How to convert nested for loop with extra command to java stream

Question

I have a nested for loop. Every message class have relation to MessageDetail class (one to many relation) and I want to convert below for loop to stream.

for (Message m:messages){
        System.out.println(m.getText());
        for (MessageDetail m2:m.getMessageDetails()){
            System.out.println(m2.getId+m2.getStatus());
        }
    }

I try this code but i can not show e.getText in upper level.

messages.stream().flatMap(e-> e.getMessageDetails().stream().map(
                e2->e.getText()+"-->{"+e2.getId()+"," +e2.getStatus()+"}"
                )).collect(Collectors.toList());

The output like:

[
 Hi-->{1,OK},
 Hi-->{2,OK},
 Hi-->{4,NOT_DELIVER},
 Bye-->{5,OK},
 Bye-->{6,NOT_DELIVER},
 Bye-->{7,NOT_DELIVER},
]

But I want to like this:

  [
    Hi [ {2,OK},{3,OK},{4,NOT_DELIVER}]
    Bye[ {5,OK},{6,NOT_DELIVER},{7,NOT_DELIVER}]
  ]

messages.stream().flatMap(e-> { System.out.println(e.getText()); return e.getMessageDetails().stream();}).map( e->e.getStatus()+e.getMessage()).forEach(System.out::println); — Naman
– Naman, Commented Jul 30, 2019 at 8:40

Sweeper · Accepted Answer · 2019-07-30 08:54:00Z

flatMap should be used only when you want each element in your stream to transform into multiple elements in the resulting stream. Here, each Message in the original stream correspond to one String in the resulting list, so you should not use flatMap.

Use map instead:

messages.stream().map(
    e -> e.getText() + "-->" + 
        e.getMessageDetails().stream().map(d -> "{" + d.getId() + ", " + d.getStatus() + "}").collect(Collectors.toList()).toString())
    .collect(Collectors.toList());

If you can edit the Message class and whatever class getMessageDetails returns, I suggest that you override their toString methods (or write your own method that returns a string) instead:

// Message class:
public String toString() {
    return getText() + "-->" + getMessageDetails().toString();
}

// MessageDetails class
public String toString() {
    return "{" + getId() + ", " + getStatus() + "}";
}

Then you could just do:

messages.stream().map(Message::toString).collect(Collectors.toList());

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