文章目录
前言问题复现解决方案前言
最近工作中发现同事写代码遗留了一个bug，在使用Collectors.toMap的时候会出现Exception in thread “main” java.lang.IllegalStateException: Duplicate key XXX的错误，下面对当时的问题代码进行复现，并提出解决方案进行解决。
问题复现
问题代码示例如下：
public class Test1 {public static void main(String[] args) {List<User> userList = new ArrayList<>();userList.add(new User(1L, "aaa"));userList.add(new User(2L, "bbb"));userList.add(new User(3L, "ccc"));userList.add(new User(2L, "ddd"));userList.add(new User(3L, "eee"));Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getName));System.out.println(map);}}}
实体User定义如下：
@AllArgsConstructor@Datapublic class User {private Long id;private String name;}
运行上述代码将会报出如下的错误：
Exception in thread "main" java.lang.IllegalStateException: Duplicate key bbbat java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)at java.util.HashMap.merge(HashMap.java:1254)at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1382)at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)at com.stream.toMapTest.Test1.main(Test1.java:27)
问题原因也很显然，是因为在将User的List集合转换为Map集合时出现了重复的Key导致，key重复的问题在工作中也经常出现，比如商品的skuId、订单id等。那么如何解决这个问题呢？
解决方案
其实方法很简单，只需要使用Collector.toMap()的另外一个api即可，具体如下：
public static <T, K, U>Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,Function<? super T, ? extends U> valueMapper,BinaryOperator<U> mergeFunction) {return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);}
public class Test1 {public static void main(String[] args) {List<User> userList = new ArrayList<>();userList.add(new User(1L, "aaa"));userList.add(new User(2L, "bbb"));userList.add(new User(3L, "ccc"));userList.add(new User(2L, "ddd"));userList.add(new User(3L, "eee"));Map<Long, String> map = userList.stream().collect(Collectors.toMap(User::getId, User::getName, (v1,v2)->v2));System.out.println(map);}}
