使用stream的Collectors.toMap()方法常见问题如何解决(stream,开发技术)

使用stream的Collectors.toMap()方法常见问题

java8开始的流式编程很大程度上简化了我们的代码，提高了开发效率。

我们经常会使用到stream的Collectors.toMap(）来将List转换Map

在使用过程中有两个小坑需要注意

1、java.lang.IllegalStateException: Duplicate key

2、java.lang.NullPointerException

第一个是由于在List转Map过程中Map集合的key重复导致的；

第二个是由于在List转Map过程中Map集合的value有null导致的（当存在value值为空时，使用Collectors.toMap()会报NPE，因为底层调用了Map的merge方法，而map方法规定了此处的vlue不能为null，从而抛出空指针异常）；

解决方案

1、Collectors.toMap(dto ->key值 , dto -> dto,(v1,v2) -> v1)

在后面添加(v1,v2)->v1 指定选取第一个值 当key值重复的时候，根据情况而定选取第一个还是第二个）

2、自定义一个Map来接收，不使用Collectors.toMap()

第一种情况示例：

importcom.google.common.collect.Lists;importjava.util.List;importjava.util.Map;importjava.util.stream.Collectors;importlombok.Data;publicclassTest{privatestaticList<User>userList=Lists.newArrayList();@DatapublicstaticclassUser{privateStringuserCode;privateStringuserName;}/***初始化数据*(这里的userCode=10002重复)*/publicstaticvoidinitData(){Useruser1=newUser();user1.setUserCode("10001");user1.setUserName("张三");Useruser2=newUser();user2.setUserCode("10002");user2.setUserName("李四");Useruser3=newUser();user3.setUserCode("10002");user3.setUserName("王五");userList.add(user1);userList.add(user2);userList.add(user3);}publicstaticvoidmain(String[]args){initData();//反例//Map<String,String>userMap=userList.stream().collect(Collectors.toMap(User::getUserCode,User::getUserName));//正例，在后面添加(u1,u2)->u1指定选取第一个值当key值重复的时候，根据情况而定选取第一个还是第二个Map<String,String>userMap=userList.stream().collect(Collectors.toMap(User::getUserCode,User::getUserName,(u1,u2)->u1));System.out.println(userMap);}}

第二种情况示例：

importcom.google.common.collect.Lists;importjava.util.HashMap;importjava.util.List;importjava.util.Map;importjava.util.stream.Collectors;importlombok.Data;publicclassTest{privatestaticList<User>userList=Lists.newArrayList();@DatapublicstaticclassUser{privateStringuserCode;privateStringuserName;}/***初始化数据*(这里的userCode=10003的userName为空)*/publicstaticvoidinitData(){Useruser1=newUser();user1.setUserCode("10001");user1.setUserName("张三");Useruser2=newUser();user2.setUserCode("10002");user2.setUserName("李四");Useruser3=newUser();user3.setUserCode("10003");user3.setUserName(null);userList.add(user1);userList.add(user2);userList.add(user3);}publicstaticvoidmain(String[]args){initData();//反例//Map<String,String>userMap=userList.stream().collect(Collectors.toMap(User::getUserCode,User::getUserName));//正例(如果对转换后的顺序有要求，这里还可以使用LinkedHashMap)Map<String,String>userMap=userList.stream().collect(HashMap::new,(map,user)->map.put(user.getUserCode(),user.getUserName()),HashMap::putAll);System.out.println(userMap);}}

Stream ToMap(Collectors.toMap) 实践

Requirements

List TO Map

List Stream 转换 Map时向collect()方法中传递Collector对象，对象由Collectors.toMap()方法返回。

如下实现List转换为Map

List<GroupBrandCateBO>list=newArrayList<>(Arrays.asList(newGroupBrandCateBO("v1","g1","b1"),newGroupBrandCateBO("v1","g1","b1"),newGroupBrandCateBO("v3","g3","b3")));Map<String,String>map=list.stream().collect(Collectors.toMap(item->item.getVersion(),item->item.getGroupCode(),(oldVal,currVal)->oldVal,LinkedHashMap::new));System.out.println(map.getClass());Map<String,String>map0=list.stream().collect(Collectors.toMap(item->item.getVersion(),item->item.getGroupCode(),(oldVal,currVal)->oldVal));System.out.println(map0.getClass());System.out.println(map0.toString());Map<String,String>map1=list.stream().collect(Collectors.toMap(GroupBrandCateBO::getVersion,GroupBrandCateBO::getGroupCode));System.out.println(map1.toString());

Console
class java.util.LinkedHashMap
class java.util.HashMap
{v1=g1, v3=g3}
Exception in thread “main” java.lang.IllegalStateException: Duplicate key g1
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
…

问题分析

toMap()函数重载：

• 未指定合并函数mergeFunction情况下，传入throwingMerger()返回BinaryOperator对象，当出现key重复时，调用合并函数！

• 未指定Supplier实例情况下，默认生成HashMap实例。

publicstatic<T,K,U>Collector<T,?,Map<K,U>>toMap(Function<?superT,?extendsK>keyMapper,Function<?superT,?extendsU>valueMapper){returntoMap(keyMapper,valueMapper,throwingMerger(),HashMap::new);}publicstatic<T,K,U>Collector<T,?,Map<K,U>>toMap(Function<?superT,?extendsK>keyMapper,Function<?superT,?extendsU>valueMapper,BinaryOperator<U>mergeFunction){returntoMap(keyMapper,valueMapper,mergeFunction,HashMap::new);}publicstatic<T,K,U,MextendsMap<K,U>>Collector<T,?,M>toMap(Function<?superT,?extendsK>keyMapper,Function<?superT,?extendsU>valueMapper,BinaryOperator<U>mergeFunction,Supplier<M>mapSupplier){BiConsumer<M,T>accumulator=(map,element)->map.merge(keyMapper.apply(element),valueMapper.apply(element),mergeFunction);returnnewCollectorImpl<>(mapSupplier,accumulator,mapMerger(mergeFunction),CH_ID);}privatestatic<T>BinaryOperator<T>throwingMerger(){return(u,v)->{thrownewIllegalStateException(String.format("Duplicatekey%s",u));};}

补充

关于合并函数

List<GroupBrandCateBO>list=newArrayList<>(Arrays.asList(newGroupBrandCateBO("v1","g1","b1"),newGroupBrandCateBO("v1","g2","b2"),newGroupBrandCateBO("v1","g2","b2"),newGroupBrandCateBO("v3","g3","b3")));Map<String,String>map00=list.stream().collect(Collectors.toMap(item->item.getVersion(),item->item.getGroupCode(),(oldVal,currVal)->currVal));Map<String,String>map01=list.stream().collect(Collectors.toMap(item->item.getVersion(),item->item.getGroupCode(),(oldVal,currVal)->oldVal+currVal));System.out.println(map00.toString());System.out.println(map01.toString());

Console
{v1=g2, v3=g3}
{v1=g1g2g2, v3=g3}

传入Lambda表达式将转化为BinaryOperator<U> mergeFunction对象，合并处理value，非Key！！！

比如：

(oldVal,currVal)->currVal)//key相同时当前值替换原始值(oldVal,currVal)->oldVal+currVal//key相同时保留原始值和当前值