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Streams

A stream in Java is a sequence of elements that can be processed declaratively. Streams allow you to perform operations like filtering, mapping, and reducing data in a concise and functional way. Streams do not store data; instead, they convey data from a source such as a collection, an array, or a file.

Streams in Java are created from collections and other data types using methods like stream() and parallelStream(). Operations on streams are divided into two types:

  • Intermediate: Transform a stream into another stream, such as filter(), map(), and sorted().
  • Terminal: Produce a result or a side-effect, such as forEach(), collect(), and reduce().

Technical example:

List<Integer> numbers = Arrays.asList(1, 2, 3, 4, 5);
List<Integer> evenNumbers = numbers.stream()
                                   .filter(n -> n % 2 == 0)
                                   .collect(Collectors.toList());
System.out.println(evenNumbers); // Prints: [2, 4]

Imagine a stream in Java is like a conveyor belt in a cookie factory. The conveyor belt doesn't store the cookies 🍪, but carries them through various stations where different operations are performed, such as adding chocolate chips, baking, and packaging 📦.

  1. Intermediate Operations: These are like the stations on the conveyor belt. Each station can do something with the cookies, like selecting only the ones with chocolate chips (filtering), transforming them into a different shape (mapping), or sorting them (sorting).

  2. Terminal Operations: These are the end of the line where the cookies are collected in a box (collecting) or counted (reducing). At the end of the conveyor belt, you get a final result, like a box full of cookies ready to be shipped.

Analogy example:

  • You have a conveyor belt (stream) 🛤️.
  • You select only the cookies with chocolate chips (filter) 🍫.
  • You pack the selected cookies in a box (collect) 📦.

Important Methods

Method Description
filter Filters the elements of the stream based on a given predicate.
map Transforms each element of the stream into another form using a given function.
flatMap Transforms each element into a stream and then flattens these streams into one.
sorted Sorts the elements of the stream based on a comparator or natural order.
distinct Removes duplicate elements from the stream.
limit Truncates the stream to contain no more than a given number of elements.
skip Skips the first given number of elements in the stream.
forEach Performs an action for each element of the stream.
collect Accumulates the elements of the stream into a collection or other mutable result.
reduce Reduces the elements of the stream to a single value using an associative accumulation function.
anyMatch Returns true if any elements of the stream match the given predicate.
allMatch Returns true if all elements of the stream match the given predicate.
noneMatch Returns true if no elements of the stream match the given predicate.
findFirst Returns the first element of the stream, if present.
findAny Returns any element of the stream, if present.
count Returns the count of elements in the stream.
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