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How Streams Work in Java?

In the world of Java programming, streams have emerged as a powerful and versatile concept for processing collections of data in a concise and efficient manner. Introduced in Java 8, streams provide a functional approach to working with data, allowing developers to perform complex operations on collections with ease. In this section, we will discuss the internal workings of streams in Java and explore their key features that make them such a valuable tool for data processing.

What is a Stream?

A stream, in Java, is a sequence of elements that can be processed in parallel or sequentially. It represents a pipeline of operations that can be performed on a data source, such as a collection, array, or I/O channel. Streams enable developers to express computations on data collections in a declarative and functional style, promoting code readability and maintainability.

Stream Operations:

Streams support two types of operations: intermediate and terminal operations. Intermediate operations are operations that transform or filter the elements of a stream and return another stream as the result. These operations do not produce a final result immediately but rather create a new stream that can be further processed. Some common intermediate operations include map, filter, sorted, and distinct.

For example, the map operation allows developers to transform each element of a stream into another object using a provided function. Similarly, the filter operation lets developers selectively retain elements from a stream based on a given condition. These operations are executed lazily, meaning they are not evaluated until a terminal operation is invoked on the stream.

On the other hand, terminal operations are operations that produce a result or a side effect. They trigger the execution of the stream pipeline and consume the elements of the stream. Terminal operations can produce a single value or a collection, or they can perform an action such as printing elements or writing to a file. Examples of terminal operations include forEach, collect, reduce, and count. Once a terminal operation is invoked, the stream cannot be reused.

Stream Creation:

Streams can be created from various sources in Java, such as collections, arrays, or I/O channels. The class provides several methods to create streams. For instance, the stream() method can be called on collections to obtain a sequential stream that processes elements in a defined order. Conversely, the parallelStream() method returns a parallel stream that allows for concurrent processing of elements, taking advantage of multi-core processors.

Additionally, the Stream.of() method allows creating streams from individual elements or arrays. It provides a convenient way to create a stream with a fixed set of elements. Moreover, streams can be generated using the Stream.generate() or Stream.iterate() methods, which generate an infinite stream based on a supplier or an iterative function, respectively.

Stream Processing:

The processing of a stream is divided into two stages: the setup stage and the processing stage. In the setup stage, the stream pipeline is constructed by applying intermediate operations to the stream source. Each intermediate operation returns a new stream, allowing for chaining of operations.

For example, consider the following pipeline: stream.filter(x -> x > 5).map(x -> x * x).forEach(System.out::println). In this pipeline, a stream is created from a collection, followed by a filter operation that retains elements greater than 5. Then, a map operation squares each element, and finally, a forEach terminal operation prints each squared element. Each operation in the pipeline is connected through the dot notation, forming a sequence of transformations on the data.

In the processing stage, terminal operations are invoked to initiate the execution of the stream pipeline. The elements flow through the pipeline, and each operation is applied to the elements as they pass. The result of the terminal operation is produced, and the stream is consumed. It's important to note that streams are designed to be consumed only once. If you need to reuse the elements of a stream, you can create a new stream from the original source.

Stream Characteristics:

Streams in Java possess several key characteristics that make them versatile and efficient. First, streams can be processed sequentially or in parallel, leveraging the power of multi-core processors. Parallel streams divide the data into multiple chunks and process them concurrently, potentially improving performance for large datasets and computationally intensive operations. By simply changing the method invoked on a stream from stream() to parallelStream(), you can leverage parallel processing capabilities.

Second, streams are often non-mutating, meaning they do not modify the source data. Instead, they produce new streams or results based on the transformations applied. This immutability promotes functional programming principles, where data is treated as immutable and operations produce new data instead of modifying existing state. By avoiding mutable state and side effects, streams enhance code readability, maintainability, and can reduce the chance of bugs caused by unexpected modifications to shared data.

Here's an example of Java code that demonstrates the usage of streams with intermediate and terminal operations, along with their corresponding output:



Example 1: Perform operations on a stream

Example 2: Use terminal operation to collect elements
[2, 4]

Example 3: Use parallel stream for concurrent processing

Example 4: Use terminal operation to calculate sum
Sum: 15

The code demonstrates four examples of stream usage. Example 1 shows how to perform operations on a stream by filtering elements and mapping them to new values. Example 2 demonstrates the use of a terminal operation (collect) to collect elements into a new list. Example 3 showcases the usage of a parallel stream for concurrent processing, where the order of output may vary. Example 4 illustrates the calculation of the sum of numbers using the reduce terminal operation.


Streams have revolutionized the way data processing is performed in Java. By offering a functional and declarative approach, streams enable developers to write elegant and concise code for working with collections. Whether it's transforming, filtering, aggregating, or performing other operations on data, streams provide a powerful and efficient toolset.

Understanding the workings of streams is essential for any Java developer looking to leverage the full potential of the language and write more expressive and efficient code. By utilizing streams, you can simplify your code, make it more readable, and take advantage of parallel processing capabilities. Streams have become a fundamental part of modern Java programming, enabling developers to tackle complex data processing tasks with ease and elegance.

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