This article shows you how to format a string in Java, via String.format(). Here is the summary. Conversion Category Description %b, %B general true of false %h, %H general hash code value of the object %s, %S general string %c, %C character unicode character %d integral decimal integer %o integral octal integer, base 8 %x, […]
Both remainder and modulo are two similar operations; they act the same when the numbers are positive but much differently when the numbers are negative. In Java, we can use Math.floorMod() to describe a modulo (or modulus) operation and % operator for the remainder operation. See the result: | rem & +divisor| rem & -divisor […]
In Java, we can use LinkedHashMap to keep the insertion order. P.S HashMap does not guarantee insertion order. 1. HashMap Generate a HashMap, UUID as key, index 0, 1, 2, 3, 4… as value. JavaHashMap.java package com.mkyong.samples; import java.util.HashMap; import java.util.Map; import java.util.UUID; import java.util.stream.IntStream; public class JavaHashMap { public static void main(String[] args) { […]
The below program is a typical Java trap, and also a popular Java interview question. It has no compiler error or warning but running very slow. Can you spot the problem? JavaSum.java package com.mkyong; import java.time.Duration; import java.time.Instant; public class JavaSum { public static void main(String[] args) { Instant start = Instant.now(); Long total = […]
An example of converting a String to LocalDateTime, but it prompts the following errors: Java8Example.java package com.mkyong.demo; import java.time.LocalDateTime; import java.time.format.DateTimeFormatter; import java.util.Locale; public class Java8Example { public static void main(String[] args) { String str = "31-Aug-2020"; DateTimeFormatter dtf = DateTimeFormatter.ofPattern("dd-MMM-yyyy", Locale.US); LocalDateTime localDateTime = LocalDateTime.parse(str, dtf); System.out.println(localDateTime); } } Output Exception in thread "main" […]
Here are a few Java 8 examples to parse date with LocalDateTime. First, find the DateTimeFormatter pattern that matches the date format, for example: String str = "2020-01-30 12:30:41"; DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"); second, parse it with LocalDateTime.parse DateTimeFormatter dtf = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"); String str = "2020-01-30 12:30:41"; LocalDateTime localDateTime = LocalDateTime.parse(str, dtf); 1. […]
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(); […]
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); […]
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 […]
In this tutorial, we will show you how to Dockerize a Java Application (an executable Jar file). Tested with Docker 19.03 Ubuntu 19 Java 8 Maven At the end of the article, we will create an executable Jar file and run inside a docker container. 1. Project Directory 1.1 A standard Maven project structure. See […]
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 […]
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 = […]
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); // […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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 […]
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! […]
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 […]
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 […]
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)); […]
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) -> { […]
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 […]
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 […]
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 […]
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 […]
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’ […]
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)
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 […]
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: […]
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 […]
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 […]
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 […]
In Java, -Xms set initial Java heap size, while -Xmx set the maximum Java heap size. 1. Some java -Xms -Xmx examples : # Start with 128MB of memory, and allow the Java process to use up to 1024MB of memory. java -Xms128m -Xmx1024m # Start with 256MB of memory, and allow the Java process […]
