Java8 list stream collect测试用例小结

测试用对象：

package com.demo.lee.util;
import lombok.AllArgsConstructor;
import lombok.Data;

@Data
@AllArgsConstructor
public class Student {
    private Integer id;
    private String groupId;
    private String name;
    private Integer age;
}

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根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根据整个实体对象分组计数,当其为String时常使用
        Map<Student, Long> result2 = list.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
        System.out.println(result2);

        //1.3根据分组的key值对结果进行排序、放进另一个map中并输出
        Map<String, Long> xMap = new HashMap<>();
        result1.entrySet().stream().sorted(Map.Entry.<String, Long>comparingByKey().reversed()) //reversed不生效
                .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.根据某个属性过滤
        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.根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);
    }

@test
public 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(",")
}

@test
public 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);
}
}```