JDK1.8 Collection.stream 工作常用方法总

最为常用的是 map方法。
具体参考下面代码示例：

package com.work.jdk8;

import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class CollectionStreamTest {
    public static void main(String[] args) {
        List<Student> studentList = initStudentList();

        // 实际工作开发，常用程度依次从高到低
        System.out.println("---寝室中的学生，提取'学生id'属性得到的list，如下：");
        mapStudenIdList(studentList).forEach(System.out::println);

        System.out.println("---寝室中[年龄等于28 且 余额大于10元]学生列表信息，如下：");
        filter(studentList).forEach(student -> System.out.println(student.toString()));

        System.out.println("---寝室中大于等于30岁的人数：" + filterCount(studentList));

        System.out.println("---寝室中的学生，['学生id'作为key、'学生姓名name'作为value]，生成的Map如下：");
        mapNewMapAndSortKey(studentList).forEach((key,value) -> System.out.println(key+":"+value));

        System.out.println("---寝室中的学生，所有'学生id'用逗号连接后，生成的String：" + mapStudentIdDistinctJoinDelimiter(studentList));

        System.out.println("---整个寝室全部余额[Double]精确为：" + mapMoneyDoubleSum(studentList));

        System.out.println("---整个寝室全部余额[BigDecimal]精确为：" + mapMoneyBigDecimalReduceSetScale(studentList));
    }


    private static List<Long> mapStudenIdList(List<Student> list){
        return list.stream()
                .map(Student::getId)
                .collect(Collectors.toList());
    }

    private static List<Student> filter(List<Student> list){
        return list.stream()
                .filter(student -> {
                    // 这样书写好处，通过变量好的命名，使得代码可读性更高
                    boolean ageIs28AndMoneyMore10 = student.getAge() == 28 && student.getMoney().compareTo(new BigDecimal(10))==1;
                    return ageIs28AndMoneyMore10;
                }).collect(Collectors.toList());
    }

    private static Long filterCount(List<Student> list){
        return list.stream()
                .filter(student -> {
                    boolean olderThan30 = student.getAge() >= 30;
                    return olderThan30;
                }).count();
    }

    private static Map<Long, String> mapNewMapAndSortKey(List<Student> list){
        return list.stream()
                .collect(
Collectors.toMap(Student::getId, Student::getName, (key1, key2) -> key2));
    }

    private static String mapStudentIdDistinctJoinDelimiter(List<Student> list){
        return list.stream()
                .map(student -> student.getId().toString())
                .distinct()
                .collect(Collectors.joining(","));
    }

    private static Double mapMoneyDoubleSum(List<Student> list){
        return list.stream()
                .mapToDouble(student -> student.getMoney().doubleValue())
                .sum();
    }

    private static BigDecimal mapMoneyBigDecimalReduceSetScale(List<Student> list){
        return list.stream()
                .map(Student::getMoney)
                // reduce方法，第一个参数是初始值；第二个参数是指定'相加'操作
                .reduce(BigDecimal.ZERO, BigDecimal::add)
                // 最终BigDecimal保留2位小数，保留具体策略(四舍五入等)
                .setScale(2, 0);
    }

    /**
     * 私有方法：初始化list列表
     * @return
     */
    private static List<Student> initStudentList(){
        List<Student> list = new ArrayList<>();
        Student stu1 = new Student(1L, "邢华阳", 29, "稳重的大象", new BigDecimal(200.66));
        Student stu2 = new Student(2L, "陈龙基", 28, "灵动的仙鹤", new BigDecimal(300.25));
        Student stu3 = new Student(3L, "李阳", 31, "神秘的小熊", new BigDecimal(166.32));
        Student stu4 = new Student(4L, "靳浩东", 28, "The One", new BigDecimal(24.10));
        list.add(stu1);
        list.add(stu2);
        list.add(stu3);
        list.add(stu4);
        return list;
    }
}

Student.java---学生实体类

package com.work.jdk8;

import java.math.BigDecimal;
import java.util.Objects;

public class Student {
    private Long id;
    private String name;
    private Integer age;
    private String nickName;
    private BigDecimal money;

    public Student(Long id, String name, Integer age, String nickName, BigDecimal money) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.nickName = nickName;
        this.money = money;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public String getNickName() {
        return nickName;
    }

    public void setNickName(String nickName) {
        this.nickName = nickName;
    }

    public BigDecimal getMoney() {
        return money;
    }

    public void setMoney(BigDecimal money) {
        this.money = money;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return Objects.equals(id, student.id) &&
                Objects.equals(name, student.name) &&
                Objects.equals(age, student.age) &&
                Objects.equals(nickName, student.nickName) &&
                Objects.equals(money, student.money);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, name, age, nickName, money);
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                ", nickName='" + nickName + '\'' +
                ", money=" + money +
                '}';
    }
}

参考文章：
1. jdk1.8新特性Stream集合详细操作