数据：nation（id,name）
需求：通过重写equals和hashCode获取nation的id

1.nation实体类

package com.example.server.pojo;

import cn.afterturn.easypoi.excel.annotation.Excel;
import com.baomidou.mybatisplus.annotation.TableName;
import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableId;
import java.io.Serializable;
import java.util.Objects;

import io.swagger.annotations.ApiModel;
import io.swagger.annotations.ApiModelProperty;

/**
 * <p>
 * 
 * </p>
 *
 * @author jiangsanjin
 * @since 2021-02-03
 */

@TableName("t_nation")

@ApiModel(value="Nation对象", description="")
public class Nation implements Serializable {

    private static final long serialVersionUID = 1L;

    @ApiModelProperty(value = "id")
    @TableId(value = "id", type = IdType.AUTO)
    private Integer id;

    @ApiModelProperty(value = "民族")
    @Excel(name = "民族")
    private String name;

    public Nation() {
    }

    public Nation(Integer id, String name) {
        this.id = id;
        this.name = name;
    }

    public Nation(String name) {
        this.name = name;
    }


    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Nation)) return false;
        Nation nation = (Nation) o;
        return name.equals(nation.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name);
    }

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }

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

    @Override
    public String toString() {
        return "Nation{" +
            "id=" + id +
            ", name=" + name +
        "}";
    }
}

2.nation实体类重写hashCode和equals分析

	public Nation() {
    }

	//传入name
    public Nation(String name) {
        this.name = name;
    }

	//根据name判断是否相等
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof Nation)) return false;
        Nation nation = (Nation) o;
        return name.equals(nation.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name);
    }

3.导入员工数据代码

    @ApiOperation(value = "导入员工数据")
    @ApiImplicitParams({@ApiImplicitParam(name="file",value = "上传文件",dataType = "MultipartFile")})
    @PostMapping("/import")
    public RespBean importEmployee(MultipartFile file){
        ImportParams params = new ImportParams();
        //去掉标题行
        params.setTitleRows(1);
        List<Nation> nations = nationService.list();
        List<PoliticsStatus> politicsStatuses = politicsStatusService.list();
        List<Position> positions = positionService.list();
        List<Department> departments = departmentService.list();
        List<Joblevel> joblevels = joblevelService.list();
        try {
            List<Employee> list = ExcelImportUtil.importExcel(file.getInputStream(), Employee.class, params);
            list.forEach(employee ->{
                //民族id
                System.out.println(new Nation(employee.getNation().getName()));
                employee.setNationId(nations.get(nations.indexOf(new Nation(employee.getNation().getName()))).getId());
                //政治面貌id
                employee.setPoliticId(politicsStatuses.get(politicsStatuses.indexOf(new PoliticsStatus(employee.getPoliticsStatus().getName()))).getId());
                //部门id
                employee.setDepartmentId(departments.get(departments.indexOf(new Department(employee.getDepartment().getName()))).getId());
                //职称id
                employee.setJobLevelId(joblevels.get(joblevels.indexOf(new Joblevel(employee.getJoblevel().getName()))).getId());
                //职位id
                employee.setPosId(positions.get(positions.indexOf(new Position(employee.getPosition().getName()))).getId());
            });
            if (employeeService.saveBatch(list)){
                return RespBean.success("导入成功");
            }
            return RespBean.error("导入失败");

        } catch (Exception e) {
            e.printStackTrace();
        }
        return RespBean.error("导入失败");
    }

4.分析

 List<Nation> nations = nationService.list();//获取nation的所有数据
 List<Employee> list = ExcelImportUtil.importExcel(file.getInputStream(), Employee.class, params);  
 
 list.forEach(employee ->{
	employee.getNation().getName()//根据employee获取nation的名字
	new Nation(employee.getNation().getName())//根据nation的名字创建nation对象
	nations.indexOf(new Nation(employee.getNation().getName())))//hashCode和equals进行比较拿到下标
	nations.get(nations.indexOf(new Nation(employee.getNation().getName())))//根据下标获取对应nation的数据
	nations.get(nations.indexOf(new Nation(employee.getNation().getName()))).getId()//获取nation的id
	employee.setNationId(nations.get(nations.indexOf(new Nation(employee.getNation().getName()))).getId());//存入employee对象
}