 冒泡排序法 发布于:2024-02-17 22:28 为数组添加新元素 发布于:2024-02-17 18:21 数组的使用 发布于:2024-02-16 20:21 1-1/2..求和 发布于:2024-02-15 19:58 大小写字母 发布于:2024-02-15 19:45 输出不被5整除的数 发布于:2024-02-15 18:48 提示登陆成功 发布于:2024-02-15 17:21 打印空心金字塔 发布于:2024-02-15 15:52 乘法口诀表 发布于:2024-02-15 14:31 for循环的嵌套使用 发布于:2024-02-15 13:51 dowhile循环的使用 发布于:2024-02-15 13:21 for循环count 发布于:2024-02-15 11:55 switch判断成绩合格 发布于:2024-02-15 11:17 if else控制语句 发布于:2024-02-15 08:46 发布于:2024-02-14 21:51 Scanner扫描器 发布于:2024-02-14 18:40 java学习 发布于:2024-02-08 01:05 我的画图脚本 发布于:2024-02-06 22:07 小红书的签名算法 发布于:2024-02-06 18:08 输入任意数字,获取大于该数字的偶数 发布于:2024-02-06 10:24 java数字排序程序 发布于:2024-01-31 14:44 23544符合法规和国家与iu哦 发布于:2024-01-31 09:55 归并排序算法 发布于:2024-01-31 09:04 加密算法-SHA256 发布于:2024-01-30 14:27 温度转换程序 发布于:2024-01-31 14:40 插入排序算法 发布于:2024-01-25 22:01 数组里有两种数是奇数个,其他都是偶数个,要求时间复杂度O(n),空间复杂度O(n) 发布于:2024-01-25 21:52 冒泡排序算法 发布于:2024-01-25 09:27 选择排序算法 发布于:2024-01-25 09:18 [更多]
显示目录

Java 8 flatMap example



学习嵌入式的绝佳套件,esp8266开源小电视成品,比自己去买开发板+屏幕还要便宜,省去了焊接不当搞坏的风险。 蜂鸣版+触控升级仅36元,更强的硬件、价格全网最低。

点击购买 固件广场

In Java 8, Stream can hold different data types, for examples:

Stream<String[]>    
Stream<Set<String>>    
Stream<List<String>>    
Stream<List<Object>>

But, the Stream operations (filter, sum, distinct…) and collectors do not support it, so, we need flatMap() to do the following conversion :

Stream<String[]>        -> flatMap ->    Stream<String>
Stream<Set<String>>    -> flatMap ->    Stream<String>
Stream<List<String>>    -> flatMap ->    Stream<String>
Stream<List<Object>>    -> flatMap ->    Stream<Object>

How flatMap() works :

{ {1,2}, {3,4}, {5,6} } -> flatMap -> {1,2,3,4,5,6}

{ {'a','b'}, {'c','d'}, {'e','f'} } -> flatMap -> {'a','b','c','d','e','f'}

1. Stream + String[] + flatMap

1.1 The below example will print an empty result, because filter() has no idea how to filter a stream of String[].

TestExample1.java

package com.mkyong.java8;

import java.util.Arrays;
import java.util.stream.Stream;

public class TestExample1 {

    public static void main(String[] args) {

        String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};

        //Stream<String[]>
        Stream<String[]> temp = Arrays.stream(data);

        //filter a stream of string[], and return a string[]?
        Stream<String[]> stream = temp.filter(x -> "a".equals(x.toString()));

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

    }

}

Output

//empty...

1.2 In above example, we should use flatMap() to convert Stream<String[]> to Stream<String>.

TestExample1.java

package com.mkyong.java8;

import java.util.Arrays;
import java.util.stream.Stream;

public class TestExample1 {

    public static void main(String[] args) {

        String[][] data = new String[][]{{"a", "b"}, {"c", "d"}, {"e", "f"}};

        //Stream<String[]>
        Stream<String[]> temp = Arrays.stream(data);

        //Stream<String>, GOOD!
        Stream<String> stringStream = temp.flatMap(x -> Arrays.stream(x));

        Stream<String> stream = stringStream.filter(x -> "a".equals(x.toString()));

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

        /*Stream<String> stream = Arrays.stream(data)
                .flatMap(x -> Arrays.stream(x))
                .filter(x -> "a".equals(x.toString()));*/

    }

}

Output

a

2. Stream + Set + flatMap

2.1 A student POJO.

Student.java

package com.mkyong.java8;

import java.util.HashSet;
import java.util.Set;

public class Student {

    private String name;
    private Set<String> book;

    public void addBook(String book) {
        if (this.book == null) {
            this.book = new HashSet<>();
        }
        this.book.add(book);
    }
    //getters and setters

}

2.2 flatMap() and Set example.

TestExample2.java

package com.mkyong.java8;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class TestExample2 {

    public static void main(String[] args) {

        Student obj1 = new Student();
        obj1.setName("mkyong");
        obj1.addBook("Java 8 in Action");
        obj1.addBook("Spring Boot in Action");
        obj1.addBook("Effective Java (2nd Edition)");

        Student obj2 = new Student();
        obj2.setName("zilap");
        obj2.addBook("Learning Python, 5th Edition");
        obj2.addBook("Effective Java (2nd Edition)");

        List<Student> list = new ArrayList<>();
        list.add(obj1);
        list.add(obj2);

        List<String> collect =
                list.stream()
                        .map(x -> x.getBook())      //Stream<Set<String>>
                        .flatMap(x -> x.stream())   //Stream<String>
                        .distinct()
                        .collect(Collectors.toList());

        collect.forEach(x -> System.out.println(x));
    }

}

Output

Spring Boot in Action
Effective Java (2nd Edition)
Java 8 in Action
Learning Python, 5th Edition

Try comments the flatMap(x -> x.stream()) the Collectors.toList() will prompts a compiler error, because it has no idea how to collect a stream of Set object.

3. Stream + Primitive + flatMapToInt

3.1 For primitive type, you can use flatMapToInt.

TestExample3.java

package com.mkyong.java8;

import java.util.Arrays;
import java.util.stream.IntStream;
import java.util.stream.Stream;

public class TestExample3 {

    public static void main(String[] args) {

        int[] intArray = {1, 2, 3, 4, 5, 6};

        //1. Stream<int[]>
        Stream<int[]> streamArray = Stream.of(intArray);

        //2. Stream<int[]> -> flatMap -> IntStream
        IntStream intStream = streamArray.flatMapToInt(x -> Arrays.stream(x));

        intStream.forEach(x -> System.out.println(x));

    }

}

Output

1
2
3
4
5
6

References

  1. Stream#flatMap JavaDoc
  2. Stackoverflow – Difference between map and flatMap methods in Java 8
  3. Java 8 – How to print an Array
  4. Java 8 – Collectors groupingBy and mapping example
由JSRUN为你提供的Java在线运行、在线编译工具
        JSRUN提供的Java 在线运行,Java 在线运行工具,基于linux操作系统环境提供线上编译和线上运行,具有运行快速,运行结果与常用开发、生产环境保持一致的特点。
yout