Streams in java

A sequence of elements supporting sequential and parallel aggregate operations. The following example illustrates an aggregate operation using Stream and IntStream: int sum = widgets.stream() .filter(w -> w.getColor() == RED) .mapToInt(w -> w.getWeight()) .sum(); In this example, widgets is a Collection. We create a stream of Widget objects via Collection.stream(), filter it to produce a stream containing only the red widgets, and then transform it into a stream of int values representing the weight of each red widget. Then this stream is summed to produce a total weight. In addition to Stream, which is a stream of object references, there are primitive specializations for IntStream, LongStream, and DoubleStream, all of which are referred to as "streams" and conform to the characteristics and restrictions described here. To perform a computation, stream stream pipeline. A stream pipeline consists of a source (which might be an array, a collection, a generator function, an I/O channel, etc), zero or more intermediate operations (which transform a stream into another stream, such as filter(Predicate)), and a terminal operation (which produces a result or side-effect, such as count() or forEach(Consumer)). Streams are lazy; computation on the source data is only performed when the terminal operation is initiated, and source elements are consumed only as needed. Collections and streams, while bearing some superficial similarities, have different goals. Collections are primaril...

In this comprehensive tutorial, we'll go through the practical uses of Java 8 Streams from creation to parallel execution. To understand this material, readers need to have a basic knowledge of Java 8 (lambda expressions, Optional, method references) and of the Stream API. In order to be more familiar with these topics, please take a look at our previous articles: An array can also be the source of a stream: Stream streamOfArray = Stream.of("a", "b", "c"); We can also create a stream out of an existing array or of part of an array: String[] arr = new String[]; Stream streamOfArrayFull = Arrays.stream(arr); Stream streamOfArrayPart = Arrays.stream(arr, 1, 3); 2.4. Stream.builder() Another way of creating an infinite stream is by using the iterate() method: Stream streamIterated = Stream.iterate(40, n -> n + 2).limit(20); The first element of the resulting stream is the first parameter of the iterate() method. When creating every following element, the specified function is applied to the previous element. In the example above the second element will be 42. 2.7. Stream of Primitives Java 8 offers the possibility to create streams out of three primitive types: int, long and double. As Stream is a generic interface, and there is no way to use primitives as a type parameter with generics, three new special interfaces were created: IntStream, LongStream, DoubleStream. Using the new interfaces alleviates unnecessary auto-boxing, which allows for increased productivity: IntStream ...

String[] arr = new String[]; Stream stream = Arrays.stream(arr); stream = Stream.of("a", "b", "c"); A stream() default method is added to the Collection interface and allows creating a Streamusing any collection as an element source : Stream stream = list.stream(); 2.2. Multi-threading With Streams Stream API also simplifies multithreading by providing the parallelStream() method that runs operations over stream's elements in parallel mode. The code below allows to run method doWork() in parallel for every element of the stream: list.parallelStream().forEach(element -> doWork(element)); In the following section, we will introduce some of the basic Stream API operations. 3. Stream Operations They are divided into intermediate operations (return Stream) and terminal operations (return a result of definite type). Intermediate operations allow chaining. It's also worth noting that operations on streams don't change the source. Here's a quick example: long count = list.stream().distinct().count(); So, the distinct() method represents an intermediate operation, which creates a new stream of unique elements of the previous stream. And the count() method is a terminal operation , which returns stream's size. 3.1. Iterating The filter() method allows us to pick a stream of elements that satisfy a predicate. For example, consider the following list: ArrayList list = new ArrayList(); list.add("One"); list.add("OneAndOnly"); list.add("Derek"); list.add("Change"); list.add("factory"); ...

Java 8 introduced the easy to create streams that execute in parallel and make use of multiple processor cores. We might think that it's always faster to divide the work on more cores. But that is often not the case. In this tutorial, we'll explore the differences between sequential and parallel streams. We'll first look at the default fork-join pool used by parallel streams. We'll also consider the performance implications of using a parallel stream, including memory locality and splitting/merging costs. Finally, we'll recommend when it makes sense to covert a sequential stream into a parallel one. By default, any stream operation in Java is processed sequentially, unless explicitly specified as parallel. Sequential streams use a single thread to process the pipeline: List listOfNumbers = Arrays.asList(1, 2, 3, 4); listOfNumbers.stream().forEach(number -> System.out.println(number + "" + Thread.currentThread().getName()) ); The output of this sequential stream is predictable. The list elements will always be printed in an ordered sequence: Any stream in Java can easily be transformed from sequential to parallel. We can achieve this by adding the parallel method to a sequential stream or by creating a stream using the parallelStream method of a collection: List listOfNumbers = Arrays.asList(1, 2, 3, 4); listOfNumbers.parallelStream().forEach(number -> System.out.println(number + "" + Thread.currentThread().getName()) ); Parallel streams enable us to execute code in paralle...

package com.w3spoint ; import java.util.ArrayList ; import java.util.List ; public class Test Output 2 strings with length less than 5 2 strings with length less than 5 Problems with the above approach: • We just want to know the strings with length less than 5 but we would also have to provide how the iteration will take place, this is also called external iteration because client program is handling the algorithm to iterate over the list. • The program is sequential in nature, there is no way we can do this in parallel easily. • There is a lot of code to do even for a simple task. To overcome these limitations Java 8 introduced a new additional package called java.util.stream which consists of classes, interfaces and enum to allow functional-style operations on the elements. Java Stream API supports internal iteration. Internal iteration provides several features like sequential and parallel execution, filtering based on the given criteria, mapping etc. The java.util.stream is a sequence of elements supporting sequential and parallel aggregate operations. Java Stream Features • Laziness-seeking: All the stream operations are lazy in nature which means they are not executed until they are needed. For example, if we want to get only the first 2 elements of a list using stream, the stream operation would stop at the end of second iteration after displaying the second element of list. • No storage: A stream is not a data structure that stores elements. It simply conveys ele...

Streams are Java’s way of integrating functional programming with its object-oriented style. There are a lot of benefits to using streams in Java, such as the ability to write functions at a more abstract level which can reduce code bugs, compact functions into fewer and more readable lines of code, and the ease they offer for parallelization. Java streams are fairly well known but not everyone knows how to take full advantage of their benefits, including the finer points of making streams, intermediate operations, and terminal operations. In this blog we’re going to use a simple example to explain how Java streams work, which will result in less verbose, more intuitive, and less error prone code. An Example of How Java Streams Work The simplest way to think of Java streams, and the way that helps me the most, is imagining a list of objects that are disconnected from each other, entering a pipeline one at a time. You can control how many of these objects enter the pipeline, what you do to these objects inside of the pipeline, and how you catch these objects as they exit the pipeline. Streams are much easier to learn by looking at the big picture first and then breaking it down. To do so, let’s make a program that takes a certain number of multiples of four, squares each of them, then takes the sum of all of the squares that are not divisible by ten: Even if you don’t know how streams work in Java, there’s a very good chance you were able to quickly figure out what the code...

Use stream operations to express sophisticated data processing queries. What would you do without collections? Nearly every Java application makes and processes collections. They are fundamental to many programming tasks: they let you group and process data. For example, you might want to create a collection of banking transactions to represent a customer’s statement. Then, you might want to process the whole collection to find out how much money the customer spent. Despite their importance, processing collections is far from perfect in Java. • Originally published in the March/April 2014 issue of First, typical processing patterns on collections are similar to SQL-like operations such as “finding” (for example, find the transaction with highest value) or “grouping” (for example, group all transactions related to grocery shopping). Most databases let you specify such operations declaratively. For example, the following SQL query lets you find the transaction ID with the highest value: "SELECT id, MAX(value) from transactions". As you can see, we don’t need to implement how to calculate the maximum value (for example, using loops and a variable to track the highest value). We only express what we expect. This basic idea means that you need to worry less about how to explicitly implement such queries—it is handled for you. Why can’t we do something similar with collections? How many times do you find yourself reimplementing these operations using loops over and over again? S...