關於stream Collectors.toMap的使用

轉map時自定義key

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

當key重復時，會報IllegalStateException異常
解決辦法

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

value為null時會拋出空指針異常

會vale進行判空
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);