函数式编程的一些魅力

Posted 2020-09-28 叶扬

例子是求一个班级中有几个学生大于20岁，共有四种方案。

（1）stutent类

package com.august.function;

/**
 * Created by baimq on 2017/7/18.
 */
public class Student {
    private String sex;
    private int age;
    private String name;
    public Student(String name,String sex,int age){
        this.name=name;
        this.sex=sex;
        this.age=age;
    }
    public String getSex() {
        return sex;
    }

    public void setSex(String sex) {
        this.sex = sex;
    }

    public int getAge() {
        return age;
    }

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

    public String getName() {
        return name;
    }

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

（2）四种实现方案

package com.august.function;

import org.apache.commons.collections.CollectionUtils;
import org.apache.commons.collections.Predicate;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

/**
 * Created by baimq on 2017/7/18.
 */
public class FunctionProgramming {
    public static void main(String[] args) {
        //求大于20岁的学生有几个人的四种写法
        List<Student> studentList=new ArrayList<Student>();
        Student student1=new Student("张三","男",23);
        Student student2=new Student("李四","女",19);
        Student student3=new Student("王五","男",21);
        studentList.add(student1);
        studentList.add(student2);
        studentList.add(student3);
        //1、for循环【java5以上均支持】【有状态记录，业务与逻辑耦合】
        int olderThanTwenty=0;
        for(Student student:studentList){
            if(student.getAge()>20){
                olderThanTwenty++;
            }
        }
        System.out.println(olderThanTwenty);
        //2、函数式思想编程【java5以上均支持】【无状态记录，业务与逻辑分离】
        olderThanTwenty =CollectionUtils.countMatches(studentList, new Predicate() {
            @Override
            public boolean evaluate(Object o) {
                Student student = (Student) o;
                return student.getAge() > 20;
            }
        });
        System.out.println(olderThanTwenty);
        //3、函数式编程【java8以上均支持】
        olderThanTwenty=CollectionUtils.countMatches(studentList, (arg0) -> ((Student) arg0).getAge()>20);
        System.out.println(olderThanTwenty);
        //4、函数式编程【java8以上均支持】
        olderThanTwenty= (int) studentList.stream().filter(student -> student.getAge()>20).count();
        System.out.println(olderThanTwenty);

    }
}