一、测试对象package com.demo.crwien.test;public class Student { private Integer id; private String groupId; private String name; private Integer age;}二、测试用例package com.demo.lee.util;import org.junit.Test;import java.util.Arrays;import java.util.HashMap;import java.util.List;import java.util.Map;import java.util.function.Function;import java.util.stream.Collectors;import static java.util.stream.Collectors.groupingBy;/** * @author lee */public class ListCollectionTest { @Test public void test1() { //1.分组计数 List<Student> list = Arrays.asList( new Student(1, "one", "zhao",11), new Student(2, "one", "qian",22), new Student(3, "two", "sun",33), new Student(4, "two", "lee",18)); //1.1依据某个属性分组计数 Map<String, Long> result1 = list.stream().collect(Collectors.groupingBy(Student::getGroupId, Collectors.counting())); System.out.println(result1); //1.2依据整个实体对象分组计数,当其为String时常应用 Map<Student, Long> result2 = list.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting())); System.out.println(result2); //1.3依据分组的key值对后果进行排序、放进另一个map中并输入 Map<String, Long> xMap = new HashMap<>(); result1.entrySet().stream().sorted(Map.Entry.<String, Long>comparingByKey().reversed()) //reversed不失效 .forEachOrdered(x -> xMap.put(x.getKey(), x.getValue())); System.out.println(xMap); } @Test public void test2() { List<Student> list = Arrays.asList( new Student(1, "one", "zhao",11), new Student(2, "one", "qian",22), new Student(3, "two", "sun",33), new Student(4, "two", "lee",18)); //2.分组，并统计其中一个属性值得sum或者avg:id总和 Map<String, Integer> result3 = list.stream().collect( Collectors.groupingBy(Student::getGroupId, Collectors.summingInt(Student::getId)) ); System.out.println(result3); } @Test public void test3(){ List<Student> list = Arrays.asList( new Student(1, "one", "zhao",11), new Student(2, "one", "qian",22), new Student(3, "two", "sun",33), new Student(4, "two", "lee",18)); //3.依据某个属性过滤 List<Student> list1 = list.stream().filter(a -> a.getName().contains("a")).collect(Collectors.toList()); System.out.println(list1); } @Test public void test4(){ List<Student> list = Arrays.asList( new Student(1, "one", "zhao",11), new Student(2, "one", "qian",22), new Student(3, "two", "sun",33), new Student(4, "two", "lee",18)); //4.依据对象属性进行分组，自行编辑分组条件 Map<String, List<Student>> collect = list.stream().collect(groupingBy(s -> { Integer age = s.getAge(); if (0<age && age<=10){ return "baby"; }else if (10<age && age<=20){ return "teenager"; }else if (20<age && age<=30){ return "youth"; }else { return "old"; } })); System.out.println(collect); }@testpublic void test5(){ List<Student> list = Arrays.asList( new Student(1, "one", "zhao",11), new Student(2, "one", "qian",22), new Student(3, "two", "sun",33), new Student(4, "two", "lee",18)); //对象属性拼字符串 String investor = list.stream().map(Student::getName).collect(Collectors.joining(",")}@testpublic void test6(){ List<Student> list = Arrays.asList( new Student(1, "one", "zhao",11), new Student(2, "one", "qian",22), new Student(3, "two", "sun",33), new Student(4, "two", "lee",18)); //对象属性求和 Integer result = list.stream().collect(Collectors.summingInt(Student::getAge)); System.out.println("所有学生年龄之和 : " + reuslt);}@testpublic void test7(){ List<Student> list = Arrays.asList( new Student(1, "one", "zhao",11), new Student(2, "one", "qian",22), new Student(3, "two", "sun",33), new Student(4, "two", "lee",18)); //list转map Map<Integer,String> map = userlist.stream().collect(Collectors.toMap(User::getAge,User::getName));}@testpublic void test8(){ //按名称形容排序 List<ECLabelEnum> enumList = Arrays.stream(ECLabelEnum.values()).sorted(Comparator.comparing(ECLabelEnum::desc)).collect(Collectors.toList());}//排序@testpublic void test9(){ //倒序，从大到小 list.stream().sorted(Comparator.comparing(Student::getAge)); //正序，从小到大 list.stream().sorted(Comparator.comparing(Student::getAge).reversed()) }}三、实战Map<String, String> dataMap = new HashMap<>(10);List<String> dataList = request.getDataList();// 获取未加密的字符串(过滤不符合条件的)List<String> unEncryptData = dataList.stream().filter(s -> !isEncryptData(s)).collect(Collectors.toList());// 移除未加密的dataList.removeAll(unEncryptData);if (encryptData != null && encryptData.size() > 0) { collect = encryptData.stream().collect(Collectors.toMap(x -> x, x -> x,(k1,k2) -> k1)); }