使用stream流生成的map进行remove操作会报错

当对stream生成的map进行remove操作时，代码报错，复现代码如下：

package com.test.testdemo01.service;import com.test.testdemo01.entity.dto.DemoData;
import org.junit.Test;import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;/*** @author : xubin* @date : 2023/12/14* @description : 针对字节流生成的map存在移除问题，针对异常情况进行测试及提供解决方案*/
public class IterationDemo {/*** 测试使用字节流会造成报错*/@Testpublic void test1(){//数据准备List<DemoData> demoDataList = new ArrayList<>();DemoData demoData1 = new DemoData();demoData1.setName("zhangsan");demoData1.setSno(123);DemoData demoData2 = new DemoData();demoData2.setName("zhangsan");demoData2.setSno(456);DemoData demoData3= new DemoData();demoData3.setName("lisi");demoData3.setSno(123);demoDataList.add(demoData1);demoDataList.add(demoData2);demoDataList.add(demoData3);Map<String, List<DemoData>> collect = demoDataList.stream().collect(Collectors.groupingBy(DemoData::getName));for (String key : collect.keySet()){if ("lisi".equals(key)){collect.remove(key);}}}}

DemoData实体：

package com.test.testdemo01.entity.dto;import com.alibaba.excel.annotation.ExcelProperty;
import lombok.Data;import java.util.UUID;/*** @author : xubin* @date : 2022/11/21* @description :*/
@Data
public class DemoData {//设置excel表头名称@ExcelProperty("学生编号")private Integer sno;@ExcelProperty("学生的姓名")private String name;public static void main(String[] args) {System.out.println(UUID.randomUUID());}
}

报错详情:

java.util.ConcurrentModificationExceptionat java.util.HashMap$HashIterator.nextNode(HashMap.java:1445)at java.util.HashMap$KeyIterator.next(HashMap.java:1469)at com.test.testdemo01.service.IterationDemo.test1(IterationDemo.java:43)at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)at java.lang.reflect.Method.invoke(Method.java:498)at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)at org.junit.runners.ParentRunner.run(ParentRunner.java:413)at org.junit.runner.JUnitCore.run(JUnitCore.java:137)at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)at com.intellij.rt.junit.IdeaTestRunner$Repeater$1.execute(IdeaTestRunner.java:38)at com.intellij.rt.execution.junit.TestsRepeater.repeat(TestsRepeater.java:11)at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:35)at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:232)at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:55)Process finished with exit code -1

使用新建的hashmap没有这个问题的发生

代码：

package com.test.testdemo01.service;import com.alibaba.fastjson.JSON;
import com.test.testdemo01.entity.dto.DemoData;
import com.test.testdemo01.entity.dto.TbOrderSendFilePO;
import org.apache.commons.collections4.map.HashedMap;
import org.junit.Test;import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;/*** @author : xubin* @date : 2023/12/14* @description : 针对字节流生成的map存在移除问题，针对异常情况进行测试及提供解决方案*/
public class IterationDemo {/*** 使用普通hashmap没有这个问题*/@Testpublic void test3(){//数据准备List<DemoData> demoDataList = new ArrayList<>();DemoData demoData1 = new DemoData();demoData1.setName("zhangsan");demoData1.setSno(123);DemoData demoData2 = new DemoData();demoData2.setName("zhangsan");demoData2.setSno(456);DemoData demoData3= new DemoData();demoData3.setName("lisi");demoData3.setSno(123);demoDataList.add(demoData1);demoDataList.add(demoData2);demoDataList.add(demoData3);Map<String, List<DemoData>> collect = new HashedMap<>();List<DemoData> demoDataList1 = new ArrayList<>();demoDataList1.add(demoData1);demoDataList1.add(demoData2);List<DemoData> demoDataList2 = new ArrayList<>();demoDataList2.add(demoData3);collect.put("zhangsan",demoDataList1);collect.put("lisi",demoDataList2);System.out.println(JSON.toJSONString(collect));for (String key : collect.keySet()){if ("lisi".equals(key)){System.out.println("need delete");collect.remove(key);}}System.out.println(JSON.toJSONString(collect));}}

输出结果：

{"zhangsan":[{"name":"zhangsan","sno":123},{"name":"zhangsan","sno":456}],"lisi":[{"name":"lisi","sno":123}]}
need delete
{"zhangsan":[{"name":"zhangsan","sno":123},{"name":"zhangsan","sno":456}]}Process finished with exit code 0

使用迭代器避免这个问题的发生

修改后代码

package com.test.testdemo01.service;import com.alibaba.fastjson.JSON;
import com.test.testdemo01.entity.dto.DemoData;
import com.test.testdemo01.entity.dto.TbOrderSendFilePO;
import org.junit.Test;import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;/*** @author : xubin* @date : 2023/12/14* @description : 针对字节流生成的map存在移除问题，针对异常情况进行测试及提供解决方案*/
public class IterationDemo {/*** 使用迭代器能够避免这个问题*/@Testpublic void test2(){//数据准备List<DemoData> demoDataList = new ArrayList<>();DemoData demoData1 = new DemoData();demoData1.setName("zhangsan");demoData1.setSno(123);DemoData demoData2 = new DemoData();demoData2.setName("zhangsan");demoData2.setSno(456);DemoData demoData3= new DemoData();demoData3.setName("lisi");demoData3.setSno(123);demoDataList.add(demoData1);demoDataList.add(demoData2);demoDataList.add(demoData3);Map<String, List<DemoData>> collect = demoDataList.stream().collect(Collectors.groupingBy(DemoData::getName));System.out.println(JSON.toJSONString(collect));Iterator<Map.Entry<String, List<DemoData>>> iterator1 = collect.entrySet().iterator();while (iterator1.hasNext()){Map.Entry<String, List<DemoData>> next = iterator1.next();if ("lisi".equals(next.getKey())){System.out.println("need delete");iterator1.remove();}}System.out.println(JSON.toJSONString(collect));}}

输出日志：

{"lisi":[{"name":"lisi","sno":123}],"zhangsan":[{"name":"zhangsan","sno":123},{"name":"zhangsan","sno":456}]}
need delete
{"zhangsan":[{"name":"zhangsan","sno":123},{"name":"zhangsan","sno":456}]}Process finished with exit code 0