Tag: stream

java8 March 20, 2020 by mkyong 0 comments

Java 8 – How to convert Iterator to Stream

In Java 8, we can use StreamSupport.stream to convert an Iterator into a Stream. // Iterator -> Spliterators -> Stream Stream<String> stream = StreamSupport.stream( Spliterators.spliteratorUnknownSize( iterator, Spliterator.ORDERED) , false); Review the StreamSupport.stream method signature, it accepts a Spliterator. StreamSupport.java public static <T> Stream<T> stream(Spliterator<T> spliterator, boolean parallel) { Objects.requireNonNull(spliterator); return new ReferencePipeline.Head<>(spliterator, StreamOpFlag.fromCharacteristics(spliterator), parallel); } […]

java March 19, 2020 by mkyong 0 comments

Java Iterator examples

A few of Java Iterator and ListIterator examples. 1. Iterator 1.1 Get Iterator from a List or Set, and loop over it. JavaIteratorExample1a.java package com.mkyong; import java.util.Arrays; import java.util.Iterator; import java.util.List; public class JavaIteratorExample1a { public static void main(String[] args) { /* Set<Integer> set = new HashSet<>(); set.add(1); set.add(2); set.add(3); set.add(4); set.add(5); Iterator<Integer> iterator = […]

java8 March 15, 2020 by mkyong 4 comments

Java 8 – Find duplicate elements in a Stream

This article shows you three algorithms to find duplicate elements in a Stream. Set.add() Collectors.groupingBy Collections.frequency At the end of the article, we use the JMH benchmark to test which one is the fastest algorithm. 1. Filter & Set.add() The Set.add() returns false if the element was already in the set; let see the benchmark […]

java8 March 14, 2020 by mkyong 0 comments

Java 8 – Get the last element of a Stream?

In Java 8, we can use reduce or skip to get the last element of a Stream. 1. Stream.reduce Java8Example1.java package com.mkyong; import java.util.Arrays; import java.util.List; public class Java8Example1 { public static void main(String[] args) { List<String> list = Arrays.asList("node", "java", "c++", "react", "javascript"); String result = list.stream().reduce((first, second) -> second).orElse("no last element"); System.out.println(result); } […]

java8 March 2, 2020 by mkyong 0 comments

Java 8 – How to convert IntStream to int or int[]

Few Java 8 examples to get a primitive int from an IntStream. 1. IntStream -> int Java8Example1.java package com.mkyong; import java.util.Arrays; import java.util.OptionalInt; import java.util.stream.IntStream; public class Java8Example1 { public static void main(String[] args) { int[] num = {1, 2, 3, 4, 5}; //1. int[] -> IntStream IntStream stream = Arrays.stream(num); // 2. OptionalInt OptionalInt […]

java8 February 25, 2020 by mkyong 7 comments

Java 8 – How to Sum BigDecimal using Stream?

In Java 8, we can use the Stream.reduce() to sum a list of BigDecimal. 1. Stream.reduce() Java example to sum a list of BigDecimal values, using a normal for loop and a stream.reduce(). JavaBigDecimal.java package com.mkyong; import java.math.BigDecimal; import java.util.LinkedList; import java.util.List; public class JavaBigDecimal { public static void main(String[] args) { List<BigDecimal> invoices = […]

java8 February 24, 2020 by mkyong 6 comments

Java 8 Stream.reduce() examples

In Java 8, the Stream.reduce() combine elements of a stream and produces a single value. A simple sum operation using a for loop. int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; int sum = 0; for (int i : numbers) { sum += i; } System.out.println("sum : " + sum); […]

java8 February 9, 2020 by mkyong 12 comments

Java 8 Predicate Examples

In Java 8, Predicate is a functional interface, which accepts an argument and returns a boolean. Usually, it used to apply in a filter for a collection of objects. @FunctionalInterface public interface Predicate<T> { boolean test(T t); } Further Reading Java 8 BiPredicate Examples 1. Predicate in filter() filter() accepts predicate as argument. Java8Predicate.java package […]

java8 February 8, 2020 by mkyong 0 comments

Java 8 Stream findFirst() and findAny()

In Java 8 Stream, the findFirst() returns the first element from a Stream, while findAny() returns any element from a Stream. 1. findFirst() 1.1 Find the first element from a Stream of Integers. Java8FindFirstExample1.java package com.mkyong.java8; import java.util.Arrays; import java.util.List; import java.util.Optional; public class Java8FindFirstExample1 { public static void main(String[] args) { List<Integer> list = […]

java July 18, 2019 by mkyong 2 comments

Java Regular Expression Examples

Java 8 stream and regular expression examples. Note Learn the basic regular expression at Wikipedia 1. String.matches(regex) 1.1 This example, check if the string is a number. JavaRegEx1.java package com.mkyong.regex; import java.util.Arrays; import java.util.List; public class JavaRegEx1 { public static void main(String[] args) { List<String> numbers = Arrays.asList("1", "20", "A1", "333", "A2A211"); for (String number […]

java July 17, 2019 by mkyong 7 comments

Java 8 Stream – Convert List<List<String>> to List<String>

As title, we can use flatMap to convert it. Java9Example1.java package com.mkyong.test; import java.util.Arrays; import java.util.List; import java.util.Scanner; import java.util.stream.Collectors; public class Java9Example1 { public static void main(String[] args) { List<String> numbers = Arrays.asList("1", "2", "A", "B", "C1D2E3"); List<List<String>> collect = numbers.stream() .map(x -> new Scanner(x).findAll("\\D+") .map(m -> m.group()) .collect(Collectors.toList()) ) .collect(Collectors.toList()); collect.forEach(x -> System.out.println(x)); […]

java July 9, 2019 by mkyong 0 comments

How to loop an enum in Java

Call the .values() method of the enum class to return an array, and loop it with the for loop: for (EnumClass obj : EnumClass.values()) { System.out.println(obj); } For Java 8, convert an enum into a stream and loop it: Stream.of(EnumClass.values()).forEach(System.out::println); 1. For Loop Enum 1.1 An enum to contain a list of the popular JVM […]

java8 June 29, 2019 by mkyong 7 comments

Java 8 Parallel Streams Examples

Few Java 8 examples to execute streams in parallel. 1. BaseStream.parallel() A simple parallel example to print 1 to 10. ParallelExample1.java package com.mkyong.java8; import java.util.stream.IntStream; public class ParallelExample1 { public static void main(String[] args) { System.out.println("Normal…"); IntStream range = IntStream.rangeClosed(1, 10); range.forEach(System.out::println); System.out.println("Parallel…"); IntStream range2 = IntStream.rangeClosed(1, 10); range2.parallel().forEach(System.out::println); } } Output Normal… 1 2 […]

java June 28, 2019 by mkyong 1 comment

Java Prime Numbers examples

The following Java examples will print a list of all the prime numbers up to 1,000: 2 3 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 […]

java8 June 14, 2019 by mkyong 2 comments

Java 8 Stream – The peek() is not working with count()?

Many examples are using the .count() as the terminal operation for .peek(), for example: Java 8 List<String> l = Arrays.asList("A", "B", "C", "D"); long count = l.stream().peek(System.out::println).count(); System.out.println(count); // 4 Output – It’s working fine. A B C D 4 However, for Java 9 and above, the peek() may print nothing: Java 9 and above […]

java May 10, 2019 by mkyong 1 comment

Java 8 – Should we close the Stream after use?

Only Streams whose source are an IO channel like Files.lines(Path, Charset) need to be closed. Read this Stream JavaDocs Streams have a BaseStream.close() method and implement AutoCloseable, but nearly all stream instances do not actually need to be closed after use. Generally, only streams whose source is an IO channel (such as those returned by […]

java8 March 20, 2019 by mkyong 1 comment

Java 8 – Convert a Stream to Array

In Java 8, we can use .toArray() to convert a Stream into an Array. 1. Stream -> String[] StreamString.java package com.mkyong; import java.util.Arrays; public class StreamString { public static void main(String[] args) { String lines = "I Love Java 8 Stream!"; // split by space, uppercase, and convert to Array String[] result = Arrays.stream(lines.split("\\s+")) .map(String::toUpperCase) […]

java8 March 19, 2019 by mkyong 1 comment

Java 8 – How to convert IntStream to Integer[]

The key is boxed() the IntStream into a Stream<Integer>, then only convert to an Array. StreamExample.java package com.mkyong; import java.util.Arrays; import java.util.stream.IntStream; import java.util.stream.Stream; public class StreamExample { public static void main(String[] args) { //int[] -> IntStream -> Stream<Integer> -> Integer[] int[] num = {3, 4, 5}; //1. int[] -> IntStream IntStream stream = Arrays.stream(num); […]

java March 18, 2019 by mkyong 0 comments

Java – How to declare and initialize an Array

Few Java examples to declare, initialize and manipulate Array in Java 1. Declares Array 1.1 For primitive types. ArrayExample1.java package com.mkyong; import java.util.Arrays; public class ArrayExample1 { public static void main(String[] args) { //declares an array of integers int[] num1 = new int[5]; int[] num2 = {1, 2, 3, 4, 5}; int[] num3 = new […]

java8 March 8, 2019 by mkyong 15 comments

Java 8 – How to sort list with stream.sorted()

Few examples to show you how to sort a List with stream.sorted() 1. List 1.1 Sort a List with Comparator.naturalOrder() package com.mkyong.sorted; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class StreamApplication { public static void main(String[] args) { List<String> list = Arrays.asList("9", "A", "Z", "1", "B", "Y", "4", "a", "c"); /* List<String> sortedList = list.stream() […]

java December 21, 2018 by mkyong 0 comments

Java – How to read a file into a list?

In Java, there are few ways to read a file line by line into a List 1. Java 8 stream List<String> result; try (Stream<String> lines = Files.lines(Paths.get(fileName))) { result = lines.collect(Collectors.toList()); } 2. Java 7 Files.readAllLines(new File(fileName).toPath(), Charset.defaultCharset()); 3. Classic BufferedReader example. List<String> result = new ArrayList<>(); BufferedReader br = null; try { br = […]

java December 1, 2018 by mkyong 3 comments

Java Fibonacci examples

Fibonacci number – Every number after the first two is the sum of the two preceding. Few Java examples to find the Fibonacci numbers. 1. Java 8 stream 1.1 In Java 8, we can use Stream.iterate to generate Fibonacci numbers like this : Stream.iterate(new int[]{0, 1}, t -> new int[]{t[1], t[0] + t[1]}) .limit(10) .forEach(x […]

java8 November 29, 2018 by mkyong 1 comment

Java 8 Stream.iterate examples

In Java 8, we can use Stream.iterate to create stream values on demand, so called infinite stream. 1. Stream.iterate 1.1 Stream of 0 – 9 //Stream.iterate(initial value, next value) Stream.iterate(0, n -> n + 1) .limit(10) .forEach(x -> System.out.println(x)); Output 0 1 2 3 4 5 6 7 8 9 1.2 Stream of odd numbers […]

java8 April 3, 2017 by mkyong 18 comments

Java 8 Streams map() examples

In Java 8, stream().map() lets you convert an object to something else. Review the following examples : 1. A List of Strings to Uppercase 1.1 Simple Java example to convert a list of Strings to upper case. TestJava8.java package com.mkyong.java8; import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors; public class TestJava8 { public static void […]

java November 19, 2016 by mkyong 4 comments

Java – Check if Array contains a certain value?

Java examples to check if an Array (String or Primitive type) contains a certain values, updated with Java 8 stream APIs. 1. String Arrays 1.1 Check if a String Array contains a certain value “A”. StringArrayExample1.java package com.mkyong.core; import java.util.Arrays; import java.util.List; public class StringArrayExample1 { public static void main(String[] args) { String[] alphabet = […]

java November 18, 2016 by mkyong 3 comments

Java – How to convert a primitive Array to List

Code snippets to convert a primitive array int[] to a List<Integer> : int[] number = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; List<Integer> list = new ArrayList<>(); for (int i : number) { list.add(i); } In Java 8, you can use the Stream APIs to do the boxing and conversion like this […]

java8 August 11, 2016 by mkyong 26 comments

Java 8 – How to sort a Map

Java 8 Stream examples to sort a Map, by keys or by values. 1. Quick Explanation Steps to sort a Map in Java 8. Convert a Map into a Stream Sort it Collect and return a new LinkedHashMap (keep the order) Map result = map.entrySet().stream() .sorted(Map.Entry.comparingByKey()) .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new)); P.S By […]

java8 August 10, 2016 by mkyong 3 comments

Java 8 – Convert a Stream to List

A Java 8 example to show you how to convert a Stream to a List via Collectors.toList Java8Example1.java package com.mkyong.java8; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class Java8Example1 { public static void main(String[] args) { Stream<String> language = Stream.of("java", "python", "node"); //Convert a Stream to List List<String> result = language.collect(Collectors.toList()); result.forEach(System.out::println); } } output […]

java8 August 9, 2016 by mkyong 6 comments

Java 8 – Filter a null value from a Stream

Review a Stream containing null values. Java8Examples.java package com.mkyong.java8; import java.util.List; import java.util.stream.Collectors; import java.util.stream.Stream; public class Java8Examples { public static void main(String[] args) { Stream<String> language = Stream.of("java", "python", "node", null, "ruby", null, "php"); List<String> result = language.collect(Collectors.toList()); result.forEach(System.out::println); } } output java python node null //

java8 April 14, 2016 by mkyong 10 comments

Java 8 flatMap example

This article explains the Java 8 Stream.flatMap and how to use it. Topic What is flatMap? Why flat a Stream? flatMap example – Find a set of books. flatMap example – Order and LineItems. flatMap example – Splits the line by spaces. flatMap and Primitive type 1. What is flatMap()? 1.1 Review the below structure. […]

java8 April 13, 2016 by mkyong 5 comments

Java – How to convert Array to Stream

In Java 8, you can either use Arrays.stream or Stream.of to convert an Array into a Stream. 1. Object Arrays For object arrays, both Arrays.stream and Stream.of returns the same output. TestJava8.java package com.mkyong.java8; import java.util.Arrays; import java.util.stream.Stream; public class TestJava8 { public static void main(String[] args) { String[] array = {"a", "b", "c", "d", […]

java8 April 12, 2016 by mkyong 9 comments

Java – Stream has already been operated upon or closed

In Java 8, Stream cannot be reused, once it is consumed or used, the stream will be closed. 1. Example – Stream is closed! Review the following example, it will throw an IllegalStateException, saying “stream is closed”. TestJava8.java package com.mkyong.java8; import java.util.Arrays; import java.util.stream.Stream; public class TestJava8 { public static void main(String[] args) { String[] […]

java8 April 7, 2016 by mkyong 36 comments

Java 8 – Stream Collectors groupingBy examples

In this article, we will show you how to use Java 8 Stream Collectors to group by, count, sum and sort a List. 1. Group By, Count and Sort 1.1 Group by a List and display the total count of it. Java8Example1.java package com.mkyong.java8; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.function.Function; import java.util.stream.Collectors; public […]

java8 March 19, 2016 by mkyong 20 comments

Java 8 – Convert List to Map

Few Java 8 examples to show you how to convert a List of objects into a Map, and how to handle the duplicated keys. Hosting.java package com.mkyong.java8 public class Hosting { private int Id; private String name; private long websites; public Hosting(int id, String name, long websites) { Id = id; this.name = name; this.websites […]

java8 February 11, 2016 by mkyong 31 comments

Java 8 Streams filter examples

In this tutorial, we will show you few Java 8 examples to demonstrate the use of Streams filter(), collect(), findAny() and orElse() 1. Streams filter() and collect() 1.1 Before Java 8, filter a List like this : BeforeJava8.java package com.mkyong.java8; import java.util.ArrayList; import java.util.Arrays; import java.util.List; public class BeforeJava8 { public static void main(String[] args) […]

java February 8, 2016 by mkyong 8 comments

Java – How to join Arrays

In this article, we will show you a few ways to join a Java Array. Apache Commons Lang – ArrayUtils Java API Java 8 Stream 1. Apache Commons Lang – ArrayUtils The simplest way is add the Apache Commons Lang library, and use ArrayUtils. addAll to join arrays. This method supports both primitive and object […]

java8 October 29, 2015 by mkyong 15 comments

Java 8 Stream – Read a file line by line

In Java 8, you can use Files.lines to read file as Stream. c://lines.txt – A simple text file for testing line1 line2 line3 line4 line5 1. Java 8 Read File + Stream TestReadFile.java package com.mkyong.java8; import java.io.IOException; import java.nio.file.Files; import java.nio.file.Paths; import java.util.stream.Stream; public class TestReadFile { public static void main(String args[]) { String fileName […]

java May 1, 2011 by mkyong 8 comments

Java – How to convert String to Char Array

In Java, you can use String.toCharArray() to convert a String into a char array. StringToCharArray.java package com.mkyong.utils; public class StringToCharArray { public static void main(String[] args) { String password = "password123"; char[] passwordInCharArray = password.toCharArray(); for (char temp : passwordInCharArray) { System.out.println(temp); } } } Output p a s s w o r d 1 […]

java June 6, 2010 by mkyong 22 comments

How to copy directory in Java

In Java, we can use the Java 1.7 FileVisitor or Apache Commons IO FileUtils.copyDirectory to copy a directory, which includes its sub-directories and files. This article shows a few of the common ways to copy a directory in Java. FileVisitor (Java 7+) FileUtils.copyDirectory (Apache commons-io) Custom Copy using Java 7 NIO and Java 8 Stream. […]

java June 5, 2010 by mkyong 15 comments

How to delete directory in Java

If we use the NIO Files.delete to delete a non-empty directory in Java, it throws DirectoryNotEmptyException; for legacy IO File.delete to delete a non-empty directory, it returns a false. The standard solution is to loop the directory recursively, and delete all its children’s contents first (sub-files or sub-directories), and delete the parent later. This example […]