Stream 处理

package com.lq.lqspringboot.common.stream;

import com.lq.lqspringboot.common.entity.Student;
import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

/**
 * @author ：liuqian
 * @date ：Created in 2022/4/6 17:53
 * @description：
 */
public class StreamTest {

    /**
     * filter 用法
     */
    public static void getFilterNull(){
        List<Student> results = new ArrayList();
        for (int i=0;i<5;i++){
            Student st = new Student();
            st.setAge(i);
            st.setName("lisi"+i);
            if(i==0 || i==3){
                st.setIsFeedback(0);
            }else if(i==1){
                st.setIsFeedback(1);
            }else{
                st.setIsFeedback(null);
            }
            results.add(st);
        }

        results = results.stream()
                .filter(r -> null != r.getIsFeedback())
                .filter(r -> 1 == r.getIsFeedback())
                .collect(Collectors.toList());// 过滤掉未反馈数据
        System.out.println("results = " + results.toString());

        List<Integer> list33 = results.stream().map(r -> r.getAge()).collect(Collectors.toList());
        List<String> nameList = results.stream().map(r -> r.getName()).collect(Collectors.toList());
        System.out.println("输出=" + nameList);
    }

    /**
     * filter 用法
     */
    public static void getFilterTest(){
        List<Integer> list = new ArrayList();
        for (int i=0;i<10;i++){
            list.add(i);
        }
        Integer ii = null;
        list.add(1);
        list.add(ii);
        list.add(1);
        list = list.stream().filter(r-> 0 == r).collect(Collectors.toList());
        System.out.println("list.count = "+list.size());
        System.out.println("list = "+list.toString());
    }

    /**
     * map用法
     */
    public static void getMapTest(){
        List<Integer> list = new ArrayList();
        for (int i=0;i<10;i++){
            list.add(i);
        }
    }

    /**
     * 去重
     */
    public static void getDistinctTest(){
        List<Integer> list = new ArrayList();
        for (int i=0;i<10;i++){
            list.add(i);
        }
        list.add(1);
        list = list.stream().distinct().collect(Collectors.toList());
        System.out.println("list="+list.toString());
    }

    /**
     * 测试分组
     */
    public static void getGroupStream(){
        // 测试数据
        List<Student> list = new ArrayList<>();
        for (int i=0;i<10;i++){
            Student st = new Student();
            st.setAge(i);
            if(i%2 == 0){
                st.setIsFeedback(0);
            }else{
                st.setIsFeedback(1);
            }
            st.setName("测试"+i);
            list.add(st);
        }

        /*
         *  Map<String, List<WorkflowParticiantBean>> rMap = list.stream().collect(Collectors.groupingBy(WorkflowParticiantBean :: getRoleCode, Collectors.toList()));
         */
        // 以isFeedback 为key，list 为value 创建 Map
        Map<Integer,List<Student>> map = list.stream().collect(Collectors.groupingBy(Student :: getIsFeedback,Collectors.toList()));
        System.out.println("打印数据结果 = "+map);

    }

    /**
     * 去重操作
     */
    public static void getFilterAttributeClass() {
        /*assessmenBeans.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(
                () -> new TreeSet<>(Comparator.comparing(o -> o.getModule() + o.getResponseRoleCode()))
        ), ArrayList::new));*/
        List<Student> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            Student st = new Student();
            st.setName("李四" + i);

            st.setAge(i + 5);
            if (i % 2 == 0) {
                st.setSex("男");
                st.setScore(2);
            } else {
                st.setScore(0);
                st.setSex("女");
            }
            list.add(st);
        }
        list = list.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(
                () -> new TreeSet<>(Comparator.comparing(o -> o.getScore() + o.getSex()))
        ), ArrayList::new));

        System.out.println("输出list=" + list);
        // list = list.stream().collect(Collectors.collectingAndThen(Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(o -> o.getScore() + o.getSex()))), ArrayList::new));
    }

    /**
     * List 转换map
     */
    public static void getListTransferMap(){
        List<Student> list = getStudentList();
        // 方法1 ,value 为属性值
        Map<Long, String> map1 = list.stream().collect(Collectors.toMap(Student::getId, Student::getName));
        System.out.println("map1="+map1);
        // 方法1.1, 属性值处理后转换
        Map<String, String> map11 = list.stream().collect(Collectors.toMap(k -> (k.getId() + k.getAge()+""), v -> v.getName()));
        System.out.println("map11="+map11);
        // 做判断
        Map<Long, String> map12 = list.stream().collect(Collectors.toMap(Student::getId, t->t.getName()));
        System.out.println("map12="+map12);

        // 方法2，key是对象中的某个属性值，value是对象本身（使用返回本身的lambda表达式）
        Map<Long, Student> map2 = list.stream().collect(Collectors.toMap(Student::getId, Student -> Student));
        System.out.println("map2="+map2);

        // 情况3，key是对象中的某个属性值，value是对象本身（使用Function.identity()的简洁写法）。
        Map<Long, Student> map3 = list.stream().collect(Collectors.toMap(Student::getId, Function.identity()));
        System.out.println("map3="+map3);

        // 情况4，key是对象中的某个属性值，value是对象本身，当key冲突时选择第二个key值覆盖第一个key值
        // 如果不正确指定Collectors.toMap方法的第三个参数（key冲突处理函数），那么在key重复的情况下该方法会报出【Duplicate Key】的错误导致Stream流异常终止
        Map<Long, Student> map4 = list.stream().collect(Collectors.toMap(Student::getId, Function.identity(), (t1, t2) -> t2));
        System.out.println("map4="+map4);

    }

    // 获取list 数组
    public static List<Student> getStudentList(){
        List<Student> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            Student st = new Student();
            st.setId(Long.valueOf(i));
            st.setName("李四" + i);
            st.setAge(i + 5);
            if (i % 2 == 0) {
                st.setSex("男");
                st.setScore(2);
            } else {
                st.setScore(0);
                st.setSex("女");
            }
            list.add(st);
        }
        return list;
    }


    public static void main(String[] args) {
        // getFilterTest();
        // getDistinctTest();
        // List<String> list = Collections.EMPTY_LIST;
        // System.out.println("list="+list);
        // getFilterNull();
        // getGroupStream();
        // 测试list 对象属性去重操作
        // getFilterAttributeClass();
        // getFilterNull();
        // 转Map
        getListTransferMap();
    }

}