For a field of the same class, I can use 

```java
stream().map(String::valueOf).collect(Collectors.joining(","));

```

How to use the stream in one line to join id of Item from the given list of RejectItem?

```
class RejectItem {
    Item item;
    String reason;
}

class Item {
    Long id;
    Date createdOn;
}

// ..................

List<RejectItem> ri;
String commaSepIdString = ri.stream ??

```