This article shows different ways to initialize HashMap in Java. Initialize a HashMap (Standard) Collections.singletonMap Java 9 Map.of Java 9 Map.ofEntries Create a function to return a Map Static Initializer Java 8, Stream of SimpleEntry Conclusion After initialized a HashMap, the result is either a mutable map or an immutable map: Mutable map – It […]
To exit JShell, type /exit. I stuck in JShell for minutes, commands like exit, ctrl + c, end, q!, q, quit all failed, the correct syntax to quit or exit the JShell is /exit. Terminal C:\Users\mkyong>jshell | Welcome to JShell — Version 11.0.1 | For an introduction type: /help intro jshell> exit | Error: | […]
In Java, Serialization means converting Java objects into a byte stream; Deserialization means converting the serialized object’s byte stream back to the original Java object. Table of contents. 1. Hello World Java Serialization 2. java.io.NotSerializableException 3. What is serialVersionUID? 4. What is transient? 5. Serialize Object to File 6. Why need Serialization in Java? 7. […]
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 […]
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)); […]
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 […]
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 […]
In Java, we can use new ArrayList(Arrays.asList(array)) to convert an Array into an ArrayList ArrayExample1.java package com.mkyong; import java.util.ArrayList; import java.util.Arrays; import java.util.List; public class ArrayExample1 { public static void main(String[] args) { String[] str = {"A", "B", "C"}; List<String> list = new ArrayList<>(Arrays.asList(str)); list.add("D"); list.forEach(x -> System.out.println(x)); } } Output A B C D […]
In the old days, we can use list.toArray(new String[0]) to convert a ArrayList<String> into a String[] StringArrayExample.java package com.mkyong; import java.util.ArrayList; import java.util.List; public class StringArrayExample { public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add("A"); list.add("B"); list.add("C"); // default, returns Object[], not what we want, // Object[] objects = list.toArray(); // […]
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 […]
This article shows you how to use try-with-resources in Java. Table of contents: 1 Java try-with-resources before and after 2. try-with-resources with single resource 3. try-with-resources with multiple resources 3.1 JDBC example. 4. Custom resource with AutoCloseable interface 5. Resource open and closing order 6. Java 9 – final or effectively final variables Download Source […]
This article shows a few ways to convert an java.io.InputStream to a String. Table of contents 1. ByteArrayOutputStream 2. InputStream#readAllBytes (Java 9) 3. InputStreamReader + StringBuilder 4. InputStreamReader + BufferedReader (modified line breaks) 5. Java 8 BufferedReader#lines (modified line breaks) 6. Apache Commons IO 7. Download Source Code 8. References What are modified line breaks? […]
Below are some Java examples of converting InputStream to a File. Plain Java – FileOutputStream Apache Commons IO – FileUtils.copyInputStreamToFile Java 7 – Files.copy Java 9 – InputStream.transferTo 1. Plain Java – FileOutputStream This example downloads the google.com HTML page and returns it as an InputStream. And we use FileOutputStream to copy the InputStream into […]
