Category: java

java January 12, 2010 by mkyong 10 comments

How to traverse a directory structure in Java

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 […]

java January 12, 2010 by mkyong 66 comments

How to open a PDF file in Java

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[] […]

java January 11, 2010 by mkyong 6 comments

How to clear / delete the content of StringBuffer()

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 […]

java January 11, 2010 by mkyong 2 comments

How to convert HTML to Javascript (.js) in Java

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 […]

java January 10, 2010 by mkyong 5 comments

How to get file last modified date in Java

In Java, we can use Files.readAttributes() to get the file metadata or attribute, and then lastModifiedTime() to display the last modified date of a file. Path file = Paths.get("/home/mkyong/file.txt"); BasicFileAttributes attr = Files.readAttributes(file, BasicFileAttributes.class); System.out.println("lastModifiedTime: " + attr.lastModifiedTime()); 1. BasicFileAttributes (NIO) This example uses the java.nio.* to display the file attributes or metadata – creation […]

java January 10, 2010 by mkyong 13 comments

How to get the current working directory in Java

In Java, we can use System.getProperty("user.dir") to get the current working directory, the directory from where your program was launched. String dir = System.getProperty("user.dir"); // directory from where the program was launched // e.g /home/mkyong/projects/core-java/java-io System.out.println(dir); One of the good things about this system property user.dir is we can easily override the system property via […]

java January 10, 2010 by mkyong 8 comments

How to read an object from file in Java (ObjectInputStream)

This example shows how to use ObjectInputStream to read a serialized object from a file in Java, aka Deserialization. public static Object readObjectFromFile(File file) throws IOException, ClassNotFoundException { Object result = null; try (FileInputStream fis = new FileInputStream(file); ObjectInputStream ois = new ObjectInputStream(fis)) { result = ois.readObject(); } return result; } // Convert byte[] to […]

java January 9, 2010 by mkyong 13 comments

How to write an object to file in Java (ObjectOutputStream)

This example shows how to use ObjectOutputStream to write objects to a file in Java, aka Serialization. public static void writeObjectToFile(Person obj, File file) throws IOException { try (FileOutputStream fos = new FileOutputStream(file); ObjectOutputStream oos = new ObjectOutputStream(fos)) { oos.writeObject(obj); oos.flush(); } } Note More Java Serialization and Deserialization examples 1. Java object We can […]

java January 9, 2010 by mkyong 5 comments

How to delete files with certain extension only

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 […]

java January 8, 2010 by mkyong 18 comments

How to check if a file exists in Java

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 […]

java December 24, 2009 by mkyong 14 comments

Java – How to get current date time

In this tutorial, we will show you how to get the current date time from the new Java 8 java.time.* like Localdate, LocalTime, LocalDateTime, ZonedDateTime, Instant and also the legacy date time APIs like Date and Calendar. Table of contents 1. Get current date time in Java 2. java.time.LocalDate 3. java.time.LocalTime 4. java.time.LocalDateTime 5. java.time.ZonedDateTime […]

java December 21, 2009 by mkyong 5 comments

How to read XML file in Java – (JDOM Parser)

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, […]

java December 20, 2009 by mkyong 4 comments

How to convert InputStream to File in Java

Below are some Java examples of converting InputStream to a File. Plain Java – FileOutputStream Apache Commons IO – FileUtils.copyInputStreamToFile Java 7 – Files.copy Java 9 – InputStream.transferTo 1. Plain Java – FileOutputStream This example downloads the google.com HTML page and returns it as an InputStream. And we use FileOutputStream to copy the InputStream into […]

java December 17, 2009 by mkyong 5 comments

SAX – Invalid byte 1 of 1-byte UTF-8 sequence

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 […]

java December 17, 2009 by mkyong 14 comments

How to read UTF-8 XML file in Java – (SAX Parser)

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 […]

java December 5, 2009 by mkyong 10 comments

SAX Error – 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. […]

java December 1, 2009 by mkyong 7 comments

Java – How to display all System properties

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 […]

java December 1, 2009 by mkyong 4 comments

Java – Convert Character to ASCII

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 […]

java December 1, 2009 by mkyong 6 comments

How to change Eclipse splash welcome screen image

Eclipse IDE has many customize components, the splash welcome image (purple color loading image) is one of those. I’m very boring to see the purple image loading everyday, a new fresh image will make my development more encouraging 🙂 How to change Eclipse IDE splash image 1) Find “config.ini” file Find the Eclipse’s configuration file […]

java October 27, 2009 by mkyong 7 comments

How to convert Java source code to HTML page

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 […]

java October 5, 2009 by mkyong 16 comments

Java – Create file checksum with SHA and MD5

In this article, we will show you how to use a SHA-256 and MD5 algorithm to generate a checksum for a file. MessageDigest.getInstance(“algorithm”) Apache Commons Codec 1. MessageDigest d:\server.log hello world 1.1 Generate a file checksum with a SHA256 algorithm. FileCheckSumSHA.java package com.mkyong.hashing; import java.io.FileInputStream; import java.io.IOException; import java.security.DigestInputStream; import java.security.MessageDigest; import java.security.NoSuchAlgorithmException; public class […]

java September 29, 2009 by mkyong 16 comments

How to resize an image in Java

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 […]

java September 29, 2009 by mkyong 26 comments

How to convert byte[] to BufferedImage in 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 […]

java September 29, 2009 by mkyong 19 comments

How to convert BufferedImage to byte[] in Java

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. […]

java September 25, 2009 by mkyong 2 comments

How to view Java class hierarchy in Eclipse IDE?

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 […]

java September 7, 2009 by mkyong 10 comments

How to Generate an unique ID in Java

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

java July 14, 2009 by mkyong 12 comments

How to get the Tomcat home directory in Java

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");

java July 11, 2009 by mkyong 5 comments

Java – How to convert Char to String

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); […]

java May 4, 2009 by mkyong 11 comments

How to detect OS in Java

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) { […]

java April 28, 2009 by mkyong 16 comments

How to read a UTF-8 file in Java

In Java, the InputStreamReader accepts a charset to decode the byte streams into character streams. We can pass a StandardCharsets.UTF_8 into the InputStreamReader constructor to read data from a UTF-8 file. import java.nio.charset.StandardCharsets; //… try (FileInputStream fis = new FileInputStream(file); InputStreamReader isr = new InputStreamReader(fis, StandardCharsets.UTF_8); BufferedReader reader = new BufferedReader(isr) ) { String str; […]

java April 28, 2009 by mkyong 8 comments

How to write a UTF-8 file in Java

In Java, the OutputStreamWriter accepts a charset to encode the character streams into byte streams. We can pass a StandardCharsets.UTF_8 into the OutputStreamWriter constructor to write data to a UTF-8 file. try (FileOutputStream fos = new FileOutputStream(file); OutputStreamWriter osw = new OutputStreamWriter(fos, StandardCharsets.UTF_8); BufferedWriter writer = new BufferedWriter(osw)) { writer.append(line); } In Java 7+, many […]

java April 23, 2009 by mkyong 7 comments

How to suppress unchecked warnings – Java

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 […]

java April 22, 2009 by mkyong 4 comments

How to insert a new line in resource bundle messages – java

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 […]

java April 11, 2009 by mkyong 11 comments

How to encode a URL string or form parameter in java

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 […]

java April 10, 2009 by mkyong 8 comments

How to escape special characters in java?

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) { […]

java March 18, 2009 by mkyong 0 comments

Convert Java objects to from JSON using Gson

This article shows how to convert Java objects to from JSON using Gson. Table of contents: 1. Download Google Gson 2. Gson Basic Usages 3. Java Object 4. Convert Object to JSON using Gson 5. Converts JSON to Java Object using Gson 6. Modify JSON with JsonElement 7. Converts Json Array to List objects 8. […]

java March 12, 2009 by mkyong 21 comments

How to run a java program in backgroud (unix / Linux)

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 […]

java February 27, 2009 by mkyong 5 comments

How to convert String to byte[] in Java?

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); […]

java February 26, 2009 by mkyong 25 comments

How to convert byte[] array to String in Java

In Java, we can use new String(bytes, StandardCharsets.UTF_8) to convert a byte[] to a String. // string to byte[] byte[] bytes = "hello".getBytes(StandardCharsets.UTF_8); // byte[] to string String s = new String(bytes, StandardCharsets.UTF_8); Table of contents 1. byte[] in text and binary data 2. Convert byte[] to String (text data) 3. Convert byte[] to String […]

java February 25, 2009 by mkyong 26 comments

JCE Encryption – Data Encryption Standard (DES) Tutorial

In this article, we show you how to use Java Cryptography Extension (JCE) to encrypt or decrypt a text via Data Encryption Standard (DES) mechanism. 1. DES Key Create a DES Key. KeyGenerator keygenerator = KeyGenerator.getInstance("DES"); SecretKey myDesKey = keygenerator.generateKey(); 2. Cipher Info Create a Cipher instance from Cipher class, specify the following information and […]