Difference between map and forEach for instance modification

Question

When I run the code below, StringBuilder instance remains empty.

String[] params = {"v1", "v2"};
StringBuilder sb = new StringBuilder();
Stream.of(params).map(param -> sb.append(param));

However, when I run the same code with forEach instead of map, it behaves as expected (appends array elements to StringBuilder instance):

String[] params = {"v1", "v2"};
StringBuilder sb = new StringBuilder();
Stream.of(params).forEach(param -> sb.append(param));

Why doesn't it change StringBuilder in the first example?

`map` is an intermediary operation, while `forEach` is a terminal operation. Since `Stream`s are evaluated lazily, the intermediary operations are ony triggered thtrough a terminal operation. — Turing85, Sep 21 '20 at 17:47
[What is the difference between intermediate and terminal operations](https://stackoverflow.com/questions/47688418/what-is-the-difference-between-intermediate-and-terminal-operations) — Michał Krzywański, Sep 21 '20 at 17:47

score 3 · Answer 1 · answered Sep 21 '20 at 17:47

3

The reason is that the stream has not been invoked.

String[] params = {"v1", "v2"};
StringBuilder sb = new StringBuilder();
Stream.of(params).map(param -> sb.append(param));

The last statement doesn't run since there is no subsequent method that activates the stream. Try this:

Stream.of(params).map(param -> sb.append(param)).forEach(System.out::println);

Now sb should have the values.

answered Sep 21 '20 at 17:47

WJS

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Thank you! Sorry but Turing85 answered first (in the comments). – Cybex Sep 21 '20 at 18:08
2

Quite alright! The important thing is that you got your answer. – WJS Sep 21 '20 at 19:29

score 1 · Accepted Answer · answered Sep 21 '20 at 17:53

Streams are evaluated lazily. This means that intermediate operations like Stream::map and Stream::filter are only evaluated when they are triggered through a terminal operation like Stream::forEach or Stream::collect.

We can rewrite the code to make it behave as expected:

String[] params = {"v1", "v2"};
StringBuilder sb = new StringBuilder();
Stream.of(params).map(sb::append).collect(Collectors.toList());
System.out.println(sb.toString());

Ideone demo

I would, however, suggest to use the forEach-example presented in the post, since this reflects the semantics of the code in a concise and clear way.

Thank you for the explanation! – Cybex Sep 21 '20 at 18:51 — Cybex, Sep 21 '20 at 18:51

Difference between map and forEach for instance modification

2 Answers2