Stream流最佳实战
stream 进行排序、分组、多级分组、交集、并集、差集等
package com.al.admin.utils;import cn.hutool.core.util.ObjectUtil;
import lombok.Data;
import org.springframework.util.StringUtils;import java.text.SimpleDateFormat;
import java.util.*;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;/*** 测试*/
public class StreamUtil {public static void main(String[] args) {List<Student> students = new ArrayList<>();for(int i=0; i<5 ;i++) {Student student = new StreamUtil().new Student();student.setId(i+"");if(i > 2) {student.setAge(1);} else {student.setAge(2);}if(i > 3) {student.setName("zhangsan");} else if(i == 0) {student.setName(null);} else {student.setName("lisi");}student.setMoney(i);student.setCreatedDate(null);students.add(student);}
//多级进行分组 当然也可以进行其他的操作Map<String, Map<Integer, List<Student>>> collectq = students.stream().collect(Collectors.groupingBy(x -> StringUtils.isEmpty(x.getName()) ? "" : x.getName(), Collectors.groupingBy(Student::getAge)));System.out.println("collectq：" + collectq);SimpleDateFormat sm = new SimpleDateFormat("yyyy-MM-dd");Map<String, List<Student>> groupBy = students.stream().collect(Collectors.groupingBy(a -> sm.format(a.getCreatedDate())));System.out.println("==============");System.out.println(groupBy);/*** 条件过滤*/System.out.println("条件过滤==============");System.out.println("student.getAge() >1 ==============");List<Student> collect = students.stream().filter(student -> student.getAge() > -1).collect(Collectors.toList());System.out.println(collect);/*** 正序排列*/System.out.println("正序排列==============");collect = collect.stream().sorted(Comparator.comparing(Student::getAge)).collect(Collectors.toList());System.out.println(collect);/*** 倒序排列*/System.out.println("倒序排列==============");collect = collect.stream().sorted(Comparator.comparing(Student::getAge).reversed()).collect(Collectors.toList());System.out.println(collect);/*** string 类型的字段 倒序排列*/System.out.println("string 类型的字段进行倒序排序 ==============");collect = collect.stream().sorted(Comparator.comparing(Student::getId).reversed()).collect(Collectors.toList());System.out.println(collect);/*** string 类型的字段 进行截取 倒序排列*/System.out.println("string 类型的字段进行倒序排序 ==============");collect = collect.stream().sorted((o1,o2) -> {try {return Integer.parseInt(o2.getName().split("-")[1].substring(0,5)) - Integer.parseInt(o1.getName().split("-")[1].substring(0,5));} catch (Exception e){e.printStackTrace();}return 0;}).collect(Collectors.toList());System.out.println(collect);/*** 多字段倒序排序*/System.out.println("多字段倒序排序 ==============");collect = collect.stream().sorted(Comparator.comparing(Student::getAge).thenComparing(Student::getId).reversed()).collect(Collectors.toList());System.out.println(collect);/*** 多字段条件*/System.out.println("age大于20 money大于3000的数据==============");Predicate<Student> predicate1 = student -> student.getAge()>20;Predicate<Student> predicate2 = student -> student.getMoney()>3000;List<Student> collect1 = collect.stream().filter(predicate1.and(predicate2)).collect(Collectors.toList());System.out.println(collect1);/*** 两个集合差集 、 并集 、 交集* 集合 students 、 collect*/System.out.println("两个集合差集");Predicate<Student> predicate =user -> collect1.stream().noneMatch(user1 -> (user.getId()).equalsIgnoreCase(user1.getId()));collect = students.stream().filter(predicate).collect(Collectors.toList());System.out.println("students - collect1的差集 注意 集合a对集合b之间的差集 和 集合b对集合a之间的差集 不一样");System.out.println(collect);System.out.println("两个集合交集");Predicate<Student> predicate3 =user -> collect1.stream().anyMatch(user1 -> (user.getId()).equalsIgnoreCase(user1.getId()));collect = students.stream().filter(predicate3).collect(Collectors.toList());System.out.println("students - collect1的交集");System.out.println(collect);System.out.println("两个集合并集");List<Student> collect2 = students.parallelStream().collect(Collectors.toList());List<Student> collect3 = collect1.parallelStream().collect(Collectors.toList());collect2.addAll(collect3);List<Student> collect4 = collect2.stream().distinct().collect(Collectors.toList());System.out.println(collect4);System.out.println("list 转 map");System.out.println("key是list对象中id，value是list对象中的年龄");Map<String, Integer> map = students.stream().collect(Collectors.toMap(Student::getId, Student::getAge));System.out.println(map);System.out.println("key是list对象中id，value是list对象");Map<String, Student> map1 = students.stream().collect(Collectors.toMap(Student::getId, student -> student));System.out.println(map1);System.out.println("key是list对象中id，value是list对象 另一种写法");Map<String, Student> map2 = students.stream().collect(Collectors.toMap(Student::getId, Function.identity()));System.out.println(map2);System.out.println("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。");Map<String, Student> map3 = students.stream().collect(Collectors.toMap(Student::getId, Function.identity(), (oldValue, newValue) -> newValue));System.out.println(map3);System.out.println("map 转 list");List<String> collect5 = map3.keySet().stream().collect(Collectors.toList());List<Student> collect6 = map3.values().stream().collect(Collectors.toList());System.out.println("map的key转list::"+collect5);System.out.println("map的value转list::"+collect6);List<Student> lst = map.entrySet().stream().map(c -> {Student student = new StreamUtil().new Student();student.setId(c.getKey());student.setAge(c.getValue());return student;}).collect(Collectors.toList());System.out.println("通过map的key，value转对象集合");System.out.println(lst);}@Datapublic class Student {private String id;private int age;private double money;private String name;private Date createdDate;public Student(){}}}
2、
java中用stream进行去重，排序，分组
3、stream排序
public void test6() {List<User> list = new ArrayList<>();//定义三个用户对象User user1 = new User();user1.setUserName("admin");user1.setAge(16);user1.setSex("男");User user2 = new User();user2.setUserName("root");user2.setAge(20);user2.setSex("女");User user3 = new User();user3.setUserName("admin");user3.setAge(18);user3.setSex("男");User user4 = new User();user4.setUserName("admin11");user4.setAge(22);user4.setSex("女");System.out.println(list);// 年龄排序(逆序)List<User> collect = list.stream().sorted(Comparator.comparing(User::getAge).reversed()).collect(Collectors.toList());System.out.println(collect);// 写法2List<User> collect1 = list.stream().sorted(Comparator.comparing(User::getAge, Comparator.reverseOrder())).collect(Collectors.toList());System.out.println(collect1);// lambda表达式List<User> collect2 = list.stream().sorted((a, b) -> b.getAge() - a.getAge()).collect(Collectors.toList());System.out.println(collect2);// 并行排序 - 排序数量大时使用并行排序，提高排序速度list.parallelStream().sorted().collect(Collectors.toList());// 多字段排序1-都进行倒序排序(先根据年龄倒序，再根据用户名倒序)List<User> collect3 = list.stream().sorted(Comparator.comparing(User::getAge).thenComparing(User::getUserName).reversed()).collect(Collectors.toList());System.out.println(collect3);// 多字段排序2-先根据年龄逆序，再根据用户名顺序--reversed()是让他前面所有字段进行倒序list.stream().sorted(Comparator.comparing(User::getAge).reversed().thenComparing(User::getUserName)).collect(Collectors.toList());// 多字段排序3list.stream().sorted(Comparator.comparing(User::getAge, Comparator.reverseOrder()).thenComparing(User::getUserName, Comparator.reverseOrder())).collect(Collectors.toList());}
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