关于stream Collectors.toMap的使用

转map时自定义key

userDOS.stream().collect(Collectors.toMap(k -> k.getUserId() + "c", Function.identity()));

当key重复时，会报IllegalStateException异常
解决办法

新的值覆盖老的值
userDOS.stream().collect(Collectors.toMap(k -> k.getUserId() + "c",Function.identity(), (oldValue, newValue) -> newValue));

value为null时会抛出空指针异常

会vale进行判空
userDOS.stream().collect(Collectors.toMap(k -> k.getUserId() + "c",UserDO-> UserDO.getUserId()==null?1l: UserDO.getUserId(), (oldValue, newValue) -> newValue));

userDOS.stream().collect(HashMap::new,(n, v) -> n.put(v.getUserId(), v.getUsername()), HashMap::putAll);