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
– LppEdd, Commented Mar 21, 2019 at 14:03
You are mutating the inside lists, why do you need anything back? Just use forEach. — Boris the Spider
– Boris the Spider, Commented Mar 21, 2019 at 14:03
@Seelenvirtuose don’t use peek for that; that’s nowhere better than map. Neither is meant to be used to manipulate the source of the Stream. — Holger
– Holger, Commented Mar 21, 2019 at 15:13

user2189998 · Accepted Answer · 2019-03-21 15:41:26Z

5

@Boris the Spider is right : use forEach :

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

edited Mar 21, 2019 at 15:41

answered Mar 21, 2019 at 14:08

user2189998

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2 Comments

Yassin Hajaj Over a year ago

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 :)

user2189998 Over a year ago

Right, always something more to remove ^^

Eran · Accepted Answer · 2019-03-21 14:03:47Z

2

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());

answered Mar 21, 2019 at 14:03

Eran

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Comments

Amit Bera · Accepted Answer · 2019-03-21 14:04:07Z

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, 2019 at 14:04

Amit Bera

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