Java拼接字符串报错Operator '+' cannot be applied to 'String','void'

使用forEach遍历对象列表拼接字符串时，触发编译错误：Operator '+' cannot be applied to 'java.lang.String', 'void'，无法定位问题原因。
报错出现在sendMailOwner方法的邮件正文拼接逻辑中，预期通过PaidRequest.getDatesRequest().forEach((p)->p.getStartDate)的写法，从PaidRequest的List<DatesRequest>类型属性datesRequest中取出所有元素的startDate、endDate字段，拼接到待发送的邮件正文中。

关联代码 #

异常方法代码 #

public void sendMailOwner(VacacionRequest PaidRequest) {
        CollaboratorDTO validator = OrganizationalUintService.findValidator(collaboratorTransformer.entityTranferToDTO(PaidRequest.getCollaborator()));
        EmailService.sendSimpleMessage(PaidRequest.getCollaborator().getEmail(),
                "EverHoliday",
                " Bonjour "+PaidRequest.getCollaborator().getFirstname()+" "+PaidRequest.getCollaborator().getLastname()+","
                + "\n Votre demande de Congé payé du date "+PaidRequest.getDatesRequest().forEach((p)->p.getStartDate)+"au"+PaidRequest.getDatesRequest().forEach((p)->p.getEndDate)+" est en attente de validation par : "
                        +validator.getLastname()+" "+validator.getFirstname()
                + " \n Cordialement.");
    }

代码中PaidRequest.getDatesRequest().forEach((p)->p.getStartDate)的预期作用是获取datesRequest列表内所有元素的startDate值

DatesRequest模型定义 #

package com.example.demo.model;

import java.time.LocalDate;

import org.springframework.data.neo4j.core.schema.GeneratedValue;
import org.springframework.data.neo4j.core.schema.Id;
import org.springframework.data.neo4j.core.schema.Node;
@Node
public class DatesRequest {
    @Id @GeneratedValue
    private Long id;    
 private LocalDate startDate;
 private LocalDate endDate;
 private double duration;
public DatesRequest(LocalDate startDate, LocalDate endDate, double duration) {
    this.startDate = startDate;
    this.endDate = endDate;
    this.duration = duration;
}
public DatesRequest() {
}
public LocalDate getStartDate() {
    return startDate;
}
public void setStartDate(LocalDate startDate) {
    this.startDate = startDate;
}
public LocalDate getEndDate() {
    return endDate;
}
public void setEndDate(LocalDate endDate) {
    this.endDate = endDate;
}
public double getDuration() {
    return duration;
}
public void setDuration(double duration) {
    this.duration = duration;
}
public Long getId() {
    return id;
}
public void setId(Long id) {
    this.id = id;
}

}

PaidRequest关联构造函数 #

public PaidRequest(LocalDate requestDate, String statut, String typeOfTime, List<DatesRequest> datesRequest,
        Collaborator collaborator, String description, double balanceUsed,String justification) {
    super(requestDate, statut, typeOfTime, datesRequest, collaborator);
    this.description = description;
    this.balanceUsed = balanceUsed;
    this.justification = justification;
}

• Java中List.forEach()方法的返回值类型为void，不返回任何执行结果，无法直接参与字符串+拼接操作，这是编译报错的直接原因：编译器检测到你试图将无返回值的forEach执行结果和String类型值做加法运算，类型不匹配因此抛出对应错误。
• 代码中写的lambda表达式(p)->p.getStartDate仅在遍历过程中读取了每个元素的startDate值，既没有对取值做存储，也没有做拼接聚合，完全无法实现“提取列表所有日期值拼接成字符串”的预期效果。

不要将forEach直接写入字符串拼接逻辑，根据业务场景选择对应实现：

• 如果datesRequest可能包含多个时间段（比如跨段请假），使用Stream流做字段提取和字符串聚合：

  // 注意类顶部需要导入 java.util.stream.Collectors 包
  public void sendMailOwner(VacacionRequest PaidRequest) {
          CollaboratorDTO validator = OrganizationalUintService.findValidator(collaboratorTransformer.entityTranferToDTO(PaidRequest.getCollaborator()));
          // 提前聚合拼接所有开始、结束日期，多个日期用逗号分隔
          String startDates = PaidRequest.getDatesRequest().stream()
                  .map(DatesRequest::getStartDate)
                  .map(LocalDate::toString)
                  .collect(Collectors.joining(", "));
          String endDates = PaidRequest.getDatesRequest().stream()
                  .map(DatesRequest::getEndDate)
                  .map(LocalDate::toString)
                  .collect(Collectors.joining(", "));
          
          EmailService.sendSimpleMessage(PaidRequest.getCollaborator().getEmail(),
                  "EverHoliday",
                  " Bonjour "+PaidRequest.getCollaborator().getFirstname()+" "+PaidRequest.getCollaborator().getLastname()+","
                  + "\n Votre demande de Congé payé du date "+ startDates +" au "+ endDates +" est en attente de validation par : "
                          +validator.getLastname()+" "+validator.getFirstname()
                  + " \n Cordialement.");
      }
  

• 如果业务上单次请假仅对应一个时间段（datesRequest列表永远只有1个元素），直接取首个元素的字段即可，无需遍历：

  DatesRequest requestDate = PaidRequest.getDatesRequest().get(0);
  // 字符串拼接时直接使用 requestDate.getStartDate()、requestDate.getEndDate()
  

内容的提问来源于stack exchange，提问作者Nouhaila Khaouti