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(); […]
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); […]
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 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 […]
Few Java examples to show you how to check if a String is numeric. 1. Character.isDigit() Convert a String into a char array and check it with Character.isDigit() NumericExample.java package com.mkyong; public class NumericExample { public static void main(String[] args) { System.out.println(isNumeric("")); // false System.out.println(isNumeric(" ")); // false System.out.println(isNumeric(null)); // false System.out.println(isNumeric("1,200")); // false System.out.println(isNumeric("1")); […]
In Java, we can use (str != null && !str.isEmpty()) to make sure the String is not empty or null. StringNotEmpty.java package com.mkyong; public class StringNotEmpty { public static void main(String[] args) { System.out.println(notEmpty("")); // false System.out.println(notEmpty(null)); // false System.out.println(notEmpty("hello")); // true System.out.println(notEmpty(" ")); // true, a space… } private static boolean notEmpty(String str) { […]
In Jackson, we can use the annotation @JsonView to control which fields of the same resource are displayed for different users. Table of contents: 1. Download Jackson 2. Define View Classes 3. Annotate model class with @JsonView 4. Testing the Jackson @JasonView 5. Download Source Code 6. References P.S Tested with Jackson 2.17.0 1. Download […]
This article uses the Jackson framework to parse JSON strings in Java. Table of contents: 1. Download Jackson 2. Parse JSON String with Jackson 3. Parse JSON Array with Jackson 4. Convert Java Object to JSON String 5. Convert JSON String to Java Object 6. Download Source Code 7. References P.S Tested with Jackson 2.17.0 […]
In Jackson, we can use @JsonInclude(JsonInclude.Include.NON_NULL) to ignore the null fields during serialization. Table of contents: 1. Jackson default include null fields 2. Ignore all null fields globally 3. Ignore null fields (Class Level) 4. Ignore null fields (Field Level) 5. Download Source Code 6. References P.S Tested with Jackson 2.17.0 1. Jackson default include […]
This article uses the Jackson framework to parse JSON Array in Java. Table of contents: 1. Download Jackson 2. Parse JSON Array with Jackson 3. Convert List of Objects to JSON Array 4. Write JSON Array with a list label 5. Parse JSON Array contains a list label 6. Download Source Code 7. References P.S […]
This article shows how to parse JSON using Gson. Table of contents: 1. Download Google Gson 2. A Java Object 3. Parse JSON String using Gson 4. Parse JSON Array using Gson 5. Convert Java Object to JSON using Gson 6. Unstructured JSON 7. Download Source Code 8. References P.S Tested with Gson 2.10.1 1. […]
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(); // […]
Find the major version of the Java class file, and we will know which JDK version compiled the Java class. Below are some popular Java LTS released and their major version numbers. Java SE Major Version Hex 17 61 0x3D 11 55 0x37 8 52 0x34 If the major version of the Java class file […]
Start a Java class and hits this java.lang.UnsupportedClassVersionError, what is class file version 52 56? Error: A JNI error has occurred, please check your installation and try again Exception in thread "main" java.lang.UnsupportedClassVersionError: TestHello has been compiled by a more recent version of the Java Runtime (class file version 56.0), this version of the Java […]
In Java, we can use java.util.Scanner to get user input from console. 1. Scanner 1.1 Read a line. UserInputExample1.java package com.mkyong; import java.util.Scanner; public class UserInputExample1 { public static void main(String[] args) { // auto close scanner try (Scanner scanner = new Scanner(System.in)) { System.out.print("Enter something : "); String input = scanner.nextLine(); // Read user […]
A Java 2d array example. ArrayExample.java package com.mkyong; public class ArrayExample { public static void main(String[] args) { int[][] num2d = new int[2][3]; num2d[0][0] = 1; num2d[0][1] = 2; num2d[0][2] = 3; num2d[1][0] = 10; num2d[1][1] = 20; num2d[1][2] = 30; //or init 2d array like this : int[][] num2dInit = { {1, 2, 3}, […]
A simple List.add() and hits the following java.lang.UnsupportedOperationException ListExample.java package com.mkyong; import java.util.Arrays; import java.util.List; public class ListExample { public static void main(String[] args) { List<String> str = Arrays.asList("A", "B", "C"); str.add("D"); System.out.println(str); } } Output Exception in thread "main" java.lang.UnsupportedOperationException at java.base/java.util.AbstractList.add(AbstractList.java:153) at java.base/java.util.AbstractList.add(AbstractList.java:111) at com.mkyong.ListExample.main(ListExample.java:14) Solution The Arrays.asList returns a fixed-size list, modify […]
In Java, for primitive types, parameters are pass-by-value; For object types, object reference is pass-by-value, however, Java is allowed to modify object’s fields via object reference. 1. Primitive Type No doubt, for primitive type, the parameters are pass by value. PrimitiveExample.java package com.mkyong; public class PrimitiveExample { public static void main(String[] args) { int value […]
A popular common error for new Java users. Terminal C:\Users\mkyong>javac ‘javac’ is not recognized as an internal or external command, operable program or batch file. C:\Users\mkyong>java -version ‘java’ is not recognized as an internal or external command, operable program or batch file. 1. Solution To fix it, update the system variable PATH to JDK_folder\bin 1. […]
A popular error message for new Java users. Error: Could not find or load main class ClassName.class Caused by: java.lang.ClassNotFoundException: ClassName.class 1. No Package 1.1 Reviews a simple Java Hello World, no package. C:\projects\HelloWorld.java public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World"); } } Terminal # Compile Java source code C:\projects> […]
In Spring, we can use ClassPathResource or ResourceLoader to get files from classpath easily. P.S Tested with Spring 5.1.4.RELEASE 1. src/main/resources/ For example, an image file in the src/main/resources/ folder 2. ClassPathResource import org.springframework.core.io.Resource; import org.springframework.core.io.ClassPathResource; import java.io.File; import java.io.InputStream; Resource resource = new ClassPathResource("android.png"); InputStream input = resource.getInputStream(); File file = resource.getFile(); 3. ResourceLoader […]
In this tutorial, we will show you how to send email via SMTP in Spring Boot. Technologies used : Spring Boot 2.1.2.RELEASE Spring 5.1.4.RELEASE Java Mail 1.6.2 Maven 3 Java 8 1. Project Directory 2. Maven To send email, declares spring-boot-starter-mail, it will pull the JavaMail dependencies. pom.xml <?xml version="1.0" encoding="UTF-8"?> <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 […]
To send email in Java, we need JavaMail pom.xml <dependency> <groupId>com.sun.mail</groupId> <artifactId>javax.mail</artifactId> <version>1.6.2</version> </dependency> 1. Send Email Send a normal email in text format. SendEmailSMTP.java package com.mkyong; import com.sun.mail.smtp.SMTPTransport; import javax.mail.Message; import javax.mail.MessagingException; import javax.mail.Session; import javax.mail.internet.InternetAddress; import javax.mail.internet.MimeMessage; import java.util.Date; import java.util.Properties; public class SendEmailSMTP { // for example, smtp.mailgun.org private static final String […]
This article focus on a few of the commonly used methods to read a file in Java. Files.lines, return a Stream (Java 8) Files.readString, returns a String (Java 11), max file size 2G. Files.readAllBytes, returns a byte[] (Java 7), max file size 2G. Files.readAllLines, returns a List<String> (Java 8) BufferedReader, a classic old friend (Java […]
In Java, we can use Files.write to create and write to a file. String content = "…"; Path path = Paths.get("/home/mkyong/test.txt"); // string -> bytes Files.write(path, content.getBytes(StandardCharsets.UTF_8)); The Files.write also accepts an Iterable interface; it means this API can write a List to a file. List<String> list = Arrays.asList("a", "b", "c"); Files.write(path, list); Short History […]
In Java, we can use Files.readAllBytes to read a file. byte[] content = Files.readAllBytes(Paths.get("app.log")); System.out.println(new String(content)); 1. Text File A Java example to write and read a normal text file. FileExample1.java package com.mkyong.calculator; import java.io.IOException; import java.nio.charset.Charset; import java.nio.charset.StandardCharsets; import java.nio.file.Files; import java.nio.file.Paths; import java.util.Arrays; import java.util.List; public class FileExample1 { public static void main(String[] […]
In IntelliJ IDEA, Maven builds a project, and hits the following warning message? Warning:java: source value 1.5 is obsolete and will be removed in a future release Warning:java: target value 1.5 is obsolete and will be removed in a future release To fix it, add maven-compiler-plugin plugin. pom.xml <build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-compiler-plugin</artifactId> <version>3.8.0</version> <configuration> […]
In Java, there are 3 ways to loop or iterate a HashMap 1. If possible, always uses the Java 8 forEach. Map<String, String> map = new HashMap<>(); map.forEach((key, value) -> System.out.println("[Key] : " + key + " [Value] : " + value)); 2. Normal for loop in entrySet() Map<String, String> map = new HashMap<>(); for […]
In this tutorial, we will show you how to use Apache Log4j 2 in Spring Boot framework. Technologies used : Spring Boot 2.1.2.RELEASE Spring 5.1.4.RELEASE Log4j 2.11.1 Maven 3 Java 8 1. Project Directory 2. Maven The key is exclude the default logback, and include log4j with spring-boot-starter-log4j2 pom.xml <?xml version="1.0" encoding="UTF-8"?> <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" […]
A JavaScript example to show you how to get the selected value or text from a dropdown list. A drop box list. <select id="country"> <option value="None">– Select –</option> <option value="ID001">China</option> <option value="ID002" selected>United State</option> <option value="ID003">Malaysia</option> </select> Get option value : var e = document.getElementById("country"); var result = e.options[e.selectedIndex].value; alert(result); //ID002 Get option text : […]
A simple log4j2.yml example, just for self-reference P.S Tested with Log4j 2.11.2 1. Jackson for YML Log4j 2 need the following libraries to parse yml file pom.xml <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-api</artifactId> <version>2.11.2</version> </dependency> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-core</artifactId> <version>2.11.2</version> </dependency> <dependency> <groupId>com.fasterxml.jackson.dataformat</groupId> <artifactId>jackson-dataformat-yaml</artifactId> <version>2.9.8</version> </dependency> <dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-databind</artifactId> <version>2.9.8</version> </dependency> 2. log4j2.yml src/resources/log4j2.yml Configuration: status: warn appenders: Console: […]
A simple log4j2.properties example, just for self-reference P.S Tested with Log4j 2.11.2 src/resources/log4j2.properties status = warn appender.console.type = Console appender.console.name = LogToConsole appender.console.layout.type = PatternLayout appender.console.layout.pattern = [%-5level] %d{yyyy-MM-dd HH:mm:ss.SSS} [%t] %c{1} – %msg%n #appender.file.type = File #appender.file.name = LogToFile #appender.file.fileName=logs/app.log #appender.file.layout.type=PatternLayout #appender.file.layout.pattern=[%-5level] %d{yyyy-MM-dd HH:mm:ss.SSS} [%t] %c{1} – %msg%n # Rotate log file appender.rolling.type = […]
In JavaScript, we can use for, forEach or for..of to loop an Array. 1.1 for //old-school, classic for loop var str = ["a", "b", "c", "d", "e"]; for(var i =0; i < str.length; i++){ console.log(str[i]); //a,b,c,d,e } output a b c d e 1.2 forEach() var num = [1, 2, 3, 4, 5]; num.forEach(anyFunction) function […]
In JavaScript, we can use forEach(function) to loop an Array. The provided function in forEach() will run once for each array element. var total = 0; var num = [1, 2, 3, 4, 5]; num.forEach(anyFunction) function anyFunction(data){ total += data; } console.log(total); //15 var words = ""; var num = ["a", "b", "c", "d", "e"]; […]
Some log4j2.xml examples, just for self-reference P.S Tested with Log4j 2.11.2 1. ConsoleAppender Logs to console. log4j2.xml <?xml version="1.0" encoding="UTF-8"?> <Configuration status="DEBUG"> <Appenders> <Console name="LogToConsole" target="SYSTEM_OUT"> <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} – %msg%n"/> </Console> </Appenders> <Loggers> <!– avoid duplicated logs with additivity=false –> <Logger name="com.mkyong" level="debug" additivity="false"> <AppenderRef ref="LogToConsole"/> </Logger> <Root level="error"> <AppenderRef ref="LogToConsole"/> </Root> […]
In JavaScript, we can combine indexOf() and splice() to remove a certain element from an Array. var array = [1, 2, 3, 4, 5]; console.log(array) // I want remove num 4, find the index first, -1 if it is not present. var index = array.indexOf(4); if (index > -1) { // found array.splice(index, 1); } […]
Enable the log4j 2 loggers to asynchronous, but hits the following order : $ mvn -Dlog4j2.contextSelector=org.apache.logging.log4j.core.async.AsyncLoggerContextSelector -jar app.jar java.lang.NoClassDefFoundError: com/lmax/disruptor/EventTranslatorVararg at java.lang.ClassLoader.defineClass1 (Native Method) at java.lang.ClassLoader.defineClass (ClassLoader.java:1009) at java.security.SecureClassLoader.defineClass (SecureClassLoader.java:174) at java.net.URLClassLoader.defineClass (URLClassLoader.java:545) at java.net.URLClassLoader.access$100 (URLClassLoader.java:83) at java.net.URLClassLoader$1.run (URLClassLoader.java:453) Solution To fix it, add disruptor pom.xml <dependency> <groupId>com.lmax</groupId> <artifactId>disruptor</artifactId> <version>3.4.2</version> </dependency> Note Read this Making […]
In JavaScript, we can use either location.replace() or location.href to redirect a page. Normally, we use location.replace() to simulate an HTTP redirect. <!DOCTYPE html> <html> <head> <meta charset="UTF-8"> </head> <body> <h1>JavaScript – How to redirect a page</h1> <h3>1. location.replace</h3> <button onclick="httpRedirect()">location.replace</button> <h3>2. location.href</h3> <button onclick="mouseClick()">location.href</button> <script> // Simulate an HTTP redirect function httpRedirect() { location.replace("https://www.mkyong.com") […]
A simple log4j 2 hello world example. Tested with Log4j 2.11.2 Maven 3 Java 8 Note Apache Log4j 2, the fastest Java logging framework, provides significant improvements over its predecessor, Log4j 1.x. 1. Project Directory 2. Maven <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-api</artifactId> <version>2.11.2</version> </dependency> <dependency> <groupId>org.apache.logging.log4j</groupId> <artifactId>log4j-core</artifactId> <version>2.11.2</version> </dependency> pom.xml <?xml version="1.0" encoding="UTF-8"?> <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 […]
