Category: java

java November 7, 2012 by mkyong 30 comments

How to count duplicated items in Java List

A Java example to show you how to count the total number of duplicated entries in a List, using Collections.frequency and Map. CountDuplicatedList.java package com.mkyong; import java.util.ArrayList; import java.util.Collections; import java.util.HashMap; import java.util.HashSet; import java.util.List; import java.util.Map; import java.util.Set; import java.util.TreeMap; public class CountDuplicatedList { public static void main(String[] args) { List<String> list = new […]

java November 5, 2012 by mkyong 14 comments

Java : getResourceAsStream in static method

To call getResourceAsStream in a static method, we use ClassName.class instead of getClass() 1. In non static method getClass().getClassLoader().getResourceAsStream("config.properties")) 2. In static method ClassName.class.class.getClassLoader().getResourceAsStream("config.properties")) 1. Non Static Method A .properties file in project classpath. src/main/resources/config.properties #config file json.filepath = /Users/mkyong/Documents/workspace/SnakeCrawler/data/ FileHelper.java package com.mkyong; import java.io.IOException; import java.io.InputStream; import java.util.Properties; public class FileHelper { public static […]

java October 21, 2012 by mkyong 19 comments

Java enum example

Some of the Java enum examples, and how to use it, nothing special, just for self-reference. Note Consider the Enum type if your program consists of a fixed set of constants, like seasons of the year, operations calculator, user status and etc. 1. Basic Enum UserStatus.java public enum UserStatus { PENDING, ACTIVE, INACTIVE, DELETED; } […]

java August 2, 2012 by mkyong 6 comments

How to list all Jobs in the Quartz Scheduler

Below are two code snippets to show you how to list all Quartz jobs. Quartz 2 APIs are changed a lot, so syntax is different from Quartz 1.x. 1. Quartz 2.1.5 example Scheduler scheduler = new StdSchedulerFactory().getScheduler(); for (String groupName : scheduler.getJobGroupNames()) { for (JobKey jobKey : scheduler.getJobKeys(GroupMatcher.jobGroupEquals(groupName))) { String jobName = jobKey.getName(); String jobGroup […]

java July 30, 2012 by mkyong 27 comments

Example to run multiple jobs in Quartz

In this example, we show you how to declare multiple Quartz jobs via Quartz APIs, Quartz XML and Spring. In Quartz scheduler framework, each job will be attached to an unique trigger, and run it by scheduler. P.S In Quartz, one trigger for multiple jobs is not possible. (Correct me if this is wrong.) 1. […]

java July 26, 2012 by Rajiv Srivastava 2 comments

How to work with Java 6’s NavigableSet and NavigableMap

You can use latest Java 6’s Collection API to navigate a set and Map collections. These API gives a lot of flexibility to find out required result from the collection. 1. NavigableMap Example package com.example.collection; import java.util.NavigableMap; import java.util.TreeMap; public class NavigableMapDemo { public static void main(String[] args) { NavigableMap<String,Integer> navigableMap=new TreeMap<String, Integer>(); navigableMap.put("X", 500); […]

java July 24, 2012 by Rene Enriquez 11 comments

How to use reflection to copy properties from Pojo to other Java Beans

Sometimes we need copy properties from a Java class to other, we can do it this manually or with our own reflection implementation, but in this case use us reflection for automate it with a utility from apache Requirements commons-beanutils , you can download from here http://commons.apache.org/beanutils/ commons-loging , you can download from here http://commons.apache.org/logging/ […]

java July 23, 2012 by Dhina Prakash 17 comments

How to run a task periodically in Java

Some java application need to execute a method between a regular interval of time. For example GUI application should update some information from database. 1. Scheduler Task For this functionality, You should create a class extending TimerTask(available in java.util package). TimerTask is a abstract class. Write your code in public void run() method that you […]

java July 18, 2012 by mkyong 8 comments

Quartz 2 JobListener example

In this tutorial, we will show you how to create a JobListener, to keep track the running jobs status, like when the job is finished. P.S This example is tested with Quartz 2.1.5 1. Quartz Job Job, print a simple message, and throw a JobExecutionException for testing. File : HelloJob.java package com.mkyong.quartz; import org.quartz.Job; import […]

java July 17, 2012 by mkyong 22 comments

Quartz 2 scheduler tutorial

Quartz, enterprise scheduler job framework, to help Java application to scheduler a job/task to run at a specified date and time. This tutorial show you how to develop a scheduler job using latest Quartz library 2.1.5. Note Quartz 2 involves significant API changed, read this for older Quartz 1.6.3 example. 1. Download Quartz You can […]

java July 9, 2012 by mkyong 4 comments

Java and zip code example

Recently, answered an question about leading zero problem ZipCode, what is the most proper data type for country’s zip code? 1. ZipCode – int In Java, some claimed it should declare as “int”, and use DecimalFormat to format and display the leading zero. For example, package com.mkyong; import java.text.DecimalFormat; public class ZipCodeExample { public static […]

java July 7, 2012 by mkyong 0 comments

Quartz : org.quartz.SchedulerConfigException: Thread count must be > 0

Working with Quartz 2, when running the project, hits following error message? org.quartz.SchedulerConfigException: Thread count must be > 0 at org.quartz.simpl.SimpleThreadPool.initialize(SimpleThreadPool.java:245) at org.quartz.impl.StdSchedulerFactory.instantiate(StdSchedulerFactory.java:1255) at org.quartz.impl.StdSchedulerFactory.getScheduler(StdSchedulerFactory.java:1484) at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4791) at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5285) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549) at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303) at java.util.concurrent.FutureTask.run(FutureTask.java:138) at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908) at java.lang.Thread.run(Thread.java:680) Solution You have defined a “quartz.properties” file, and override the […]

java July 4, 2012 by mkyong 37 comments

How to get URL content in Java

In this Java example, we show you how to get content of a page from URL “mkyong.com” and save it into local file drive, named “test.html”. package com.mkyong; import java.io.BufferedReader; import java.io.BufferedWriter; import java.io.File; import java.io.FileWriter; import java.io.IOException; import java.io.InputStreamReader; import java.net.MalformedURLException; import java.net.URL; import java.net.URLConnection; public class GetURLContent { public static void main(String[] args) […]

java April 27, 2012 by mkyong 7 comments

Convert PNG to JPEG image in Java

This article shows how to convert an PNG image format to an JPEG image format in Java. 1. PNG Image Below is a PNG image with a transparent background. 2. Java Convert PNG to JPEG Since the JPEG image format doesn’t support the transparent background, we will create a white background as a replacement. Below […]

java February 20, 2012 by mkyong 6 comments

How to check if Date is within a certain range in Java?

A Java example to check if a provided date is within a range of 3 momths before and after current date. The idea is quite simple, just use Calendar class to roll the month back and forward to create a “date range”, and use the Date.before() and Date.after() to check if the Date is within […]

java February 20, 2012 by mkyong 10 comments

How to check if date is valid in Java

This article shows how to use the Java 8 DateTimeFormatter to check if a date format is valid in Java. Below are the requirements for a valid date. Year format, 1900, 2099 Month format, 1, 01, 2, 02… 12 Day format, 1, 01… 31 Leap year, February 29 days. Common year, February 28 days. Month […]

java February 3, 2012 by mkyong 10 comments

Java try-with-resources example

This article shows you how to use try-with-resources in Java. Table of contents: 1 Java try-with-resources before and after 2. try-with-resources with single resource 3. try-with-resources with multiple resources 3.1 JDBC example. 4. Custom resource with AutoCloseable interface 5. Resource open and closing order 6. Java 9 – final or effectively final variables Download Source […]

java January 20, 2012 by mkyong 7 comments

How to read file in Java – FileInputStream

In Java, we use FileInputStream to read bytes from a file, such as an image file or binary file. Topics FileInputStream – Read a file FileInputStream – Remaining bytes FileInputStream – Better performance FileInputStream vs BufferedInputStream InputStreamReader – Convert FileInputStream to Reader FileInputStream – Read a Unicode file Note However, all the below examples use […]

java August 11, 2011 by mkyong 16 comments

How to loop ArrayList in Java

No nonsense, four ways to loop ArrayList in Java For loop For loop (Advance) While loop Iterator loop package com.mkyong.core; import java.util.ArrayList; import java.util.Iterator; import java.util.List; public class ArrayListLoopingExample { public static void main(String[] args) { List<String> list = new ArrayList<String>(); list.add("Text 1"); list.add("Text 2"); list.add("Text 3"); System.out.println("#1 normal for loop"); for (int i = […]

java August 11, 2011 by mkyong 0 comments

Convert JSON string to Map using Jackson

This article uses the Jackson framework to parse JSON strings to a Map in Java. Table of contents: 1. Download Jackson 2. Convert JSON string to Map 3. Convert Map to JSON string 4. Download Source Code 5. References P.S Tested with Jackson 2.17.0 1. Download Jackson Simply declare jackson-databind in the pom.xml, and it […]

java August 11, 2011 by mkyong 1 comment

Read and write JSON using JSON.simple

This article shows how to read and write JSON using JSON.simple. Table of contents: 1. Setup JSON.simple 2. Write JSON to File using JSON.simple 3. Read JSON from File using JSON.simple 4. Java object to JSON using JSON.simple 5. JSON to Java Object using JSON.simple 6. Download Source Code 7. References P.S Tested with json-simple […]

java August 11, 2011 by mkyong 0 comments

Gson Streaming APIs to read and write JSON

Streaming APIs are efficient ways of processing large JSON files or data without loading the entire document into memory. Gson provides JsonReader and JsonWriter classes for JSON streaming. Gson’s Streaming API JsonWriter – Write JSON as a stream. JsonReader – Read JSON as a stream. Table of contents: 1. Setup Google Gson 1. Write JSON […]

java August 10, 2011 by mkyong 0 comments

Jackson Streaming API examples

This article shows how to use Jackson’s Streaming API for both reading from and writing to JSON. Jackson’s Streaming API JsonGenerator – Write JSON data JsonParser – Read and Parse JSON data Table of contents: 1. Download Jackson 2. Write JSON using JsonGenerator 2. Write JSON Array using JsonGenerator 3. Read JSON using JsonParser 4. […]

java August 10, 2011 by mkyong 18 comments

Jackson Tree Model examples

Jackson’s Tree Model provides a flexible way to work with JSON data compared to the data-binding method. The Tree Model works well when the JSON data or JSON’s schema is unknown until runtime or the JSON structure does not map properly to Java classes. This article shows how to use Jackson’s Tree Model JsonNode and […]

java August 10, 2011 by mkyong 1 comment

How to enable pretty print JSON with Jackson

In Jackson, we can use writerWithDefaultPrettyPrinter() to pretty print the JSON strings. Table of contents: 1. Setup Jackson 2. Compact print JSON (Default) 3. Pretty print JSON with Jackson 4. Pretty print JSON with Jackson (Globally) 5. Download Source Code 6. References P.S Tested with Jackson 2.17.0 1. Setup Jackson Puts jackson-databind at pom.xml. pom.xml […]

java August 9, 2011 by mkyong 55 comments

How to convert Java object to / from JSON (Jackson)

In this tutorial, we show you how to use Jackson 1.x data binding to convert Java object to / from JSON. Note Jackson 1.x is a maintenance project, please use Jackson 2.x instead. Note This tutorial is obsolete, no more update, please refer to the latest Jackson 2 tutorial – Object to / from JSON. […]

java August 9, 2011 by mkyong 1 comment

How to pretty print JSON using Gson

This article shows how to pretty print JSON using Gson. Table of contents: 1. Setup Google Gson 2. Default Compact Print JSON 3. Pretty Print JSON using Gson 4. Download Source Code 5. References P.S Tested with Gson 2.10.1 1. Setup Google Gson Declare gson in the pom.xml. pom.xml <dependency> <groupId>com.google.code.gson</groupId> <artifactId>gson</artifactId> <version>2.10.1</version> </dependency> 2. […]

java August 4, 2011 by mkyong 11 comments

JAXB hello world example

Jakarta XML Binding (JAXB; formerly Java Architecture for XML Binding) is an XML binding framework to convert Java objects to and from XML. XML Marshalling – Convert Java objects into XML. XML Unmarshalling – Convert XML back into Java Objects. Note This article will focus on Java 11, JAXB 3 and EclipseLink MOXy JAXB RI. […]

java August 4, 2011 by mkyong 10 comments

How to modify XML file in Java – (DOM Parser)

This tutorial shows how to use the Java built-in DOM Parser to modify an XML file. Table of contents 1. The XML file, before and after 2. Dom Parser modify XML file 3. Download Source Code 4. References P.S Tested with Java 11. Note Please read DOM parse XML and DOM write XML. 1. The […]

java August 4, 2011 by mkyong 1 comment

How to write XML file in Java – (JDOM)

This tutorial shows how to use the JDOM to write data to an XML file. Table of contents 1. Download JDOM 2.x 2. Write XML String to Console 3. Write XML to a file 4. Write XML attribute, comment, CDATA and etc 5. Change XML encoding 6. Download Source Code 7. References P.S Tested with […]

java August 4, 2011 by mkyong 2 comments

How to count XML elements in Java

This article shows how to use DOM parser, SAX parser, and XPath to count the number of a specified XML element in Java. Table of contents 1. An XML file 2. Count the XML elements (DOM Parser) 3. Count the XML elements (XPath) 4. Count the XML elements (SAX Parser) 5. Download Source Code 6. […]

java May 1, 2011 by mkyong 2 comments

Java – How to convert Char[] to String

In Java, we can use String.valueOf() to convert a char array to a String. JavaSample1.java package com.mkyong.markdown; public class JavaSample1 { public static void main(String[] args) { char[] charArrays = new char[]{‘1’, ‘2’, ‘3’, ‘A’, ‘B’, ‘C’}; String str = new String(charArrays); System.out.println(str); // 123ABC String str2 = String.valueOf(charArrays); System.out.println(str2); // 123ABC } } Output […]

java May 1, 2011 by mkyong 8 comments

Java – How to convert String to Char Array

In Java, you can use String.toCharArray() to convert a String into a char array. StringToCharArray.java package com.mkyong.utils; public class StringToCharArray { public static void main(String[] args) { String password = "password123"; char[] passwordInCharArray = password.toCharArray(); for (char temp : passwordInCharArray) { System.out.println(temp); } } } Output p a s s w o r d 1 […]

java February 26, 2011 by mkyong 5 comments

java.lang.ClassNotFoundException: com.sun.mail.util.MessageRemovedIOException

Problem Use JavaMail API to send Email via GMail smtp server, but hit the following error message : Caused by: java.lang.ClassNotFoundException: com.sun.mail.util.MessageRemovedIOException at java.net.URLClassLoader$1.run(URLClassLoader.java:200) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:188) at java.lang.ClassLoader.loadClass(ClassLoader.java:307) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:252) at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320) … 1 more P.S The javaee.jar library is included. Solution To solve it, you need to include the […]

java January 17, 2011 by mkyong 4 comments

java.lang.ClassNotFoundException: org.objectweb.asm.Type

Problem In Hibernate development, it hits… java.lang.ClassNotFoundException: org.objectweb.asm.Type Solution This is caused by the missing of asm.jar, you can get it from asm official website, or get it via Maven <dependency> <groupId>asm</groupId> <artifactId>asm</artifactId> <version>3.3.1</version> </dependency>

java December 30, 2010 by mkyong 49 comments

How to get MAC address in Java

Since JDK 1.6, Java developers are able to access network card detail via NetworkInterface class. In this example, we show you how to get the localhost MAC address in Java. App.java – Get MAC Address via NetworkInterface.getByInetAddress() package com.mkyong; import java.net.InetAddress; import java.net.NetworkInterface; import java.net.SocketException; import java.net.UnknownHostException; public class App{ public static void main(String[] args){ […]

java December 30, 2010 by mkyong 20 comments

How to get Server IP address in Java

In Java, you can use InetAddress.getLocalHost() to get the Ip Address of the current Server running the Java app. package com.mkyong; import java.net.InetAddress; import java.net.UnknownHostException; public class Test { public static void main(String[] args) { InetAddress ip; try { ip = InetAddress.getLocalHost(); System.out.println("Current IP address : " + ip.getHostAddress()); } catch (UnknownHostException e) { e.e.printStackTrace(); […]

java December 30, 2010 by mkyong 5 comments

How to convert negative number to positive in Java

To convert negative number to positive number (this is called absolute value), uses Math.abs(). This Math.abs() method is work like this “number = (number < 0 ? -number : number);". See a complete example : package com.mkyong; public class app{ public static void main(String[] args) { int total = 1 + 1 + 1 + […]

java November 28, 2010 by mkyong 43 comments

Java HttpsURLConnection example

Here’s a simple Java HTTPS client to demonstrate the use of HttpsURLConnection class to send a HTTP GET request yo get the https URL content and certificate detail. P.S You may interest at this example – automate login a website with HttpsURLConnection. HttpsClient.java package com.mkyong.client; import java.net.MalformedURLException; import java.net.URL; import java.security.cert.Certificate; import java.io.*; import javax.net.ssl.HttpsURLConnection; […]

java November 1, 2010 by mkyong 13 comments

How to validate URL in Java

A short example to show the use of apache.commons.validator.UrlValidator class to validate an URL in Java. import org.apache.commons.validator.UrlValidator; public class ValidateUrlExample{ public static void main(String[] args) { UrlValidator urlValidator = new UrlValidator(); //valid URL if (urlValidator.isValid("https://mkyong.com")) { System.out.println("url is valid"); } else { System.out.println("url is invalid"); } //invalid URL if (urlValidator.isValid("http://invalidURL^$&%$&^")) { System.out.println("url is valid"); […]