In this tutorial, we will show you how to install Oracle JDK 8 On CentOS. Environment : CentOS 6.8 Oracle JDK 8u102 Note This guide should work on Fedora and RedHat. 1. Get Oracle JDK 8 1.1 Visit Oracle JDK download page, look for RPM version. 1.2 Copy the download link of jdk-8u102-linux-x64.rpm and wget […]
In Java, there are two types of exceptions – checked and unchecked exception. Here’s the summary : Checked – Extends java.lang.Exception, for recoverable condition, try-catch the exception explicitly, compile error. Unchecked – Extends java.lang.RuntimeException, for unrecoverable condition, like programming errors, no need try-catch, runtime error. 1. Custom Checked Exception Note Some popular checked exception : […]
Some Java reflection API examples. 1. Display all fields and data type A Java reflection example to loop over all the fields declared by a class. 1.1 A POJO. CompanyA.java package com.mkyong.test; import java.util.List; import java.util.Map; import java.util.Set; public class CompanyA { String orgName; int count; List<String> comments; Set<String> branches; Map<String, String> extra; //… } […]
Some Java examples to show you how to override equals and hashCode. 1. POJO To compare two Java objects, we need to override both equals and hashCode (Good practice). User.java public class User { private String name; private int age; private String passport; //getters and setters, constructor } User user1 = new User("mkyong", 35, "111222333"); […]
In Java, you can use == operator to compare Enum value. 1. Java Enum example Language.java package com.mkyong.java public enum Language { JAVA, PYTHON, NODE, NET, RUBY } 2. Compare with == Example to compare enum value with == operator. Test.java package com.mkyong.java public class Test { public static void main(String[] args) { // Covert […]
Java object Serialization is an API provided by Java Library stack as a means to serialize Java objects. Serialization is a process to convert objects into a writable byte stream. Once converted into a byte-stream, these objects can be written to a file. The reverse process of this is called de-serialization. A Java object is […]
In this article, we will show you a few ways to join a Java Array. Apache Commons Lang – ArrayUtils Java API Java 8 Stream 1. Apache Commons Lang – ArrayUtils The simplest way is add the Apache Commons Lang library, and use ArrayUtils. addAll to join arrays. This method supports both primitive and object […]
This article shows you the difference between Mutable and Immutable objects in Java 1. Mutable object – You can change the states and fields after the object is created. For examples: StringBuilder, java.util.Date and etc. 2. Immutable object – You cannot change anything after the object is created. For examples: String, boxed primitive objects like […]
In this article, we will show you a few ways to print a Java Array. Table of contents. 1. JDK 1.5 Arrays.toString 2. Java 8 Stream APIs 3. Java 8 Stream APIs – Collectors.joining 4. Jackson APIs 5. References 1. JDK 1.5 Arrays.toString We can use Arrays.toString to print a simple array and Arrays.deepToString for […]
RMI stands for Remote Method Invocation and it is the object-oriented equivalent of RPC (Remote Procedure Calls). RMI was designed to make the interaction between applications using the object-oriented model and run on different machines seem like that of stand-alone programs. The code below will give you the basis to Java RMI with a very […]
In this tutorial, we will show you how to install Oracle JDK 8 On Debian, manually. Environment : Debian 7 OpenJDK 1.7 is installed. (Switch to Oracle JDK 8 later) At the time of writing, OpenJDK 1.8 is not included in the default apt-get repository yet. I just don’t like the default apt repository schedule, […]
In Java, the System.getenv() returns an unmodifiable string Map view of the current system environment. Map<String, String> env = System.getenv(); // get PATH environment variable String path = System.getenv("PATH"); Table of contents. 1. Get a specified environment variable 2. UnsupportedOperationException 3. Display all environment variables 4. Sort the environment variables 5. Download Source Code 6. […]
In this tutorial, we will show you how to use Jackson 2.x to convert Java objects to / from a JSON. 1. Basic 1.1 Convert a Staff object to from JSON. writeValue(…) – Java Objects to JSON ObjectMapper mapper = new ObjectMapper(); // Java object to JSON file mapper.writeValue(new File("c:\\test\\staff.json"), new Staff()); // Java object […]
In Java, we can use String#split() to split a string. String phone = "012-3456789"; // String#split(string regex) accepts regex as the argument String[] output = phone.split("-"); String part1 = output[0]; // 012 String part2 = output[1]; // 3456789 Table of contents 1. Split a string (default) 2. Split a string and retain the delimiter 3. […]
In Java, we can use TimeUnit.SECONDS.sleep() or Thread.sleep() to delay few seconds. 1. TimeUnit JavaDelayExample.java package com.mkyong; import java.util.Date; import java.util.concurrent.TimeUnit; public class JavaDelayExample { public static void main(String[] args) { try { System.out.println("Start…" + new Date()); // delay 5 seconds TimeUnit.SECONDS.sleep(5); System.out.println("End…" + new Date()); // delay 0.5 second //TimeUnit.MICROSECONDS.sleep(500); // delay 1 minute […]
In Java, you can use Map.containsKey() to check if a key exists in a Map. TestMap.java package com.mkyong.examples; import java.util.HashMap; import java.util.Map; public class TestMap { public static void main(String[] args) { Map<String, Integer> fruits = new HashMap<>(); fruits.put("apple", 1); fruits.put("orange", 2); fruits.put("banana", 3); fruits.put("watermelon", null); System.out.println("1. Is key ‘apple’ exists?"); if (fruits.containsKey("apple")) { //key […]
Learn how to set the JAVA_HOME environment variable on Windows 10 or 11 with easy-to-follow steps and screenshots.
In this article, we will show you three ways to generate random integers in a range. java.util.Random.nextInt Math.random java.util.Random.ints (Java 8) 1. java.util.Random This Random().nextInt(int bound) generates a random integer from 0 (inclusive) to bound (exclusive). 1.1 Code snippet. For getRandomNumberInRange(5, 10), this will generates a random integer between 5 (inclusive) and 10 (inclusive). private […]
In Java, we can use Integer.parseInt(String) to convert a String to an int; For unparsable String, it throws NumberFormatException. Integer.parseInt("1"); // ok Integer.parseInt("+1"); // ok, result = 1 Integer.parseInt("-1"); // ok, result = -1 Integer.parseInt("100"); // ok Integer.parseInt(" 1"); // NumberFormatException (contains space) Integer.parseInt("1 "); // NumberFormatException (contains space) Integer.parseInt("2147483648"); // NumberFormatException (Integer max 2,147,483,647) […]
In Java, there are few ways to display double in 2 decimal places. Table of contents. 1. DecimalFormat("0.00") 2. String.format("%.2f") 3. BigDecimal 4. References Possible Duplicate How to round double / float value to 2 decimal places 1. DecimalFormat(“0.00”) We can use DecimalFormat("0.00") to ensure the number is round to 2 decimal places. DecimalExample.java package […]
A simple Math.pow example, display 2 to the power of 8. In Math 2^8 = 2x2x2x2x2x2x2x2 =256 In Java Math.pow(2, 8) Note In Java, the symbol ^ is a XOR operator, DON’T use this for the power calculation. TestPower.java package com.mkyong.test; import java.text.DecimalFormat; public class TestPower { static DecimalFormat df = new DecimalFormat(".00"); public static […]
A code snippet to show you how to get the number of available processors / cores / CPUs in your environment. int processors = Runtime.getRuntime().availableProcessors(); System.out.println(processors); Output 8 P.S Tested with Intel(R) Core(TM) i7-4770 CPU @3.40GHz
In this article, we will show you how to use Jsoup to check if an URL is going to redirect. 1. URL Redirection Normally, a redirect URL will return an HTTP code of 301 or 307, and the target URL will be existed in the response header “location” field. Review a sample of HTTP Response […]
In this tutorial, we will show you few examples (ZonedDateTime (Java 8), Date, Calendar and Joda Time) to convert a date and time between different time zones. All examples will be converting the date and time from (UTC+8:00) Asia/Singapore – Singapore Time Date : 22-1-2015 10:15:55 AM to (UTC-5:00) America/New_York – Eastern Standard Time Date […]
This Java example shows you how to display a list of TimeZone with GMT in front. P.S Tested with JDK 1.7 Java 8 You may interest at this example – Display all ZoneId and its UTC offset TimeZoneExample.java package com.mkyong.test; import java.util.TimeZone; import java.util.concurrent.TimeUnit; public class TimeZoneExample { public static void main(String[] args) { String[] […]
In Java, you can use calendar.setTime(date) to convert a Date object to a Calendar object. Calendar calendar = Calendar.getInstance(); Date newDate = calendar.setTime(date); A Full example DateAndCalendar.java import java.text.ParseException; import java.text.SimpleDateFormat; import java.util.Calendar; import java.util.Date; public class DateAndCalendar { public static void main(String[] argv) throws ParseException { //1. Create a Date from String SimpleDateFormat sdf […]
A code snippet to use Joda Time to convert a java.util.Date to different timezone : //java.util.Date : 22-1-2015 10:15:55 AM //System TimeZone : Asia/Singapore //Convert java.util.Date to America/New_York TimeZone DateTime dt = new DateTime(date); DateTimeZone dtZone = DateTimeZone.forID("America/New_York"); DateTime dtus = dt.withZone(dtZone); //21-1-2015 09:15:55 PM – Correct! //Convert Joda DateTime back to java.util.Date, and print […]
In Java, you can use Enum valueOf() to convert a String to an Enum object, review the following case study : 1. Java Enum example WhoisRIR.java package com.mkyong.whois.utils; public enum WhoisRIR { ARIN("whois.arin.net"), RIPE("whois.ripe.net"), APNIC("whois.apnic.net"), AFRINIC("whois.afrinic.net"), LACNIC("whois.lacnic.net"), JPNIC("whois.nic.ad.jp"), KRNIC("whois.nic.or.kr"), CNNIC("ipwhois.cnnic.cn"), UNKNOWN(""); private String url; WhoisRIR(String url) { this.url = url; } public String url() { […]
Quartz is good, but often times we just need a simple scheduler system to run a jar file periodically. On *unix system, you can use the build-in cron to schedule a scheduler job easily. In this example, we will show you how to create a cron job on *nix to run a jar file, by […]
This article covers 3 ways to read a text file line by line : Java NIO libraries – FileChannel to read the files. It is a non-blocking mode of reading files BufferedReader – Which is a blocking operation i.e it blocks any other read/write request to the file until it has completed the task. Apache […]
Normally, we are using the following ways to generate a random number in Java. 1. ThreadLocalRandom (JDK 1.7) //Generate number between 0-9 int index = ThreadLocalRandom.current().nextInt(10); 2. Random() //Generate number between 0-9 Random random = new Random(); int index = random.nextInt(10); 3. Math.random() //Generate number between 0-9 int index = (int)(Math.random()*10); Note 1. For single […]
This example shows you how to use jsoup to send a search query to Google. Document doc = Jsoup .connect("https://www.google.com/search?q=mario"); .userAgent("Mozilla/5.0") .timeout(5000).get(); Unusual traffic from your computer network Don’t use this example to spam Google, you will get above message from Google, read this Google answer. 1. jsoup example Example to send a “mario” search […]
In Java, we can use getResourceAsStream or getResource to read a file or multiple files from a resources folder or root of the classpath. The getResourceAsStream method returns an InputStream. // the stream holding the file content InputStream is = getClass().getClassLoader().getResourceAsStream("file.txt"); // for static access, uses the class name directly InputStream is = JavaClassName.class.getClassLoader().getResourceAsStream("file.txt"); The […]
Deployed a “war” file on Tomcat, and hits following error messages : Jul 17, 2014 7:59:55 PM org.apache.catalina.loader.WebappClassLoader validateJarFile INFO: validateJarFile(D:\apache-tomcat-7.0.53\webapps\hc\WEB-INF\lib\servlet-api-2.5.jar) – jar not loaded. See Servlet Spec 3.0, section 10.7.2. Offending class: javax/servlet/Servlet.class Tools used : JDK1.7 Maven 3 Tomcat 7 1. Reason The Tomcat’s container comes with own version of servlet-api.jar, and the […]
In this tutorial, we will show you how to get the nameservers of a website using Java + dig command. 1. Using Dig Command 1.1 On Linux, we can use dig command to query a DNS lookup of a website, for example : $ dig any mkyong.com //… mkyong.com. 299 IN A 162.159.x.x mkyong.com. 299 […]
In this article, we will show you how to use the -XX:+PrintFlagsFinal to find out your heap size detail. In Java, the default and maximum heap size are allocated based on this – ergonomics algorithm. Heap sizes Initial heap size of 1/64 of physical memory up to 1Gbyte Maximum heap size of 1/4 of physical […]
In this example, we will show you how to append values in Object[] and int[] array. Object[] obj = new Object[] { "a", "b", "c" }; ArrayList<Object> newObj = new ArrayList<Object>(Arrays.asList(obj)); newObj.add("new value"); newObj.add("new value 2"); 1. Object[] Array Example Example to append values with ArrayList : TestApp.java package com.mkyong.test; import java.util.ArrayList; import java.util.Arrays; public […]
Examples show you how to convert between int[] and its’ wrapper class Integer[]. 1. Convert int[] to Integer[] public static Integer[] toObject(int[] intArray) { Integer[] result = new Integer[intArray.length]; for (int i = 0; i < intArray.length; i++) { result[i] = Integer.valueOf(intArray[i]); } return result; } 2. Convert Integer[] to int[] public static int[] toPrimitive(Integer[] […]
Eclipse IDE prompts warning on new DefaultHttpClient, mark this class as deprecated. package com.mkyong.web.controller; import org.apache.http.Header; import org.apache.http.HttpResponse; import org.apache.http.client.HttpClient; import org.apache.http.client.methods.HttpGet; import org.apache.http.impl.client.DefaultHttpClient; public class WebCrawler { public static void main(String[] args) throws Exception { HttpClient client = new DefaultHttpClient(); HttpGet request = new HttpGet("https://mkyong.com"); HttpResponse response = client.execute(request); //… } } Solution Dive […]
