使用Collectors.toMap报java.lang.Ill

查看下面的代码：

public class DailyTest {

    public static void main(String[] args) {
        //
        List<User> users = Lists.newArrayList(
                new User(12L,"花花2"),
                new User(13L,"花花3"),
                new User(14L,"花花4"),
                new User(15L,"花花5"),
                new User(16L,"花花6"),
                new User(17L,"花花7"),
                new User(17L,"花花7")
        );
        // 转成map
        Map<Long, User> map = users.stream().collect(Collectors.toMap(User::getId, user -> user));
        System.out.println(map);
    }
}

@Data
@AllArgsConstructor
@NoArgsConstructor
class User{
    private Long id;
    private String username;
}

我们使用的是java8的特性使用stream中的api吧list转换为map
但是运行的时候报了异常
异常如下：

Exception in thread "main" java.lang.IllegalStateException: Duplicate key User(id=17, username=花花7)
    at 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.wangye.powermock.DailyTest.main(DailyTest.java:26)

那么这是为什么呢?
是因为，你把user对象的id作为map的key，但是你的list中有两个id为17L的对象，所以就会报这个重复的key的异常
需要get的代码是下面这两行：

                new User(17L,"花花7"),
                new User(17L,"花花7")

所以使用这个Collectors.toMap的时候，一定要确定，作为可以的属性，不能重复