java8 stream操作

111

package com.aaa.demo.testStream;

import com.aaa.pojo.Student;

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

/**
 * projectName: testSpring
 *
 * @author: 黄睿武
 * time: 2023/6/3 13:16
 * description:
 */
public class TestSteam {

    public static void main(String[] args) {
        Student student = new Student("段誉",18,"男");
        Student student2 = new Student("萧峰",19,"男");
        Student student3 = new Student("虚竹",20,"男");
        Student student4 = new Student("王语嫣",18,"女");
        Student student5 = new Student("阿朱",null,null);
        Student student6 = new Student("鸠摩智",45,"男");
        Student student7 = new Student("扫地僧",50,"男");
        Student student8 = new Student("段誉",48,"男");

        List<Student> list = new ArrayList();
        list.add(student);
        list.add(student2);
        list.add(student3);
        list.add(student4);
        list.add(student5);
        list.add(student6);
        list.add(student7);
        list.add(student8);

        //过滤元素，并生成新的集合
//        List<Student> collect = list.stream().filter(ss -> ss.getGender() != null && ss.getAge() <= 20).collect(Collectors.toList());
//        for (Student o : collect) {
//            System.out.println(o.getName());
//        }

        //过滤后获取数量
//        long count = list.stream().filter(ss -> ss.getGender() != null && ss.getAge() <= 20).count();
//        System.out.println("count = " + count);

        //在流中进行循环操作
//        list.stream().filter(ss -> ss.getGender() != null && ss.getAge() <= 20).forEach(ss -> {
//            System.out.print(ss.getName());
//            System.out.print(" 年龄：" + ss.getAge());
//            System.out.println(" 性别：" + ss.getGender());
//        });



        //对流中的元素进行排序
        //方法1 studenet对象内要提前实现Comparable并重写compareTo方法即可,需要倒序的话则sorted里加上Comparator.reverseOrder()
//        list.stream().sorted(Comparator.reverseOrder()).forEach(ss->{System.out.println(ss.getName() + ss.getAge());});

        //方法2 在sorted直接实现排序方法，要倒序则接上reversed()方法即可
//        list.stream().sorted(Comparator.comparing(Student::getName).reversed()).forEach(ss->{System.out.println(ss.getName() + ss.getAge());});

        //排序字段如果可能出现空值，则要加上nullsFirst（）或 nullsLast（）方法指定空值放前面还是放后面
//        list.stream().sorted(Comparator.comparing(Student::getAge,Comparator.nullsFirst(Integer::compareTo))).forEach(ss->{System.out.println(ss.getName() + ss.getAge());});


//        List<Student> collect1 = list.stream().filter(ss -> ss.getAge() > 18).collect(Collectors.toList());


        //list转map，简单模式，如果有重复key则报错
//        Map<Integer, String> map = list.stream().collect(Collectors.toMap(Student::getAge, Student::getName));

        //list转map，key有重复时，新值替换旧值
//        Map<Integer, String> map = list.stream().collect(Collectors.toMap(Student::getAge, Student::getName, (v1, v2)->v2));
//        Set<Integer> integers = map.keySet();
//        for (Integer integer : integers) {
//            String name = map.get(integer);
//            System.out.println("age = " + integer + "  name = " + name);
//        }

        //list转map，key有重复时，处理已存在的key的数据，这里示例是直接拼接value值
        Map<Integer, String> map = list.stream().collect(Collectors.toMap(Student::getAge, Student::getName, (v1, v2)-> v1 + v2));
        Set<Integer> integers = map.keySet();
        for (Integer integer : integers) {
            String name = map.get(integer);
            System.out.println("age = " + integer + "  name = " + name);
        }

    }
}