<\/p>\n
Introduction<\/h2>\n
Let’s start responsibly by Returns a stream consisting of the elements of this stream, additionally performing the provided action<\/strong> on each element as elements are consumed from the resulting stream. The result of such operation is not surprising:<\/p>\n but… what will happen if we replace the terminal collect()<\/em> operation with a forEach()?<\/em><\/p>\n It might be tempting to think that we’ll see a series of peek() logs followed by a series of for-each logs but this is not the case:<\/p>\n It gets even more interesting if we get rid of the forEach<\/em> call:<\/p>\n the result would be nothing:<\/p>\n As JavaDoc<\/a> states:<\/p>\n Intermediate operations return a new stream. They are always lazy; executing an intermediate operation such as filter() does not actually perform any filtering, but instead creates a new stream that, when traversed, contains the elements of the initial stream that match the given predicate. Traversal of the pipeline source does not begin until the terminal operation of the pipeline is executed. This is why it’s possible to represent and manipulate infinite\u00a0sequences<\/a> using Streams. (Because where do you store an infinite sequence? In the cloud?):<\/p>\n The above operation completes almost immediately because of the lazy character of Stream traversal. Of course, if you try to collect the whole infinite sequence to some data structure, even laziness will not save you.<\/p>\n So, we can see that peek() can’t be treated as an intermediate for-each replacement because it invokes the passed Consumer only on elements that are visited by the Stream.<\/strong><\/p>\n Unfortunately, Streams do not always behave entirely lazily.<\/a><\/p>\n Further inspection of the official docs reveals a note<\/a>:<\/p>\n This method exists mainly<\/strong> to support debugging, where you want to see the elements as they flow past a certain point in a pipeline We can see that non-debugging usages are not forbidden nor discouraged.<\/strong><\/p>\n So, technically, we should be able to, e.g., modify the Stream elements on the fly which would not be possible in the immutable, functional world. Let’s use the infamous mutable java.util.Date:<\/a><\/p>\n and we can observe that the result is far away from the standard epoch, which means the mutating operation was indeed applied.<\/p>\n The problem is that this behaviour is highly deceiving because certain Stream implementations can optimize out peek() calls.<\/strong><\/p>\n This gets clarified in the early draft of JDK 9’s docs<\/a>\u00a0eventually:<\/p>\nRTFM<\/del> inspecting peek()’s<\/em> user’s manual.<\/a><\/p>\n
\nSounds pretty straightforward. We can use it for applying side-effects for every consumed Stream element:<\/p><\/blockquote>\nStream.of(\"one\", \"two\", \"three\")\r\n .peek(e -> System.out.println(e + \" was consumed by peek().\"))\r\n .collect(Collectors.toList());<\/pre>\n
one was consumed by peek().\r\ntwo was consumed by peek().\r\nthree was consumed by peek().<\/pre>\n
Stream\/peek Lazy Evaluation<\/h4>\n
Stream.of(\"one\", \"two\", \"three\")\r\n .peek(e -> System.out.println(e + \" was consumed by peek().\"))\r\n .forEach(System.out::println);<\/pre>\n
one was consumed by peek().\r\none\r\ntwo was consumed by peek().\r\ntwo\r\nthree was consumed by peek().\r\nthree<\/pre>\n
Stream.of(\"one\", \"two\", \"three\")\r\n .peek(e -> System.out.println(e + \" was consumed by peek().\"));<\/pre>\n
<\/pre>\n
\nSo, because of the\u00a0lazy evaluation<\/a> Stream pipelines are always being traversed “vertically” and not “horizontally” which allows to avoid doing unnecessary calculations. Additionally, the traversal is triggered only when a terminal method is present. Hence, the observed behaviour.<\/p><\/blockquote>\nStream.iterate(0, i -> i + 1)\r\n .peek(System.out::println)\r\n .findFirst();<\/pre>\n
Proper Usage<\/h2>\n
\nThe point of confusion is the “mainly” word. Let’s have a peek(pun intended) at the English dictionary:<\/p><\/blockquote>\n![\"\"](\"http:\/\/4comprehension.com\/wp-content\/uploads\/2017\/08\/Screen-Shot-2017-08-24-at-12.45.09.png\")
<\/p>\nStream.of(Date.from(Instant.EPOCH))\r\n .peek(d -> d.setTime(Long.MAX_VALUE))\r\n .forEach(System.out::println);\r\n\r\n\/\/ Sun Aug 17 08:12:55 CET 292278994<\/pre>\n