How does Java stream API work?

Norman 55 Published: 07/26/2024

How does Java stream API work?

I'd be happy to explain how the Java Stream API works.

The Java Stream API is a set of classes and interfaces in Java Standard Edition (Java SE) that allows you to process data as a series of operations, one after another, with an emphasis on functional programming techniques such as filtering, mapping, and reducing. This API is designed for handling collections of objects, and it provides many useful methods for operating on those collections.

To understand how the Java Stream API works, let's start by looking at a simple example. Suppose you have a list of integers representing some scores:

List scores = Arrays.asList(90, 80, 70, 95, 85, 75);

And suppose you want to get the average score (after filtering out any scores below 80). Here's how you might do it using the Stream API:

double averageScore = scores.stream()

.filter(score -> score >= 80)


.mapToInt(Integer::intValue)


.average()


.orElse(0.0);

Now, let's break this down step by step.

First, we call scores.stream() to convert the list of integers into a Stream. The Stream interface represents a sequence of elements supporting parallel and functional-style operations. It's the core interface for processing data as a stream in Java.

Next, we apply a filter using .filter(score -> score >= 80). This method creates a new Stream that contains only those elements from the original stream that meet the condition specified by the lambda function (in this case, scores greater than or equal to 80).

Then, we map each integer to an int value using .mapToInt(Integer::intValue) and then call .average() to calculate the average of all these filtered integers. The average method returns an OptionalInt that contains the result, so we use .orElse(0.0) to provide a default value if the stream is empty (in this case, 0.0).

This is a simple example of how you might use the Java Stream API to process a collection of objects. The API provides many other useful methods for filtering, mapping, reducing, and more.

Here are some key concepts to keep in mind when working with streams:

Streams can be created from collections, arrays, or I/O channels. You can think of a stream as a pipeline that you can pipe data through. Each method call adds another step to the pipeline. Most methods on Stream return a new Stream that you must chain together. Some methods on Stream produce a terminal result, which means they don't return a Stream but instead produce some final value. Examples include collect, reduce, and count. Streams can be parallelized with the .parallel() method.

With this knowledge, you should have a good grasp of how to use the Java Stream API for processing collections of objects.

Java stream api tutorial w3schools

I'm happy to help!

The Java Stream API is a powerful tool for processing data streams in a concise and efficient manner. Introduced in Java 8, this API revolutionized the way developers work with collections and data streams.

Here's a comprehensive guide on using the Java Stream API from W3Schools:

Stream Basics

A stream is a sequence of elements supporting parallel and functional-style operations. It allows you to process data without having to write explicit loops or recursion. The key components are:

Source: Where your data comes from. Intermediate Operations: Methods like filter(), map(), distinct(), etc., that transform the stream. Terminal Operation: A method that produces a result, such as collect().

Creating Streams

To start working with streams, you need to create one. You can do this using various methods:

Collections.stream(collection): Creates a stream from a collection like List or Set. IntStream.range(start, end): Creates an integer stream for ranges. Files.lines(path): Reads lines from a file.

Intermediate Operations

These are the building blocks of your stream pipeline. Here are some examples:

Filter: Removes elements that don't meet a condition (stream.filter(predicate)). Map: Transforms each element to something else (stream.map(function)). Distinct: Removes duplicate elements. sorted(): Sorts the elements.

Terminal Operations

These produce the final result. Here are some examples:

collect(): Collects all elements into a new collection (e.g., List or Set). **forEach()`: Performs an operation on each element, like printing. reduce(): Combines elements to produce a single value.

Example Use Cases

Find the average of numbers: Use stream.mapToInt(), then average(). Get unique words from a text file: Use Files.lines(path).collect(Collectors.toSet()). Calculate sum and average of scores: 
List scores = Arrays.asList(90, 85, 78);

int sum = scores.stream().mapToInt(i -> i).sum();


double avg = sum / scores.size();

Best Practices

Use stream() instead of stream().collect(toList()) when you don't need to store the results in a collection. Avoid using streams for small collections, as they can be less efficient.

That's it! W3Schools provides an extensive Java Stream API tutorial, covering all the basics and more. This guide should give you a solid foundation to start working with streams in your Java projects.

Do you have any specific questions about this topic or would you like me to elaborate on certain aspects?

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