Stream的使用

Stream的使用

Stream 流主要用来对集合数据进行过滤，映射，排序。也可用于数组
Stream 可以用来代替for循环对集合数据进行遍历处理，可以极大地简化代码

用法

常用的一个是过滤器filter，添加过滤条件
filter函数输入是谓词Predicate的类型

另一个是映射器map
peek可以提供map+return的功能
flatMap可以提供展开Stream流嵌套Stream流的功能

实践

package com.zimuge.stream;

import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import org.junit.Test;

/**
 * 功能描述
 *
 * @since 2022-06-13
 */
public class StreamTest {
    @Test
    public void Test1() {

        // list 流过滤
        List<String> strings = Arrays.asList("zhangsan", "lisi", "wangwu", "zhouliu");
        List<String> sorted = strings.stream().filter(str -> str.startsWith("z")).map(String::toUpperCase).sorted().collect(Collectors.toList());
        System.out.println(sorted);

        // Array 数组流过滤
        String[] str1 = {"zhangsan", "lisi", "wangwu", "zhouliu"};
        List<String> sorted1 = Stream.of(str1).filter(str -> str.endsWith("u")).map(String::toLowerCase).sorted().collect(Collectors.toList());
        System.out.println(sorted1);

        // Set 流过滤
        HashSet<String> strings1 = new HashSet<>(strings);
        Set<String> z = strings1.stream().filter(str -> str.startsWith("z")).map(String::toUpperCase).sorted().collect(Collectors.toSet());
        System.out.println(z);
    }
}


    @Test
    public void Test2() {
        Employee e1 = new Employee(1, 21, "zhangsan", "F");
        Employee e2 = new Employee(2, 45, "lisi", "M");
        Employee e3 = new Employee(3, 60, "wangwu", "M");
        Employee e4 = new Employee(4, 32, "zhouliu", "F");
        Employee e5 = new Employee(5, 28, "zhaoqi", "M");
        Employee e6 = new Employee(6, 43, "qianba", "F");

        List<Employee> list = Arrays.asList(e1, e2, e3, e4, e5, e6);

//        List<Employee> f = list.stream()
//                .filter(e -> e.getAge() > 35 || e.getGender().equals("M"))
//                .collect(Collectors.toList());

        // 两个实现都可以，下面的实现谓词可以复用
        List<Employee> f = list.stream()
                .filter(Employee.ageGreaterThan35.or(Employee.genderM))
                .collect(Collectors.toList());
        System.out.println(f);

    }

    @Test
    public void Test3() {
        Employee e1 = new Employee(1, 21, "zhangsan", "F");
        Employee e2 = new Employee(2, 45, "lisi", "M");
        Employee e3 = new Employee(3, 60, "wangwu", "M");
        Employee e4 = new Employee(4, 32, "zhouliu", "F");
        Employee e5 = new Employee(5, 28, "zhaoqi", "M");
        Employee e6 = new Employee(6, 43, "qianba", "F");

        List<Employee> list = Arrays.asList(e1, e2, e3, e4, e5, e6);

        List<Employee> m = list.stream()
                .filter(v -> v.getAge() > 25)
                .map(e -> {
                    e.setAge(e.getAge() + 1);
                    e.setGender(e.getGender().equals("M") ? "Male" : "Female");
                    return e;
                })
                .collect(Collectors.toList());
        System.out.println(m);

        // 与上面map+return一样
        List<Employee> m1 = list.stream()
                .filter(v -> v.getAge() > 25)
                .peek(e -> {
                    e.setAge(e.getAge() + 1);
                    e.setGender(e.getGender().equals("M") ? "Male" : "Female");
                })
                .collect(Collectors.toList());
        System.out.println(m1);
    }

    @Test
    public void Test4() {
        List<String> list = Arrays.asList("Hello", "World");
        list.stream()
                .map(e -> e.split(""))
                .forEach(System.out::println);

        list.stream()
                .map(e -> Arrays.stream(e.split("")))
                .forEach(System.out::println);

        list.stream()
                .flatMap(e -> Arrays.stream(e.split("")))
                .forEach(System.out::println);

    }