Lambda表达式应用

一、list转map

- （1）key：list[i]某个字段，value：list[i]

 list.stream().collect(Collectors.toMap(Object::getXX,Function.identity()))

- （2）key：list[i]某个字段，value：list[i]某个字段

list.stream().collect(Collectors.toMap(Object::getXX,Object::getYY))

- （3）将list按某个字段分组

list.stream().collect(Collectors.toMap(Collectors.groupingBy(Object::getXX)))

二、list排序

list.sort((index2,index1)->index1.getXX.compareTo(index2.getXX))
从小到大：list.stream().sorted(Comparator.comparing(Object::getXX)).collect(Collectors.toList())
从大到小：list.stream().sorted(Comparator.comparing(Object::getXX).reversed()).collect(Collectors.toList())

三、List条件filter过滤

- 获取list中符合条件的数目

list.stream().filter(item->item.getXX>YY).count()

- 获取list中符合条件list

list.stream().filter(item->item.getXX>YY).collect(Collectors.toList())

四、List获取list中对象某个字段集合

list.stream().map(EmployeeRelationEntity::getId).collect(Collectors.toList())

下面根据简单实例来看看lambda的具体应用：

1、创建实体类：部门类、员工类

package com.dust.service.LambdaTest;

import java.io.Serializable;

/**
 * @author 朝雨
 * @description 部门类
 * @date 2019/11/10
 */
public class DeptEntity implements Serializable {
    private Integer deptId;//部门id
    private String deptName;//部门名称
    private String address;//部门的地址

    public Integer getDeptId() {
        return deptId;
    }

    public void setDeptId(Integer deptId) {
        this.deptId = deptId;
    }

    public String getDeptName() {
        return deptName;
    }

    public void setDeptName(String deptName) {
        this.deptName = deptName;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }
}

　　

package com.dust.service.LambdaTest;

import java.io.Serializable;
import java.math.BigDecimal;

/**
 * @author 朝雨
 * @description 员工类
 * @date 2019/11/10
 */
public class EmpEntity implements Serializable {
    private Integer empId;//员工id
    private String empName;//员工姓名
    private String sex;//性别
    private String phone;//电话
    private BigDecimal saleMoney;//工资
    private Integer deptId;//所属部门

    public Integer getDeptId() {
        return deptId;
    }

    public void setDeptId(Integer deptId) {
        this.deptId = deptId;
    }

    public Integer getEmpId() {
        return empId;
    }

    public void setEmpId(Integer empId) {
        this.empId = empId;
    }

    public String getEmpName() {
        return empName;
    }

    public void setEmpName(String empName) {
        this.empName = empName;
    }

    public String getSex() {
        return sex;
    }

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

    public String getPhone() {
        return phone;
    }

    public void setPhone(String phone) {
        this.phone = phone;
    }

    public BigDecimal getSaleMoney() {
        return saleMoney;
    }

    public void setSaleMoney(BigDecimal saleMoney) {
        this.saleMoney = saleMoney;
    }
}

2、lambda应用

package com.dust.service.LambdaTest;

import com.dust.util.JsonUtils;
import org.springframework.stereotype.Service;

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

/**
 * @author 朝雨
 * @description
 * @date 2019/11/10
 */
@Service
public class SearchService {

    public void lambdaTest(){
        List<DeptEntity> deptEntities = initDeptList();
        List<EmpEntity> empEntities = initEmpList();
        //获取部门列表中的所有部门ID列表
        List<Integer> deptIdList = deptEntities.stream().map(DeptEntity::getDeptId).collect(Collectors.toList());
        System.out.println(String.format("获取部门列表中的所有部门ID列表：deptIdList:%s",deptIdList));
        //获取员工id对应员工信息的map
        Map<Integer, EmpEntity> empEntityMap = empEntities.stream().collect(Collectors.toMap(EmpEntity::getEmpId, Function.identity()));
        System.out.println(String.format("获取员工id对应员工信息的map：empEntityMap:%s",JsonUtils.objectToJson(empEntityMap)));
        //获取员工id对应的员工工资的map
        Map<Integer, BigDecimal> empSaleMoneyMap = empEntities.stream().collect(Collectors.toMap(EmpEntity::getEmpId, EmpEntity::getSaleMoney));
        System.out.println(String.format("获取员工id对应的员工工资的map：empSaleMoneyMap:%s",JsonUtils.objectToJson(empSaleMoneyMap)));
        //获取各部门员工：将员工按部门分组
        Map<Integer, List<EmpEntity>> deptEmpMap = empEntities.stream().collect(Collectors.groupingBy(EmpEntity::getDeptId));
        System.out.println(String.format("获取各部门员工：将员工按部门分组：deptEmpMap:%s",JsonUtils.objectToJson(deptEmpMap)));
        //员工按工资正序排序
        List<EmpEntity> empSaleAscSortList = empEntities.stream().sorted(Comparator.comparing(EmpEntity::getSaleMoney)).collect(Collectors.toList());
        System.out.println(String.format("员工按工资正序排序：empSaleAscSortList:%s",JsonUtils.objectToJson(empSaleAscSortList)));
        //员工按工资逆序
        List<EmpEntity> empSaleDescSortList = empEntities.stream().sorted(Comparator.comparing(EmpEntity::getSaleMoney).reversed()).collect(Collectors.toList());
        System.out.println(String.format("员工按工资逆序：empSaleDescSortList:%s",JsonUtils.objectToJson(empSaleDescSortList)));
        //获取技术部员工数量
        long count = empEntities.stream().filter(item -> item.getDeptId() == 1001).count();
        System.out.println(String.format("获取技术部员工数量：count:%s",count));
        //获取技术部员工信息
        List<EmpEntity> techEmpList = empEntities.stream().filter(item -> item.getDeptId() == 1001).collect(Collectors.toList());
        System.out.println(String.format("获取技术部员工信息：techEmpList:%s",JsonUtils.objectToJson(techEmpList)));
    }

    private List<EmpEntity> initEmpList(){
        EmpEntity empEntity1 = new EmpEntity();
        empEntity1.setEmpId(1);
        empEntity1.setEmpName("小一");
        empEntity1.setPhone("12022020202");
        empEntity1.setSaleMoney(new BigDecimal("3000"));
        empEntity1.setSex("M");
        empEntity1.setDeptId(1001);

        EmpEntity empEntity2 = new EmpEntity();
        empEntity2.setEmpId(2);
        empEntity2.setEmpName("小二");
        empEntity2.setPhone("12022020202");
        empEntity2.setSaleMoney(new BigDecimal("4000"));
        empEntity2.setSex("F");
        empEntity2.setDeptId(1001);

        EmpEntity empEntity3 = new EmpEntity();
        empEntity3.setEmpId(3);
        empEntity3.setEmpName("小三");
        empEntity3.setPhone("12999999999");
        empEntity3.setSaleMoney(new BigDecimal("5000"));
        empEntity3.setSex("M");
        empEntity3.setDeptId(1002);

        List<EmpEntity> resultList = new ArrayList<>();
        resultList.add(empEntity1);
        resultList.add(empEntity3);
        resultList.add(empEntity2);
        return resultList;
    }
    //初始化部门信息
    private List<DeptEntity> initDeptList(){
        DeptEntity deptEntity1 = new DeptEntity();
        deptEntity1.setDeptId(1001);
        deptEntity1.setDeptName("技术部");
        deptEntity1.setAddress("北京");

        DeptEntity deptEntity2 = new DeptEntity();
        deptEntity2.setDeptId(1002);
        deptEntity2.setDeptName("产品部");
        deptEntity2.setAddress("北京");
        List<DeptEntity> result = new ArrayList<>();
        result.add(deptEntity1);
        result.add(deptEntity2);
        return result;
    }
}

3、测试

package com.dust.test;

import com.dust.service.LambdaTest.SearchService;
import com.dust.service.proxyTest.BaseService;
import com.dust.service.proxyTest.Person;
import com.dust.service.proxyTest.ProxyFactory;
import org.checkerframework.checker.units.qual.A;
import org.junit.Test;
import org.springframework.beans.factory.annotation.Autowired;

/**
 * @author 朝雨
 * @description
 * @date 2019/11/11
 */
public class LambdaTest extends TestApplication {

    @Autowired
    private SearchService searchService;

    @Test
    public void test(){
        searchService.lambdaTest();
    }
}

4、结果

开始测试-----------------
获取部门列表中的所有部门ID列表：deptIdList:[1001, 1002]
获取员工id对应员工信息的map：empEntityMap:{"1":{"empId":1,"empName":"小一","sex":"M","phone":"12022020202","saleMoney":3000,"deptId":1001},"2":{"empId":2,"empName":"小二","sex":"F","phone":"12022020202","saleMoney":4000,"deptId":1001},"3":{"empId":3,"empName":"小三","sex":"M","phone":"12999999999","saleMoney":5000,"deptId":1002}}
获取员工id对应的员工工资的map：empSaleMoneyMap:{"1":3000,"2":4000,"3":5000}
获取各部门员工：将员工按部门分组：deptEmpMap:{"1001":[{"empId":1,"empName":"小一","sex":"M","phone":"12022020202","saleMoney":3000,"deptId":1001},{"empId":2,"empName":"小二","sex":"F","phone":"12022020202","saleMoney":4000,"deptId":1001}],"1002":[{"empId":3,"empName":"小三","sex":"M","phone":"12999999999","saleMoney":5000,"deptId":1002}]}
员工按工资正序排序：empSaleAscSortList:[{"empId":1,"empName":"小一","sex":"M","phone":"12022020202","saleMoney":3000,"deptId":1001},{"empId":2,"empName":"小二","sex":"F","phone":"12022020202","saleMoney":4000,"deptId":1001},{"empId":3,"empName":"小三","sex":"M","phone":"12999999999","saleMoney":5000,"deptId":1002}]
员工按工资逆序：empSaleDescSortList:[{"empId":3,"empName":"小三","sex":"M","phone":"12999999999","saleMoney":5000,"deptId":1002},{"empId":2,"empName":"小二","sex":"F","phone":"12022020202","saleMoney":4000,"deptId":1001},{"empId":1,"empName":"小一","sex":"M","phone":"12022020202","saleMoney":3000,"deptId":1001}]
获取技术部员工数量：count:2
获取技术部员工信息：techEmpList:[{"empId":1,"empName":"小一","sex":"M","phone":"12022020202","saleMoney":3000,"deptId":1001},{"empId":2,"empName":"小二","sex":"F","phone":"12022020202","saleMoney":4000,"deptId":1001}]
测试结束-----------------