最近用到 Java8 新特性时候，发现流函数非常好用，特地总结了一下常用的方法和应用范围，下面通过一个例子来展示
1、实体类代码

package com.test;

import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

/**
 * Author:LSR
 * Date:2020/4/30
 * DESCRIPTION:
 */


public class Employee {

    private String name;
    private int salary;
    private String office;

    public String getName() {
        return name;
    }

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

    public int getSalary() {
        return salary;
    }

    public void setSalary(int salary) {
        this.salary = salary;
    }

    public String getOffice() {
        return office;
    }

    public void setOffice(String office) {
        this.office = office;
    }

    public Employee(String name, int salary, String office) {
        this.name = name;
        this.salary = salary;
        this.office = office;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name='" + name + '\'' +
                ", salary=" + salary +
                ", office='" + office + '\'' +
                '}';
    }
}

2、Java8 流函数各种方法应用测试代码

package com.test;
import java.util.*;
import java.util.stream.Collectors;
public class Java8Test2 {
    private static List<Employee> employeeList = new ArrayList<>();
    static{
        employeeList.add(new Employee("Matt",5000,"New York"));
        employeeList.add(new Employee("Steve",6000,"London"));
        employeeList.add(new Employee("Carrie",20000,"New York"));
        employeeList.add(new Employee("Peter",7000,"New York"));
        employeeList.add(new Employee("Pat",8000,"London"));
        employeeList.add(new Employee("Tammy",29000,"Shanghai"));

        /*employeeList.add(Employee.builder().name("Matt").salary(5000).office("New York").build());
        employeeList.add(Employee.builder().name("Steve").salary(6000).office("London").build());
        employeeList.add(Employee.builder().name("Carrie").salary(20000).office("New York").build());
        employeeList.add(Employee.builder().name("Peter").salary(7000).office("New York").build());
        employeeList.add(Employee.builder().name("Pat").salary(8000).office("London").build());
        employeeList.add(Employee.builder().name("Tammy").salary(29000).office("Shanghai").build());*/
    }

    public static void main(String[] args) {
        //anyMatch
        boolean isMatch = employeeList.stream().anyMatch(employee -> employee.getOffice().equals("London"));
        System.out.println(isMatch);

        //返回所有salary大于6000
        boolean matched = employeeList.stream().allMatch(employee -> employee.getSalary()>4000);
        System.out.println("返回所有salary大于6000:"+matched);

        //找出工资最高
        Optional<Employee> hightestSalary = employeeList.stream().max((e1, e2)->Integer.compare(e1.getSalary(),e2.getSalary()));
        System.out.println("找出工资最高:"+hightestSalary);

        //返回姓名列表
        List<String> names = employeeList.stream().map(employee -> employee.getName()).collect(Collectors.toList());
        System.out.println("返回姓名列表:"+names);

        //List转换成Map 其中 (k1, k2)->k2 表示最后的值覆盖前面的值可以达到去重目的
        Map<String,Employee> employeeMap = employeeList.stream().collect(Collectors.toMap((key->key.getName()),(value->value),(k1, k2)->k2));
        employeeMap.forEach((key,value)-> System.out.println(key + "=" + value.toString()));

        //统计办公室是New York的个数
        long officeCount = employeeList.stream().filter(employee -> employee.getOffice().equals("Shanghai")).count();
        System.out.println("统计办公室是New York的个数:"+officeCount);

        //List转换为Set
        Set<String> officeSet = employeeList.stream().map(employee -> employee.getOffice()).distinct().collect(Collectors.toSet());
        System.out.println("List转换为Set:"+officeSet.toString());

        //查找办公室地点是New York的员工
        Optional<Employee> allMatchedEmployees = employeeList.stream().filter(employee -> employee.getOffice().equals("New York")).findAny();
        System.out.println("查找办公室地点是New York的员工:"+allMatchedEmployees.toString());

        //按照工资的降序来列出员工信息
        List<Employee> sortEmployeeList =  employeeList.stream().sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary())).collect(Collectors.toList());
        //按照名字的升序列出员工信息
        List<Employee> sortEmployeeByName = employeeList.stream().sorted((e1,e2)->e1.getName().compareTo(e2.getName())).collect(Collectors.toList());
        System.out.println("按照工资的降序来列出员工信息:" +sortEmployeeList);
        System.out.println("按照名字的升序列出员工信息:" + sortEmployeeByName.toString());

        //获取工资最高的前2条员工信息
        List<Employee> top2EmployeeList= employeeList.stream()
                .sorted((e1,e2)->Integer.compare(e2.getSalary(),e1.getSalary()))
                .limit(2)
                .collect(Collectors.toList());
        System.out.println("获取工资最高的前2条员工信息:"+top2EmployeeList.toString());

        //获取平均工资
        OptionalDouble averageSalary = employeeList.stream().mapToInt(employee->employee.getSalary()).average();
        System.out.println("平均工资:" + averageSalary);

        //查找New York
        OptionalDouble averageSalaryByOffice = employeeList.stream().filter(employee -> employee.getOffice()
                .equals("New York"))
                .mapToInt(employee->employee.getSalary())
                .average();
        System.out.println("New York办公室平均工资:" + averageSalaryByOffice);

    }
}