Issue with java8 lambda expression and foreach

Question

I have a list like below

List<String> fruits = new ArrayList<>();
fruits.add("apple");
fruits.add("mango");
fruits.add("grapes");
System.out.println(fruits.toString());

I am using lambda expression for printing the list like

fruits.forEach(item->System.out.println(item));

and its working fine my requirement is I need to iterate over the list and concatenate the items to a string

String stringFruits = "";
fruits.forEach(item->stringFruits = stringFruits+item);

this is giving a compile time error saying variable values used in lambda should be effectively final is there any way I can do it in java 8 ?

String stringFruits = fruits.stream().collect(Collectors.joining()); — Eran
– Eran, Commented May 4, 2017 at 7:55

Eugene · Accepted Answer · 2017-05-04 07:57:03Z

7

You need to join via a delimiter. In this case the delimiter is going to be ,; but you can choose any you want.

 String result = fruits.stream().collect(Collectors.joining(","));
 System.out.println(result);

answered May 4, 2017 at 7:57

Eugene

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1 Comment

Holger Over a year ago

Big gun for a small problem. In that case, there’s the convenient simple method String.join(…)…

ΦXocę 웃 Пepeúpa ツ · Accepted Answer · 2017-05-05 05:58:58Z

4

Java8 introduced a StringJoiner that does what you need (is another alternative to erans comment)

StringJoiner sj = new StringJoiner("-,-");
fruits.forEach(item -> sj.add(item));

here the doc

edit:

for the posterity you can do as well:

String result = fruits.stream().collect(Collectors.joining(","));
System.out.println(result);

or

String stringFruits = String.join(", ", fruits);
System.out.println(stringFruits);

credits and thanks to @Holger

edited May 5, 2017 at 5:58

answered May 4, 2017 at 7:59

ΦXocę 웃 Пepeúpa ツ

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1 Comment

Holger Over a year ago

And since this is so practical, there’s String.join(…) already doing that…

Holger · Accepted Answer · 2017-05-04 13:55:14Z

3

You don’t need to do that by hand at all. If your source is a List, you can just use

String stringFruits = String.join(", ", fruits);
System.out.println(stringFruits);

The first argument to String.join(…) is the delimiter.

answered May 4, 2017 at 13:55

Holger

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Comments

Robin Topper · Accepted Answer · 2017-05-04 07:59:25Z

1

Or a pre-Java8 solution:

String combined = "";
for (String s : fruits) {
   combined += s;
}

... and one with delimiter:

String combined = "";
for (String s : fruits) {
   combined = combined + ", " + s;
}

answered May 4, 2017 at 7:59

Robin Topper

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1 Comment

shmosel Over a year ago

It's usually best to use a StringBuilder when you're looping. Concatentation gets exponentially slower in a loop.

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