A very important question in mathematics and security is telling whether a number is prime or not. This is pretty useful when encrypting a password. In this tutorial, you will learn how to find whether a number is prime in simple cases. Trivial Cases We learned numbers are prime if the only divisors they have […]
This article shows a few ways to save a byte[] into a file. For JDK 1.7 and above, the NIO Files.write is the simplest solution to save byte[] to a file. // bytes = byte[] Path path = Paths.get("/path/file"); Files.write(path, bytes); FileOutputStream is the best alternative. try (FileOutputStream fos = new FileOutputStream("/path/file")) { fos.write(bytes); //fos.close […]
In Java, we can use Files.readAllBytes(path) to convert a File object into a byte[]. import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; String filePath = "/path/to/file"; // file to byte[], Path byte[] bytes = Files.readAllBytes(Paths.get(filePath)); // file to byte[], File -> Path File file = new File(filePath); byte[] bytes = Files.readAllBytes(file.toPath()); P.S The NIO Files class is […]
Quartz is a powerful and advance scheduler framework, to help Java developer to scheduler a job to run at a specified date and time. This tutorial show you how to develop a scheduler job using Quartz 1.6.3. Note This example is a bit outdate, unless you are still using the old Quartz 1.6.3 library, otherwise, […]
This tutorial shows how to use the JDOM to modify an XML file. Table of contents 1. Download the JDOM 2.x 2. The XML file, before and after 3. Modify XML file 4. Find and remove XML elements 5. Download Source Code 6. References P.S Tested with JDOM 2.0.6 Note We need to understand how […]
This tutorial shows how to use the Java built-in DOM APIs to write data to an XML file. Table of contents 1. Write XML to a file 2. Pretty Print XML 3. Write XML elements, attributes, comments CDATA, etc 4. DOM FAQs 4.1 How to disable the XML declaration? 4.2 How to change the XML […]
JDK Timer is a simple scheduler for a specified task for repeated fixed-delay execution. To use this, you have to extends the TimerTask abstract class, override the run() method with your scheduler function. RunMeTask.java package com.mkyong.common; import java.util.TimerTask; public class RunMeTask extends TimerTask { @Override public void run() { System.out.println("Run Me ~"); } } Now, […]
In this article, we will show you how to send an email via Gmail SMTP server. 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. Gmail SMTP via TLS SMTP = smtp.gmail.com Port = 587 SendEmailTLS.java package com.mkyong; import javax.mail.*; import javax.mail.internet.InternetAddress; import javax.mail.internet.MimeMessage; import java.util.Properties; public class SendEmailTLS […]
The native2ascii is a handy tool build-in in the JDK, which is used to convert a file with ‘non-Latin 1’ or ‘non-Unicode’ characters to ‘Unicode-encoded’ characters. Native2ascii example 1. Create a file (source.txt) Create a file named “source.txt”, put some Chinese characters inside, and save it as “UTF-8” format. 2. native2ascii Use native2ascii command to […]
By default, Eclipse will output Chinese or non-English characters as question marks (?) or some weird characters. This is because the Eclipse’s default console encoding is Cp1252 or ASCII, which is unable to display other non-English words. To enable Eclipse to display Chinese or other non-English characters correctly, do following : 1. In Eclipse, right […]
This article shows how to get the current date time or timestamps in Java. import java.sql.Timestamp; import java.time.Instant; import java.util.Date; // 2025-03-07 21:34:46.504 // Get current java.sql.Timestamp Timestamp timestamp = new Timestamp(System.currentTimeMillis()); // 2025-03-07 21:34:46.504 // Get current java.sql.Timestamp from a Date Date date = new Date(); Timestamp timestamp2 = new Timestamp(date.getTime()); // convert Instant […]
In Java, we can use MessageDigest to get a SHA-256 or SHA3-256 hashing algorithm to hash a string. MessageDigest md = MessageDigest.getInstance("SHA3-256"); byte[] result = md.digest(input); This article shows how to use Java SHA-256 and SHA3-256 algorithms to generate a hash value from a given string and checksum from a file. Note The hashing is […]
The MD5, defined in RFC 1321, is a hash algorithm to turn inputs into a fixed 128-bit (16 bytes) length of the hash value. Note MD5 is not collision-resistant – Two different inputs may producing the same hash value. Read this MD5 vulnerabilities. There are many fast and secure hashing algorithms like SHA3-256 or BLAKE2; […]
Here’s a tip to demonstrate how to know from where a Java Class was loaded in Java. Java Example Here’s an example to load a Java class called “Address “, package in “com.mkyong.io“, and print out the location from where this class was loaded. import java.net.URL; import java.security.CodeSource; import java.security.ProtectionDomain; public class App{ public static […]
The “javax.imageio” package is used to deal with the Java image stuff. Here’s two “ImageIO” code snippet to read an image file. 1. Read from local file File sourceimage = new File("c:\\mypic.jpg"); Image image = ImageIO.read(sourceimage); 2. Read from URL URL url = new URL("https://mkyong.com/image/mypic.jpg"); Image image = ImageIO.read(url); ImageIO Example In this example, you […]
In last section, you learn about how to compress a serialized object into a file, now you learn how to decompress it from a Gzip file. FileInputStream fin = new FileInputStream("c:\\address.gz"); GZIPInputStream gis = new GZIPInputStream(fin); ObjectInputStream ois = new ObjectInputStream(gis); address = (Address) ois.readObject(); GZIP example In this example, you will decompress a compressed […]
In last section, you learn about how to write or serialized an object into a file. In this example , you can do more than just serialized it , you also can compress the serialized object to reduce the file size. The idea is very simple, just using the “GZIPOutputStream” for the data compression. FileOutputStream […]
In this tutorial, we will learn how to unzip a ZIP file using Java’s built-in java.util.zip package, while also discussing the Zip Slip vulnerability and how to avoid it. Table of Contents: 1. Extracting a ZIP File in Java 2. Understanding the Zip Slip Vulnerability What is Zip Slip? How to Prevent Zip Slip? Secure […]
Collection object has a constructor that accept a Collection object to initial the value. Since both Set and List are extend the Collection, the conversion is quite straightforward. It’s just pass a List into Set constructor or vice verse. Convert List to Set Set set = new HashSet(list); Convert Set to List List list = […]
Set and List explanation Set – Stored elements in unordered or shuffles way, and does not allow duplicate values. List – Stored elements in ordered way, and allow duplicate values. Set and List Example import java.util.ArrayList; import java.util.HashSet; import java.util.List; import java.util.Set; public class SetAndListExample { public static void main( String[] args ) { System.out.println("List […]
Learn how to easily zip files and directories in Java using `java.util.zip`, `Files.copy`, and the Zip File System Provider.
Here are a few Java examples of converting between String or ASCII to and from Hexadecimal. Apache Commons Codec – Hex Integer Bitwise 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F 1. Apache Commons Codec Both Hex.encodeHex and Hex.decodeHex can convert String to Hex and vice versa. […]
This article shows how to Java 8 Files.walk to walk a file tree and stream operation filter to find files that match a specific file extension from a folder and its subfolders. // find files matched `png` file extension from folder C:\\test try (Stream<Path> walk = Files.walk(Paths.get("C:\\test"))) { result = walk .filter(p -> !Files.isDirectory(p)) // […]
Normally, Java properties file is used to store project configuration data or settings. In this tutorial, we will show you how to read and write to/from a .properties file. Properties prop = new Properties(); // set key and value prop.setProperty("db.url", "localhost"); prop.setProperty("db.user", "mkyong"); prop.setProperty("db.password", "password"); // save a properties file prop.store(outputStream, ""); // load a […]
As far as i know, there are 4 ways to get the Java Class object. 1. “.class” Class cls = Address.class; 2. object.getClass() Address address = new Address(); Class cls = address.getClass(); 3. Class.forName() Class cls = Class.forName("com.mkyong.io.Address"); 4. ClassLoader.loadClass() ClassLoader cl = ClassLoader.getSystemClassLoader(); Class cls = cl.loadClass("com.mkyong.io.Address");
In this tutorial, we will show you how to create a executable JAR – When you double click on it, it runs the defined main class in manifest file. 1. AWT Example Create a simple AWT Java application, just display label and print out some funny characters ~ AwtExample.java package com.mkyong.awt; import java.awt.Frame; import java.awt.Label; […]
This article shows few examples to compare two dates in Java. Updated with Java 8 examples. 1. Compare two date 1.1 Date.compareTo 1.2 Date.before(), Date.after() and Date.equals() 1.3 Check if a date is within a certain range 2. Compare two calendar 3. Compare two date and time (Java 8) 3.1 Compare two LocalDate 3.2 Compare […]
Often times, this error is causing by the mismatch order between “m” and “f” Jar options. For example, jar -cvfm manifest.txt example.jar com/mkyong/awt/*.class The above command will causing the following error : java.io.IOException: invalid header field at java.util.jar.Attributes.read(Attributes.java:406) at java.util.jar.Manifest.read(Manifest.java:199) at java.util.jar.Manifest.<init>(Manifest.java:69) at sun.tools.jar.Main.run(Main.java:150) at sun.tools.jar.Main.main(Main.java:1044) Did you spot the error? The “m” and “manifest” […]
In Java, we use StringTokenizer to split a string into multiple tokens. Note The StringTokenizer is a legacy class, try the split method of String, read this How to split a string in Java. 1. StringTokenizer examples 1.1 By default, the StringTokenizer uses default delimiters: space, tab, newline, carriage-return, and form-feed characters to split a […]
Reflection is a very useful approach to deal with the Java class at runtime, it can be use to load the Java class, call its methods or analysis the class at runtime. In this example, you will load a class called “AppTest” and call each of its methods at runtime. 1. AppTest.java This Java class […]
In Java, we can use the javax.imageio.ImageIO class to read and write an image. 1. Read an image Read an image from a file. BufferedImage image = ImageIO.read(new File("c:\\test\\image.png")); Read an image from an URL. BufferedImage image = ImageIO.read(new URL("https://example.com/image.png")); 2. Write or save an image Write or save an image in different image formats. […]
Here’s the project structure. /workspace/test/classes/com/mkyong/awt/AwtExample.class /workspace/test/classes/com/mkyong/awt/AwtExample2.class /workspace/test/classes/com/mkyong/awt/AwtExample3.class /workspace/test/classes/manifest.txt P.S Assume you are in “/workspace/test/classes/“ 1. Create a jar file -c create new archive -v generate verbose output on standard output -f specify archive file name 1.1 Create a Jar file which include AwtExample.class only. jar -cvf test.jar com/mkyong/awt/AwtExample.class 1.2 Create a Jar file which include […]
File.listRoots() will list out all the available file system roots / drives in your current system. Example package com.mkyong.io; import java.io.File; public class App{ public static void main (String args[]) { File[] rootDrive = File.listRoots(); for(File sysDrive : rootDrive){ System.out.println("Drive : " + sysDrive); } } } Output Drive : A:\ Drive : C:\ Drive […]
This article shows you a few ways to generate the serialVersionUID for serialization class. 1. serialver JDK has a built-in command serialver to generate a serialVersionUID automatically. In this example, we use serialver to generate a serialVersionUID for an Address class. Terminal $ serialver Address Address: static final long serialVersionUID = -687991492884005033L; 2. Eclispe IDE […]
In certain scenario, you may need to load some classes which are not in your classpath. Java Example Assume folder “c:\\other_classes\\” is not in your project classpath, here’s an example to show how to load a Java class from this folder. The code and comments are self-explanatory. import java.io.File; import java.net.URL; import java.net.URLClassLoader; import java.security.CodeSource; […]
In Java, you can use manifest file to define application’s entry point, adding classpath or package version for a JAR file. In this short tutorial , we will show you how to add a custom manifest file into a Jar file. 1. Project Structure Assume this is your project folder structure /workspace/test/classes/com/mkyong/awt/AwtExample.class /workspace/test/classes/manifest.txt 2. Jar […]
Java application usually required a main() method as entry point to run it. Here’s an example by using static initializer to run a Java application even without the main() method. P.S The static initializer is called while the Java class is loaded. This is for fun only, do not use this example in real environment […]
Java’s “SystemClassLoader” can use to pint out the current project classpath , indirectly display the library dependency as well. Example package com.mkyong.io; import java.net.URL; import java.net.URLClassLoader; public class App{ public static void main (String args[]) { ClassLoader cl = ClassLoader.getSystemClassLoader(); URL[] urls = ((URLClassLoader)cl).getURLs(); for(URL url: urls){ System.out.println(url.getFile()); } } } Output /E:/workspace/HibernateExample/target/test-classes/ /E:/workspace/HibernateExample/target/classes/ /D:/maven/repo/antlr/antlr/2.7.7/antlr-2.7.7.jar […]
Note This post is duplicated, please refer to this – 3 ways to read input from console in Java. A quick example to show you how to read the standard input in Java. package com.mkyong.pageview; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; public class Test { public static void main(String[] args) { BufferedReader br = null; […]
Most people will read the file content and assign to StringBuffer or String line by line. Here’s another trick that may interest you – how to assign whole file content into a variable with one Java’s statement, try it 🙂 Example In this example, you will use DataInputStreamto convert all the content into bytes, and […]
