第一篇 ：针对写代码中，不习惯熟悉使用 java 8 lambda表达式的同学快熟上手，简易开发的分享。

我开始使用java8的时候，也是新到一家公司，当时使用java版本比较新，就引用了java8新的特性， lambda表达式处理集合遍历，收集，分组，查询等操作，开始觉得脱离了原有的for循环繁琐的遍历，也是挺感兴趣的，于是买了一本java8  lambda 处理流的书籍，其实最后也是没有认真完全看下来，其中说了很多底层的数据结构。然而实际开发中关于 lambda 表达式其实只要会几种操作就完全可以应付日常的开发工作。下面我会介绍针对我们日常开发的小伙伴说lambda表达式的开发中经常使用的操作。

// Java 8之前：
new Thread(new Runnable() {
    @Override
    public void run() {
    System.out.println("hello world !");
    }
}).start();


// Java 8的使用方式：

 new Thread( () -> System.out.println("hello，world!") ).start();

大家相比较是不是觉得代码整洁简单许多。接下来我也不介绍java8的新特性的原理和比较，我只针对在实际开发中我们会用 java8怎么处理我业务逻辑代码。

首先我们创建一个学生成绩类

/***
 * 学生成绩表
 * @author lovewell
 *
 */
@Data
@ApiModel
public class StudentGrade implements Serializable{
    /**
     *
     */
    private static final long serialVersionUID = 5656670486884778407L;

    @ApiModelProperty(value = "id")
    private Integer id;

    @ApiModelProperty(value = "学生姓名")
    private String name;

    @ApiModelProperty(value = "课程科目")
    private String subject;

    @ApiModelProperty(value = "成绩")
    private double grade;


}

我们书写个测试类给对象添加数据，然后我们使用java8 表达式类处理常见的操作

public class StudentTest {


    public static void main(String[] args) {

        List<StudentGrade> studentGrades = new ArrayList<StudentGrade>();

         studentGrades.add(new StudentGrade(1, "張三", "语文", 90));
         studentGrades.add(new StudentGrade(2, "張三", "数学", 95));
         studentGrades.add(new StudentGrade(3, "張三", "英语", 100));
         studentGrades.add(new StudentGrade(4, "李四", "语文", 85));
         studentGrades.add(new StudentGrade(5, "李四", "数学", 80));
         studentGrades.add(new StudentGrade(6, "李四", "英语", 90));

        System.out.println(studentGrades);
   编译后 ：

[StudentGrade(id=1, name=張三, subject=语文, grade=90), StudentGrade(id=2, name=張三, subject=数学, grade=95), StudentGrade(id=3, name=張三, subject=英语, grade=100),

 StudentGrade(id=4, name=李四, subject=语文, grade=85), StudentGrade(id=5, name=李四, subject=数学, grade=80), StudentGrade(id=6, name=李四, subject=英语, grade=90)]

    




    }

    //比如，我们想找 id=1 或者张三学生语文成绩 的数据 ，可以这样写
        StudentGrade grade = studentGrades.stream().filter(studentGrade -> Objects.equals(1, studentGrade.getId())).collect(Collectors.toList()).get(0);
        System.out.println(grade);

        StudentGrade grade1 = studentGrades.stream().filter(studentGrade -> Objects.equals("張三", studentGrade.getName())&&Objects.equals("语文", studentGrade.getSubject()))
                                                      .collect(Collectors.toList()).get(0);
        System.out.println(grade1);

 编译后 ：

         StudentGrade(id=1, name=張三, subject=语文, grade=90)
         StudentGrade(id=1, name=張三, subject=语文, grade=90)

    //比如我们想对每个学生分组 就可以这样处理  

        Map<String, List<StudentGrade>> map = new HashMap<String, List<StudentGrade>>();
        map = studentGrades.stream().collect(Collectors.groupingBy(StudentGrade::getName));
        System.out.println(map.size());
        System.out.println(map.get("張三"));
        System.out.println(map.get("李四"));

编译后 ：

       2
      [StudentGrade(id=1, name=張三, subject=语文, grade=90), StudentGrade(id=2, name=張三, subject=数学, grade=95), StudentGrade(id=3, name=張三, subject=英语, grade=100)]
     [StudentGrade(id=4, name=李四, subject=语文, grade=85), StudentGrade(id=5, name=李四, subject=数学, grade=80), StudentGrade(id=6, name=李四, subject=英语, grade=90)]

	//比如我们想遍历集合 ，且想把所有人的成绩改成100分 ，可以这样操作
		studentGrades.forEach(studentGrade->{

			studentGrade.setGrade(new BigDecimal(100));

		});
		System.out.println(studentGrades);

编译后 ：[StudentGrade(id=1, name=張三, subject=语文, grade=100), StudentGrade(id=2, name=張三, subject=数学, grade=100), StudentGrade(id=3, name=張三, subject=英语, grade=100), StudentGrade(id=4, name=李四, subject=语文, grade=100), StudentGrade(id=5, name=李四, subject=数学, grade=100), StudentGrade(id=6, name=李四, subject=英语, grade=100)]

　　

//比如，我们想计算李四同学的所有科目的总成绩

          double  totalGrade = studentGrades.stream()
                .filter(studentGrade->Objects.equals("李四", studentGrade.getName())).mapToDouble(StudentGrade::getGrade).sum();
          System.out.println(totalGrade);


编译后 ：255.0

  //例如我想收集所有同学所有科目中最大分数和最小分数

		 double max = studentGrades.stream().collect(Collectors.maxBy(Comparator.comparingDouble(StudentGrade::getGrade))).map(StudentGrade::getGrade).get();

		 double min = studentGrades.stream().collect(Collectors.minBy(Comparator.comparingDouble(StudentGrade::getGrade))).map(StudentGrade::getGrade).get();

           // 求平均值
           double avg = studentGrades.stream().collect(Collectors.averagingDouble(StudentGrade::getGrade));
          // 对集合一级分组
          Map<String, List<StudentGrade>> collect = studentGrades.stream().collect(Collectors.groupingBy(StudentGrade-> {
              if (StudentGrade.getGrade()<60) {
                  return "不及格";
              } else if (StudentGrade.getGrade() >= 60 && StudentGrade.getGrade()<70) {
                  return "及格";
              }else if (StudentGrade.getGrade()>= 70 && StudentGrade.getGrade()<90) {
                  return "中等";
              }else {
                  return "优秀";
              }
          }));
          //对集合二级分组
          Map<String, Map<String, List<StudentGrade>>> collect1 = studentGrades.stream().collect(Collectors.groupingBy(StudentGrade -> {
              if (StudentGrade.getGrade()<60) {
                  return "不及格";
              } else if (StudentGrade.getGrade() >= 60 && StudentGrade.getGrade()<70) {
                  return "及格";
              }else if (StudentGrade.getGrade()>= 70 && StudentGrade.getGrade()<90) {
                  return "中等";
              }else {
                  return "优秀";
              }
          }, Collectors.groupingBy(StudentGrade::getSubject)));
          // 对集合分组进行统计数量
          final Map<String, Long> collect2 = studentGrades.stream().collect(Collectors.groupingBy(StudentGrade -> {
              if (StudentGrade.getGrade()<60) {
                  return "不及格";
              } else if (StudentGrade.getGrade() >= 60 && StudentGrade.getGrade()<70) {
                  return "及格";
              }else if (StudentGrade.getGrade()>= 70 && StudentGrade.getGrade()<90) {
                  return "中等";
              }else {
                  return "优秀";
              }
          }, Collectors.counting()));
          // 将对象的属性转化成相应的key value形式
          Map<Object, Double> ljMap2 = studentGrades.stream()
                  .collect(Collectors.toMap(lj -> lj.getName(), StudentGrade::getGrade));


          //将对象集合中的属性数据转换到新的集合（集合间数据转换）
          Set<String> names = studentGrades.stream().map(StudentGrade::getName).collect(Collectors.toSet());



          // 通过条件过滤数据
          List<StudentGrade> List = studentGrades.stream()
                  .filter(emp -> emp.getGrade() >= 80 && emp.getGrade() <= 100)
                  .collect(Collectors.toList());


          // 单条件排序
          List<StudentGrade> sortList = studentGrades.stream().sorted(Comparator.comparing(StudentGrade::getGrade))
                  .collect(Collectors.toList());

          // 多条件排序
          List<StudentGrade> sortListTwo = studentGrades.stream().sorted(Comparator.comparing(StudentGrade::getGrade)
                  .thenComparing(StudentGrade::getName).reversed())
                  .collect(Collectors.toList());
          // 统计计数
          Long count=studentGrades.stream().collect(Collectors.counting());

          // 取最大或小值(大maxBy 小 minBy)
          StudentGrade studentGrade = studentGrades.stream().collect(Collectors.maxBy(Comparator.comparingDouble(StudentGrade::getGrade))).get();

          // 成绩求和
          double sum = studentGrades.stream().collect(Collectors.summingDouble(StudentGrade::getGrade));






                                            End 谢谢各位小伙伴阅读，略有帮助的话加个关注，私下有问题可以讨教。