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Questions tagged [java-stream]

Use this tag for questions related to the use of the Stream API. It was introduced in Java 8 and supports functional-style operations on streams of values, such as filter-map-reduce pipelines on collections.

The Streams API is an API introduced in Java 8 that supports functional-style operations on streams of values, such as filter-map-reduce pipelines on collections. A primary design goal of the Streams API is to support parallel processing.

Features

  • Mapping and flat-mapping
  • Filtering
  • Reducing and aggregating such as count, sum or average
  • Collecting and grouping to Map, List and Set.
  • Summarizing

References

11473 questions
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Java 8 lambdas, Function.identity() or t->t

I have a question regarding the usage of the Function.identity() method. Imagine the following code: Arrays.asList("a", "b", "c") .stream() .map(Function.identity()) // <- This, .map(str -> str) // <- is the…
user4464654
340
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5 answers

What is difference between Collection.stream().forEach() and Collection.forEach()?

I understand that with .stream(), I can use chain operations like .filter() or use parallel stream. But what is difference between them if I need to execute small operations (for example, printing the elements of the…
VladS
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Java 8 Streams: multiple filters vs. complex condition

Sometimes you want to filter a Stream with more than one condition: myList.stream().filter(x -> x.size() > 10).filter(x -> x.isCool()) ... or you could do the same with a complex condition and a single filter: myList.stream().filter(x -> x.size() >…
deamon
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24 answers

Filter Java Stream to 1 and only 1 element

I am trying to use Java 8 Streams to find elements in a LinkedList. I want to guarantee, however, that there is one and only one match to the filter criteria. Take this code: public static void main(String[] args) { LinkedList users = new…
ryvantage
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306
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Using Java 8's Optional with Stream::flatMap

The new Java 8 stream framework and friends make for some very concise Java code, but I have come across a seemingly-simple situation that is tricky to do concisely. Consider a List things and method Optional resolve(Thing thing). I…
Yona Appletree
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306
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9 answers

Why does Stream not implement Iterable?

In Java 8 we have the class Stream, which curiously have a method Iterator iterator() So you would expect it to implement interface Iterable, which requires exactly this method, but that's not the case. When I want to iterate over a Stream…
roim
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3 answers

Java 8 method references: provide a Supplier capable of supplying a parameterized result

I'd like to use java.util.Optional.orElseThrow() with an Exception type that asks for a constructor parameter. Something like this: .orElseThrow(MyException::new(someArgument)) // obviously NOT working Is there a way to create a Supplier that…
mdo
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20 answers

How to convert List to Map?

Recently I have conversation with a colleague about what would be the optimal way to convert List to Map in Java and if there any specific benefits of doing so. I want to know optimal conversion approach and would really appreciate if any one can…
Rachel
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10 answers

Java8: HashMap to HashMap using Stream / Map-Reduce / Collector

I know how to "transform" a simple Java List from Y -> Z, i.e.: List x; List y = x.stream() .map(s -> Integer.parseInt(s)) .collect(Collectors.toList()); Now I'd like to do basically the same with a Map,…
Benjamin M
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15 answers

Using streams to convert a list of objects into a string obtained from the toString method

There are a lot of useful new things in Java 8. E.g., I can iterate with a stream over a list of objects and then sum the values from a specific field of the Object's instances. E.g. public class AClass { private int value; public int getValue()…
mat_boy
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3 answers

Why does Iterable not provide stream() and parallelStream() methods?

I am wondering why the Iterable interface does not provide the stream() and parallelStream() methods. Consider the following class: public class Hand implements Iterable { private final List list = new ArrayList<>(); private…
skiwi
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13 answers

How to use a Java8 lambda to sort a stream in reverse order?

I'm using java lambda to sort a list. how can I sort it in a reverse way? I saw this post, but I want to use java 8 lambda. Here is my code (I used * -1) as a hack Arrays.asList(files).stream() .filter(file -> isNameLikeBaseLine(file,…
Elad Benda2
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8 answers

Java Stream API - Best way to transform a list: map or forEach?

I have a list, myListToParse, where I want to filter the elements and apply a method on each element, and add the result in another list, myFinalList. With the Stream API (added in Java 8), I noticed that I can do it in 2 different ways. I would…
Emilien Brigand
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248
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6 answers

Why are Java Streams once-off?

Unlike C#'s IEnumerable, where an execution pipeline can be executed as many times as we want, in Java a stream can be 'iterated' only once. Any call to a terminal operation closes the stream, rendering it unusable. This 'feature' takes away a lot…
Vitaliy
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232
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7 answers

Java 8: How do I work with exception throwing methods in streams?

Suppose I have a class and a method class A { void foo() throws Exception() { ... } } Now I would like to call foo for each instance of A delivered by a stream like: void bar() throws Exception { Stream as = ... as.forEach(a ->…
Bastian
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