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 […]
Normal practice Constant value comes second in comparison. private static final String COMPARE_VALUE = "VALUE123"; public boolean compareIt(String input){ if(input.equals(COMPARE_VALUE)){ return true; }else{ return false; } } Problem This is fine to compare a constant value with the above method, however it will potentially causing a NullPointerException, if user pass a “null” value for the […]
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 […]
In this example, the program will traverse the given directory and print out all the directories and files absolute path and name one by one. Example package com.mkyong.io; import java.io.File; public class DisplayDirectoryAndFile{ public static void main (String args[]) { displayIt(new File("C:\\Downloads")); } public static void displayIt(File node){ System.out.println(node.getAbsoluteFile()); if(node.isDirectory()){ String[] subNote = node.list(); for(String […]
In this article, we show you two ways to open a PDF file with Java. 1. rundll32 – Windows Platform Solution In Windows, you can use “rundll32” command to launch a PDF file, see example : package com.mkyong.jdbc; import java.io.File; //Windows solution to view a PDF file public class WindowsPlatformAppPDF { public static void main(String[] […]
The StringBuffer is always been using for the String concatenation. However this class didn’t provide a method to clear the existing content. Why there are no clear() function? You can use the delete(int start, int end) as dirty trick : sb.delete(0, sb.length()); 1 . Example package com.mkyong.io; public class App{ public static void main (String […]
For some security reasons, you may need to convert your HTML file into Javascript (js) file, and display the JS file instead of the HTML file directly. The concept is quite simple – using the document.write HTML <h1>Convert HTML to Javascript file</h1> Javascript (js) document.write(‘<h1>Convert HTML to Javascript file</h1>’); 1. Test.html Create a simple HTML […]
In Java, you can implements the FilenameFilter, override the accept(File dir, String name) method, to perform the file filtering function. In this example, we show you how to use FilenameFilter to list out all files that are end with “.txt” extension in folder “c:\\folder“, and then delete it. package com.mkyong.io; import java.io.*; public class FileChecker […]
In Java, we can use Files.exists(path) to test whether a file exists. The path can be a file or a directory. It is better to combine with !Files.isDirectory(path) to ensure the existing file is not a directory. Path path = Paths.get("/home/mkyong/test/test.log"); // file exists and it is not a directory if(Files.exists(path) && !Files.isDirectory(path)) { System.out.println("File […]
JDOM is open-source Java-based XML parser framework built on top of DOM and SAX, also a document object model(DOM) in-memory representation of an XML document. JDOM makes navigating XML documents easier by providing more straightforward APIs and a standard Java-based collection interface. If you don’t mind downloading a small library to parse an XML file, […]
Note Since Java 8, the JDK built-in Apache Xerces XML parser has been improved a lot, and the majority of the IO classes now default to UTF-8. We cannot simulate the same error anymore, and the internal Xerces APIs are now printing ? for unknown characters instead of showing the error Invalid byte 1 of […]
This article shows how to use the SAX parser to read or parse a UTF-8 XML file. Table of contents 1. SAX parser to parse a UTF-8 XML file. 2. Character Encoding in XML and code 3. SAX common errors 3.1 Invalid byte 1 of 1-byte UTF-8 sequence 3.2 Content is not allowed in prolog […]
We use SAX parser to parse an XML file, and hist the following error message: Terminal org.xml.sax.SAXParseException; systemId: ../src/main/resources/staff.xml; lineNumber: 1; columnNumber: 1; Content is not allowed in prolog. In short, invalid text or BOM before the XML declaration or different encoding will cause the SAX Error – Content is not allowed in prolog. 1. […]
In Java, you can use System.getProperties() to get all the system properties. Properties properties = System.getProperties(); properties.forEach((k, v) -> System.out.println(k + ":" + v)); // Java 8 1. Example DisplayApp.java package com.mkyong.display; import java.util.Properties; public class DisplayApp { public static void main(String[] args) { Properties properties = System.getProperties(); // Java 8 properties.forEach((k, v) -> System.out.println(k […]
In Java, we can cast the char to int to get the ASCII value of the char. char aChar = ‘a’; //int ascii = (int) aChar; // explicitly cast, optional, improves readability int ascii = aChar; // implicit cast, auto cast char to int, System.out.println(ascii); // 97 The explicit cast (int)char is optional, if we […]
Java2HTML is a tool to converts Java source code into a colorized and browsable HTML page, and the steps are quite straight forward. Steps to convert it 1) Download Java2HTML – http://www.java2html.com/download.html 2) Unzip it , e.g “D:\Java2HTML” 3) Edit the “j2h.bat” file and modify the CLASSPATH variable. Make sure it point to the correct […]
This article shows two ways to resize an image (create a thumbnail) in Java. 1. Original Image, 544×184 Below is a Google logo image, width, height 544×184, and file size 14k. Later, we will resize the below image to a new width and height 300×150. 2. Resize an image or create a thumbnail. This Java […]
This article shows how to convert a byte[] to a BufferedImage in Java. InputStream is = new ByteArrayInputStream(bytes); BufferedImage bi = ImageIO.read(is); The idea is puts the byte[] into an ByteArrayInputStream object, and we can use ImageIO.read to convert it to a BufferedImage. 1. Convert byte[] to BufferedImage. The below example shows how to convert […]
This article shows how to convert a BufferedImage to a byte array or byte[]. BufferedImage bi = ImageIO.read(new File("c:\\image\\mypic.jpg")); ByteArrayOutputStream baos = new ByteArrayOutputStream(); ImageIO.write(bi, "jpg", baos); byte[] bytes = baos.toByteArray(); The idea is uses the ImageIO.write to write the BufferedImage object into a ByteArrayOutputStream object, and we can get the byte[] from the ByteArrayOutputStream. […]
Eclipse IDE, press CTRL + T or F4 to view the Java class hierarchy, no need to search the JavaDoc API online. Note For Mac OS X, press Command + T Steps to show class hierarchy 1. Clicks on a class, press CTRL + T to view the subtype hierarchy. 2. Press CTRL + T […]
Example UUID is the fastest and easiest way to generate unique ID in Java. import java.util.UUID; public class UniqueIDTest { public static void main(String[] args) { UUID uniqueKey = UUID.randomUUID(); System.out.println (uniqueKey); } } Result 0a6a3de7-b974-42d0-af9e-9c23473b09b9
Q : Is there a function in Java to retrieve the Tomcat (Catalina) home directory? A : Yes, Tomcat home directory or Catalina directory is stored at the Java System Property environment. If the Java web application is deployed into Tomcat web server, we can get the Tomcat directory with the following command System.getProperty("catalina.base");
A Java example to show you how to convert a Char into a String and vise verse. ConvertCharToString.java package com.mkyong.utils; public class ConvertCharToString { public static void main(String[] args) { String website = "https://mkyong.com"; //convert a String into char char charH = website.charAt(0); //h char charP = website.charAt(3);//p char charM = website.charAt(11);//m System.out.println(charH); System.out.println(charP); System.out.println(charM); […]
This article shows a handy Java class that uses System.getProperty("os.name") to detect which type of operating system (OS) you are using now. 1. Detect OS (Original Version) This code can detect Windows, Mac, Unix, and Solaris. OSValidator.java package com.mkyong.system; public class OSValidator { private static String OS = System.getProperty("os.name").toLowerCase(); public static void main(String[] args) { […]
The ‘unchecked warnings’ is quite popular warning message in Java. However, if you insist this is an invalid warning, and there are no ways to solve it without compromising the existing program functionality. You may just use @SuppressWarnings(“unchecked”) to suppress unchecked warnings in Java. 1. In Class If applied to class level, all the methods […]
Here is a simple ResourceBundle message files hello = "Hi \n \n Good Morning \n \n thanks" If we retrieve the resource bundle key “hello”, we expected the result as following Hi Good Morning thanks Unfortunately, Java doesn’t work in this way, “String” will not translate the \n into new line by default. If we […]
The “context-param” tag is define in “web.xml” file and it provides parameters to the entire web application. For example, store administrator’s email address in “context-param” parameter to send errors notification from our web application. web.xml <context-param> <param-name>AdministratorEmail</param-name> <param-value>[email protected]</param-value> </context-param> We can get the above “AdministratorEmail” context-param value with the following java code. String email= getServletContext().getInitParameter("AdministratorEmail"); […]
This is always advisable to encode URL or form parameters; plain form parameter is vulnerable to cross site attack, SQL injection and may direct our web application into some unpredicted output. A URL String or form parameters can be encoded using the URLEncoder class – static encode (String s, String enc) method. For example, when […]
In Java, we can use Apache commons-text to escape the special characters in HTML entities. Special characters as follow: < > ” & pom.xml <dependency> <groupId>org.apache.commons</groupId> <artifactId>commons-text</artifactId> <version>1.8</version> </dependency> EscapeSpecialChar.java package com.mkyong.html; import org.apache.commons.text.StringEscapeUtils; // @deprecated as of 3.6, use commons-text StringEscapeUtils instead //import org.apache.commons.lang3.StringEscapeUtils; public class EscapeSpecialChar { public static void main(String[] args) { […]
Oftentimes, we use SSH to remote access into the server to run a Java program. The problem is, we can’t type anything After the Java program is executed like this : $ java -jar example.jar In addition, when the remote access session is expired or terminated, the executed Java program will be killed. Solution To […]
In Java, we can use str.getBytes(StandardCharsets.UTF_8) to convert a String into a byte[]. String str = "This is a String"; // default charset, a bit dangerous byte[] output1 = str.getBytes(); // in old days, before java 1.7 byte[] output2 = str.getBytes(Charset.forName("UTF-8")); // the best , java 1.7+ , new class StandardCharsets byte[] output3 = str.getBytes(StandardCharsets.UTF_8); […]
Believe it or not, Java contains Integer buffer overflow as well. I’m not sure is this the correct word to describe it or not, may be you can suggest some 🙂 OK, please take a look at below program, this program print the number of microseconds in a day. public class JavaLongOverflow { public static […]
There are many monetary values calculation in the financial or e-commerce application, and there is one question that arises for this – Should we use double or float data type to represent the monetary values? Answer: Always uses java.math.BigDecimal to represent the monetary values. 1. Double or Float? Here is an example of using double […]
In Java, you can use following methods to get time in milliseconds Date class – getTime() method Calendar class – getTimeInMillis() method TimeMilisecond.java package com.mkyong.test; import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Calendar; import java.util.Date; public class TimeMilisecond { public static void main(String[] argv) throws ParseException { SimpleDateFormat sdf = new SimpleDateFormat("dd-M-yyyy hh:mm:ss"); String dateInString = "22-01-2015 […]
Regular expression pattern in Java always is the best method to validate an user’s phone number. Here i provide a regex pattern to determines if the phone number is in correct format, the pattern force starting with 3 digits follow by a “-” and 7 digits at the end. \\d{3}-\\d{7} Explanation \\d = only digit […]
