Java stream how to add value in nested List of List

Question

I have a List<List<Object>> type data. I need to add the same value in the List list. I tried the following but it doesn't work as its type is List<Boolean>.

List<List<Object>> data = ...;
data.stream().map(v -> v.add("test")).collect(Collectors.toList());

How can I get in same type List<List<Object>> ?

I have the following data:

"data": [
        [
            5,
            "Johnny",
            "Lollobrigida"                
        ],
        [
            6,
            "Bette",
            "Nicholson"               
        ],
        [
            7,
            "Grace",
            "Mostel"                
        ],
        [
            8,
            "Matthew",
            "Johansson"                
        ]
     ]

I want to change it to:

"data": [
        [
            5,
            "Johnny",
            "Lollobrigida",
            "test"
        ],
        [
            6,
            "Bette",
            "Nicholson",
            "test"               
        ],
        [
            7,
            "Grace",
            "Mostel" ,
            "test"               
        ],
        [
            8,
            "Matthew",
            "Johansson",
            "test"                
        ]
     ]

I can't get what you want to do. Ahhh okay, then don't use `map`. — LppEdd, Mar 21 '19 at 14:03
You are mutating the inside lists, why do you need _anything_ back? Just use `forEach`. — Boris the Spider, Mar 21 '19 at 14:03
@Seelenvirtuose [don’t use `peek`](https://stackoverflow.com/q/33635717/2711488) for that; that’s nowhere better than `map`. Neither is meant to be used to manipulate the source of the Stream. — Holger, Mar 21 '19 at 15:13

user2189998 · Answer 1 · 2019-03-21T15:41:26.590

5

@Boris the Spider is right : use forEach :

data.forEach(v -> v.add("test"));

edited Mar 21 '19 at 15:41

answered Mar 21 '19 at 14:08

user2189998

718
6
10

1

I don't get why this has been tackled so complex when the answer was just to use a `forEach`. There is no need in creating a `stream` even here :) – Yassin Hajaj Mar 21 '19 at 14:23
1

Right, always something more to remove ^^ – user2189998 Mar 21 '19 at 15:41

score 2 · Answer 2 · answered Mar 21 '19 at 14:03

List.add() returns a boolean, but you want your map() to return the List to which you added the new element.

You need:

List<List<Object>> out = 
    data.stream()
        .map(v -> {v.add("test"); return v;})
        .collect(Collectors.toList());

Note that if you don't want to mutate the original inner Lists, you can create a copy of them:

List<List<Object>> out = 
    data.stream()
        .map(v -> {
            List<Object> l = new ArrayList<>(v);
            l.add("test"); 
            return l;
        })
        .collect(Collectors.toList());

score 2 · Answer 3 · answered Mar 21 '19 at 14:04

2

You need to return the list from map after adding the element. Below solution should work for you:

List<List<Object>> dataModified = data.stream().map(v -> {
    v.add("test");
    return v;
}).collect(Collectors.toList());

answered Mar 21 '19 at 14:04

Amit Bera

7,075
1
19
42

Java stream how to add value in nested List of List

3 Answers3