Java stream map to int

Interface Stream

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 operations are composed into a 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.

A stream implementation is permitted significant latitude in optimizing the computation of the result. For example, a stream implementation is free to elide operations (or entire stages) from a stream pipeline — and therefore elide invocation of behavioral parameters — if it can prove that it would not affect the result of the computation. This means that side-effects of behavioral parameters may not always be executed and should not be relied upon, unless otherwise specified (such as by the terminal operations forEach and forEachOrdered ). (For a specific example of such an optimization, see the API note documented on the count() operation. For more detail, see the side-effects section of the stream package documentation.)

Collections and streams, while bearing some superficial similarities, have different goals. Collections are primarily concerned with the efficient management of, and access to, their elements. By contrast, streams do not provide a means to directly access or manipulate their elements, and are instead concerned with declaratively describing their source and the computational operations which will be performed in aggregate on that source. However, if the provided stream operations do not offer the desired functionality, the BaseStream.iterator() and BaseStream.spliterator() operations can be used to perform a controlled traversal.

A stream pipeline, like the «widgets» example above, can be viewed as a query on the stream source. Unless the source was explicitly designed for concurrent modification (such as a ConcurrentHashMap ), unpredictable or erroneous behavior may result from modifying the stream source while it is being queried.

• must be non-interfering (they do not modify the stream source); and
• in most cases must be stateless (their result should not depend on any state that might change during execution of the stream pipeline).

Such parameters are always instances of a functional interface such as Function , and are often lambda expressions or method references. Unless otherwise specified these parameters must be non-null.

A stream should be operated on (invoking an intermediate or terminal stream operation) only once. This rules out, for example, «forked» streams, where the same source feeds two or more pipelines, or multiple traversals of the same stream. A stream implementation may throw IllegalStateException if it detects that the stream is being reused. However, since some stream operations may return their receiver rather than a new stream object, it may not be possible to detect reuse in all cases.

Streams have a BaseStream.close() method and implement AutoCloseable . Operating on a stream after it has been closed will throw IllegalStateException . Most stream instances do not actually need to be closed after use, as they are backed by collections, arrays, or generating functions, which require no special resource management. Generally, only streams whose source is an IO channel, such as those returned by Files.lines(Path) , will require closing. If a stream does require closing, it must be opened as a resource within a try-with-resources statement or similar control structure to ensure that it is closed promptly after its operations have completed.

Stream pipelines may execute either sequentially or in parallel. This execution mode is a property of the stream. Streams are created with an initial choice of sequential or parallel execution. (For example, Collection.stream() creates a sequential stream, and Collection.parallelStream() creates a parallel one.) This choice of execution mode may be modified by the BaseStream.sequential() or BaseStream.parallel() methods, and may be queried with the BaseStream.isParallel() method.

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Java 8 Stream mapToInt(ToIntFunction) Method Examples | Convert Stream to IntStream

A quick practical example to Java 8 Stream API mapToInt() Method and how to convert Stream to IntStream. Syntax: IntStream mapToInt​(ToIntFunction ; mapper).

1. Overview

In this java 8 Streams series, We’ll learn today what is Stream API mapToInt() method and when to use this. This method is used to convert the Stream to IntStream. Let us see it’s syntax and few examples on mapToInt() method. This is also part of intermediate operations such as map() and filter() methods.

2. Syntax

IntStream mapToInt​(ToIntFunction mapper)

Returns an IntStream consisting of the results of applying the given function to the elements of this stream.

3. Stream mapToInt Examples

We will see now a few working examples using filter(), map(), forEach(), mapToInt() methods.

3.1 Finding the length of each string a collection

package com.java.w3schools.blog.java.program.to.java8.datetime; import java.util.stream.IntStream; import java.util.stream.Stream; public class StreamMapToIntExample1 < public static void main(String[] args) < String[] stringArray = < "java", "program", "to", "com", "w3schools" >; // convert string array to Stream Stream stringStream = Stream.of(stringArray); IntStream intStream = stringStream.mapToInt(value -> value.length()); intStream.forEach(s -> System.out.println(s)); > >

3.2 Sleeping 1 second in processing and adding each value into List

package com.java.w3schools.blog.java.program.to.java8.stream; import java.util.ArrayList; import java.util.List; import java.util.stream.IntStream; import java.util.stream.Stream; public class StreamMapToIntExample2 < public static void main(String[] args) < Listlengths = new ArrayList<>(); String[] stringArray = < "java", "program", "to", "com", "w3schools" >; // convert string array to Stream Stream stringStream = Stream.of(stringArray).peek(StreamMapToIntExample2::doSleeo); IntStream intStream = stringStream.mapToInt(value -> value.length()); intStream.forEach(lengths::add); lengths.forEach(n -> System.out.println(n)); > private static void doSleeo(String value) < try < System.out.println("Sleeping for a second"); Thread.sleep(1000); >catch (InterruptedException e) < e.printStackTrace(); >> >

Sleeping for a second Sleeping for a second Sleeping for a second Sleeping for a second Sleeping for a second 4 7 2 3 9

3.3 Getting Student Ages into a List

package com.java.w3schools.blog.java.program.to.java8.stream; import java.util.ArrayList; import java.util.Collections; import java.util.List; import java.util.stream.IntStream; public class StreamMapToIntExample3 < public static void main(String[] args) < Liststudents = new ArrayList<>(); students.add(new Student(100, "Nani", 30)); students.add(new Student(101, "priyanka arul", 25)); students.add(new Student(102, "mohan", 35)); students.add(new Student(103, "vikram kumar", 45)); IntStream ageStream = students.stream().parallel().mapToInt(s -> s.getAge()); List ages = Collections.synchronizedList(new ArrayList<>()); ageStream.forEach(ages::add); ages.forEach(age -> System.out.println(age)); > > class Student < private int id; private String name; private int age; public Student(int id, String name, int age) < super(); this.id = id; this.name = name; this.age = age; >public int getId() < return id; >public void setId(int id) < this.id = id; >public String getName() < return name; >public void setName(String name) < this.name = name; >public int getAge() < return age; >public void setAge(int age) < this.age = age; >@Override public String toString() < return "Student [id=" + id + ", name=" + name + ", age=" + age + "]"; >>

Here we have used a parallel() method on stream. So output can not be expected in order. Every time you execute the above program, you will see the different order of ages.

4. Conclusion

In this article, We’ve seen how to convert a Stream to IntStream using mapToInt() method. This method is part of the java 8 Stream API. And also shown 3 working examples on this method.

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JavaProgramTo.com: Java 8 Stream mapToInt(ToIntFunction) Method Examples | Convert Stream to IntStream

A quick practical example to Java 8 Stream API mapToInt() Method and how to convert Stream to IntStream. Syntax: IntStream mapToInt​(ToIntFunction ; mapper).

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