java

How to change the JVM default locale?

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In Java, we can use Locale.setDefault() to change the JVM default locale. JavaLocaleExample.java package com.mkyong.locale; import java.util.Locale; public class JavaLocaleExample { public static void main(String[] args) { // get jvm default locale Locale defaultLocale = Locale.getDefault(); System.out.println(defaultLocale); // set jvm locale to china Locale.setDefault(Locale.CHINA); // or like this //Locale.setDefault(new Locale("zh", "cn"); Locale chinaLocale = Locale.getDefault(); …

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Java Scanner examples

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Long live the Scanner class, a few examples for self-reference. 1. Read Input 1.1 Read input from the console. JavaScanner1.java package com.mkyong.io; import java.util.Scanner; public class JavaScanner1 { public static void main(String[] args) { try (Scanner scanner = new Scanner(System.in)) { System.out.print("Please enter your name: "); String input = scanner.nextLine(); System.out.println("name : " + input); …

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Java – How to print a name 10 times?

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This article shows you different ways to print a name ten times. 1. Looping 1.1 For loop JavaSample1.java package com.mkyong.samples; public class JavaSample1 { public static void main(String[] args) { for (int i = 0; i < 10; i++) { System.out.println("Java "); } } } Output Java Java Java Java Java Java Java Java Java …

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Java – How to download web page from internet?

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A simple Java source code to download a web page from the internet. JavaDownloadWebPage.java package com.mkyong; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStream; import java.io.InputStreamReader; import java.net.URL; import java.net.URLConnection; public class JavaDownloadWebPage { public static void main(String[] args) throws IOException { String result = downloadWebPage("https://mkyong.com"); System.out.println(result); } private static String downloadWebPage(String url) throws IOException { StringBuilder …

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How to escape HTML in Java

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In Java, we can use Apache commons-text, StringEscapeUtils.escapeHtml4(str) to escape HTML characters. pom.xml <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-text</artifactId> <version>1.8</version> </dependency> JavaEscapeHtmlExample.java package com.mkyong.html; // make sure import the correct commons-text package import org.apache.commons.text.StringEscapeUtils; // @deprecated as of 3.6, use commons-text StringEscapeUtils instead //import org.apache.commons.lang3.StringEscapeUtils; public class JavaEscapeHtmlExample { public static void main(String[] args) { String html = …

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Java – Pretty Print HTML

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In Java, we can use jsoup, a Java HTML parser, to parse a HTML code and pretty print it. pom.xml <dependency> <groupId>org.jsoup</groupId> <artifactId>jsoup</artifactId> <version>1.12.1</version> </dependency> JavaPrettyPrintHTML.java package com.mkyong; import org.jsoup.Jsoup; import org.jsoup.nodes.Document; public class JavaPrettyPrintHTML { public static void main(String[] args) { String html = "<html><body><h1>hello world</h1></body></html>"; System.out.println(html); // original Document doc = Jsoup.parse(html); // …

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Java – How to read input from console using Scanner

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This example shows you how to read input from the console using the standard java.util.Scanner class. JavaScanner.java package com.mkyong.io; import java.util.Scanner; public class JavaScanner { public static void main(String[] args) { try (Scanner scanner = new Scanner(System.in)) { System.out.print("Please enter your name: "); String input = scanner.nextLine(); System.out.println("name : " + input); System.out.print("Please enter your …

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Java – How to read input from System.console()

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In Java, we can use System.console() to read input from the console. JavaSample.java package com.mkyong.io; import java.io.Console; public class JavaSample { public static void main(String[] args) { // jdk 1.6 Console console = System.console(); System.out.print("Enter your name: "); String name = console.readLine(); System.out.println("Name is: " + name); System.out.print("Enter your password: "); // Reads a password …

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Java – Server returned HTTP response code: 403 for URL

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A simple Java program tries to download something from the internet: try (InputStream is = new URL(url).openStream(); BufferedReader br = new BufferedReader(new InputStreamReader(is))) { while ((line = br.readLine()) != null) { result.append(line); } } But, some web servers return the following HTTP 403 errors? Exception in thread "main" java.io.IOException: Server returned HTTP response code: 403 …

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Java – How to print name 1000 times without looping?

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Here’s a Java example to print a name 1000 times without looping or recursion, instead, use String concatenation and simple math. Just for fun. JavaNoLoop.java package com.mkyong.samples; public class JavaNoLoop { public static void main(String[] args) { String s1 = "Mkyong\n"; String s3 = s1 + s1 + s1; String s10 = s3 + s3 …

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Java – How to enable the preview language features?

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This article shows you how to use –enable-preview to enable the preview language features in Java 12, 13 and above. Note Read this JEP 12: Preview Language and VM Features P.S All preview features are disabled by default. On JDK 12: # compile javac Example.java // Do not enable any preview features javac –release 12 …

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Java Ternary Operator examples

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This example shows you how to write Java ternary operator. Here’s the syntax condition ? get_this_if_true : get_this_if_false Java ternary operator syntax (n > 18) ? true : false; (n == true) ? 1 : 0; (n == null) ? n.getValue() : 0; 1. Java ternary operator 1.1 Java example without ternary operator. JavaExample1.java package …

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Java – How to search a string in a List?

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In Java, we can combine a normal loop and .contains(), .startsWith() or .matches() to search for a string in ArrayList. JavaExample1.java package com.mkyong.test; import java.util.ArrayList; import java.util.List; public class JavaExample1 { public static void main(String[] args) { List<String> list = new ArrayList<>(); list.add("Java"); list.add("Kotlin"); list.add("Clojure"); list.add("Groovy"); list.add("Scala"); List<String> result = new ArrayList<>(); for (String s …

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Java 11 HttpClient Examples

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This article shows you how to use the new Java 11 HttpClient APIs to send HTTP GET/POST requests, and some frequent used examples. HttpClient httpClient = HttpClient.newBuilder() .version(HttpClient.Version.HTTP_2) .followRedirects(HttpClient.Redirect.NORMAL) .connectTimeout(Duration.ofSeconds(20)) .proxy(ProxySelector.of(new InetSocketAddress("proxy.yourcompany.com", 80))) .authenticator(Authenticator.getDefault()) .build(); 1. Synchronous Example This example sends a GET request to https://httpbin.org/get, and print out the response header, status code, and …

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OkHttp – How to send HTTP requests

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This article shows you how to use the OkHttp library to send an HTTP GET/POST requests and some frequent used examples. P.S Tested with OkHttp 4.2.2 pom.xml <dependency> <groupId>com.squareup.okhttp3</groupId> <artifactId>okhttp</artifactId> <version>4.2.2</version> </dependency> 1. Synchronous Get Request OkHttpExample1.java package com.mkyong.http; import okhttp3.Headers; import okhttp3.OkHttpClient; import okhttp3.Request; import okhttp3.Response; import java.io.IOException; public class OkHttpExample1 { // only …

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Apache HttpClient Basic Authentication Examples

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This article shows you how to use Apache HttpClient to perform an HTTP basic authentication. P.S Tested with HttpClient 4.5.10 pom.xml <dependency> <groupId>org.apache.httpcomponents</groupId> <artifactId>httpclient</artifactId> <version>4.5.10</version> </dependency> Start a simple Spring Security WebApp providing HTTP basic authentication, and test it with the HttpClient 1. Basic Authentication The key is to configure CredentialsProvider and pass it to …

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Java 13 – Switch Expressions

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In Java 13, the JEP 354: Switch Expressions extends the previous Java 12 Switch Expressions by adding a new yield keyword to return a value from the switch expression. P.S Switch expressions are a preview feature and are disabled by default. Note This is a standard feature in Java 14. 1. No more value breaks! …

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What is new in Java 13

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Java 13 reached General Availability on 17 September 2019, download Java 13 here or this openJDK archived. Java 13 features. 1. JEP 350 Dynamic CDS Archives 2. JEP 351 ZGC: Uncommit Unused Memory 3. JEP-353 Reimplement the Legacy Socket API 4. JEP-354 Switch Expressions (Preview) 5. JEP-355 Text Blocks (Preview) Java 13 developer features. Switch …

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Java 12 – Switch Expressions

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Java 12, JEP 325: Switch Expressions enhanced the traditional switch statement to support the following new features: Multiple case labels Switch expression returning value via break (replaced with yield in Java 13 switch expressions) Switch expression returning value via label rules (arrow) P.S Switch expressions are a preview feature and are disabled by default. A …

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Java – Get the last element of a list

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In Java, index starts at 0, we can get the last index of a list via this formula: list.size() – 1 JavaExample1.java package com.mkyong.test; import java.util.Arrays; import java.util.List; public class JavaExample1 { public static void main(String[] args) { List<Integer> list = Arrays.asList(1, 2, 3, 4, 5); System.out.println(list.get(list.size() – 1)); System.out.println(list.get(list.size() – 2)); System.out.println(list.get(list.size() – 3)); …

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Java – How to get keys and values from Map

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In Java, we can get the keys and values via map.entrySet() Map<String, String> map = new HashMap<>(); // Get keys and values for (Map.Entry<String, String> entry : map.entrySet()) { String k = entry.getKey(); String v = entry.getValue(); System.out.println("Key: " + k + ", Value: " + v); } // Java 8 map.forEach((k, v) -> { …

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java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.Long

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Below example, the jdbcTemplate.queryForList returns an object of Integer and we try to convert it into a Long directly: public List<Customer> findAll() { String sql = "SELECT * FROM CUSTOMER"; List<Customer> customers = new ArrayList<>(); List<Map<String, Object>> rows = jdbcTemplate.queryForList(sql); for (Map row : rows) { Customer obj = new Customer(); obj.setID(((Long) row.get("ID"))); // the …

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Java – How to lock a file before writing

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In Java, we can combine RandomAccessFile and FileChannel to lock a file before writing. LockFileAndWrite.java package com.mkyong; import java.io.File; import java.io.IOException; import java.io.RandomAccessFile; import java.nio.channels.FileLock; import java.util.concurrent.TimeUnit; public class LockFileAndWrite { public static void main(String[] args) { writeFileWithLock(new File("D:\\server.log"), "mkyong"); } public static void writeFileWithLock(File file, String content) { // auto close and release the …

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Java – How to read last few lines of a File

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In Java, we can use the Apache Commons IO ReversedLinesFileReader to read the last few lines of a File. pom.xml <dependency> <groupId>commons-io</groupId> <artifactId>commons-io</artifactId> <version>2.6</version> </dependency> 1. Test Data A server log file sample d:\\server.log a b c d 1 2 3 4 5 2. Read Last Line 2.1 Read the last 3 lines of a …

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How to loop an enum in Java

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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 …

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java.sql.SQLException: The server time zone value ‘xx time’ is unrecognized

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Making a JDBC connection to the MySQL server with the latest mysql-connector-java:8.0.16 and hits the following SQLException: Tested with MySQL 5.7 Java 8 JDBC driver, mysql-connector-java 8.0.16 try (Connection conn = DriverManager.getConnection( "jdbc:mysql://127.0.0.1:3306/test", "root", "password")) { //… } catch (Exception e) { e.printStackTrace(); } Output java.sql.SQLException: The server time zone value ‘Malay Peninsula Standard Time’ …

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Java – Convert Integer to Long

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In Java, we can use Long.valueOf() to convert an Integer to a Long TestInteger.java package com.mkyong.example; public class TestInteger { public static void main(String[] args) { Integer num = 1; System.out.println(num); // 1 Long numInLong = Long.valueOf(num); System.out.println(numInLong); // 1 } } References Long.valueOf JavaDocs)

Java Events 2019

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Some Java Conferences and Events 2019. 1. JAX London (October 7–10, 2019) JAX London is a four-day conference for cutting edge software engineers and enterprise-level professionals. JAX brings together the world’s leading innovators in the fields of JAVA, microservices, continuous delivery and DevOps. Date: October 7–10, 2019 Location: Business Design Centre, London Discount(15%): mky_jxl19#15 Twitter: …

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JDBC – How to print all table names from a database?

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A JDBC example to connect to a PostgreSQL, and print out all the tables from the default database postgres pom.xml <dependencies> <dependency> <groupId>org.postgresql</groupId> <artifactId>postgresql</artifactId> <version>42.2.5</version> </dependency> </dependencies> P.S Tested with Java 8 and postgresql jdbc driver 42.2.5 PrintAllTables.java package com.mkyong.jdbc; import java.sql.*; public class PrintAllTables { public static void main(String[] argv) { System.out.println("PostgreSQL JDBC Connection …

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Java – How to change date format in a String

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If Java 8, DateTimeFormatter, else SimpleDateFormat to change the date format in a String. 1. DateTimeFormatter (Java 8) Convert the String to LocalDateTime and change the date format with DateTimeFormatter DateFormatExample1.java package com.mkyong; import java.time.LocalDateTime; import java.time.format.DateTimeFormatter; public class DateFormatExample1 { // date format 1 private static final DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.S"); // date …

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Java – Add new line in String

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Different operating system has a different new line or line separator string: UNIX, Linux or Mac OSX = \n Windows = \r\n NewLineExample.java package com.mkyong; public class NewLineExample { public static void main(String[] args) { String original = "Hello World Java"; System.out.println(original); // add new line String originalNewLine = "Hello\nWorld\nJava"; System.out.println(originalNewLine); } } Output Hello …

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FastJson – Convert Java objects to / from JSON

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FastJson provides easily APIs to convert Java objects to / from JSON JSON.toJSONString – Java objects to JSON JSON.parseObject – JSON to Java objects JSON.parseArray – JSON array to List of Java objects Note You may have interest to read this How to parse JSON with Jackson Overall, the FastJson is really simple and easy …

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Java password generator example

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Here’s the Java password generator to generate a secure password that consists of two lowercase chars, two uppercase chars, two digits, two special chars, and pad the rest with random chars until it reaches the length of 20 characters. Secure password requirements: Password must contain at least two digits [0-9]. Password must contain at least …

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Java – How to generate a random String

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Few Java examples to show you how to generate a random alphanumeric String, with a fixed length. 1. Random [a-ZA-Z0-9] 1.1 Generate a random alphanumeric String [a-ZA-Z0-9], with a length of 8. RandomExample.java package com.mkyong; import java.security.SecureRandom; public class RandomExample { private static final String CHAR_LOWER = "abcdefghijklmnopqrstuvwxyz"; private static final String CHAR_UPPER = CHAR_LOWER.toUpperCase(); …

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Java – Convert String to double

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In Java, we can use Double.parseDouble() to convert a String to double 1. Double.parseDouble() Example to convert a String 3.142 to an primitive double. String str = "3.142"; double pi = Double.parseDouble(str); System.out.println(pi); Output 3.142 2. Double.valueOf() Example to convert a String 3.142 to an Double object. String str = "3.142"; Double pi = Double.valueOf(str); …

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Java – Convert Array to ArrayList

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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 …

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JSON.simple – How to parse JSON

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In this tutorial, we will show you how to parse JSON with JSON.simple Note You may have interest to read – How to parse JSON with Jackson or Gson JSON.simple short history This project was formerly JSON.simple 1.x from a Google code project by Yidong, now maintaining by Clifton Labs, read this JSON.simple history P.S …

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