A Java example to print half and full pyramid, for fun. CreatePyramid.java package com.mkyong; import java.util.Collections; public class CreatePyramid { public static void main(String[] args) { int rows = 5; System.out.println("\n1. Half Pyramid\n"); for (int i = 0; i < rows; i++) { for (int j = 0; j <= i; j++) { System.out.print("*"); } […]
This article shows you how to use Apache PDFBox to read a PDF file in Java. 1. Get PDFBox pom.xml <dependency> <groupId>org.apache.pdfbox</groupId> <artifactId>pdfbox</artifactId> <version>2.0.6</version> </dependency> 2. Print PDF file Example to extract all text from a PDF file. ReadPdf.java package com.mkyong; import org.apache.pdfbox.pdmodel.PDDocument; import org.apache.pdfbox.text.PDFTextStripper; import org.apache.pdfbox.text.PDFTextStripperByArea; import java.io.File; import java.io.IOException; public class ReadPdf { […]
This article shows you how to download a file from an URL by using the following methods : Apache Commons IO Java NIO 1. Apache Commons IO 1.1 This is still my prefer way to download a file from the Internet, simple and clean. Read the signature : org.apache.commons.io.FileUtils //int = number of milliseconds public […]
In Java, you can use the Jackson library to convert a Java object into a Map easily. 1. Get Jackson pom.xml <dependency> <groupId>com.fasterxml.jackson.core</groupId> <artifactId>jackson-databind</artifactId> <version>2.6.3</version> </dependency> 2. Convert Object to Map 2.1 A Jackson 2 example to convert a Student object into a java.util.Map Student.java package com.mkyong.examples; import java.util.List; public class Student { private String […]
The static keyword in Java is a modifier used to create memory efficient code. It helps in managing the memory occupied by objects, variables and method definitions. The static keyword makes sure that there is only one instance of the concerned method, object or variable created in memory. It is used when one needs to […]
In Java, there is no ready make API to compare two java.util.Set 1. Solution Here’s my implementation, combine check size + containsAll : SetUtils.java package com.mkyong.core.utils; import java.util.Set; public class SetUtils { public static boolean equals(Set<?> set1, Set<?> set2){ if(set1 == null || set2 ==null){ return false; } if(set1.size()!=set2.size()){ return false; } return set1.containsAll(set2); } […]
Final keyword in Java is a modifier used to restrict the user from doing unwanted code or preventing from the code or value from being changed. It is possible to use this keyword in 3 contexts. They are: Final keyword as a variable modifier Final keyword as a method modifier Final keyword as a class […]
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 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 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 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 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 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 […]
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() { […]
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 […]
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 […]
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 tutorial, we show you how to convert an IP address to its decimal equivalent in Java, and vice versa. For examples : 255.255.255.255 <-> 4294967295 192.168.1.2 <-> 3232235778 1. IP Address to Decimal We show you two ways to convert an IP address to a decimal number Normal power of 256 Bit shifting […]
Before you understand what is & 0xFF, make sure you know following stuffs : Bitwise AND operator, link. Converts hex to/from binary, and decimal to/from binary. In short, & 0xFF is used to make sure you always get the last 8 bits. Let’s see an example to convert an IP address to/from decimal number. 1. […]
In this example, we show you how to find a location (country, city, latitude, longitude) using an IP address. 1. GeoLite Database The MaxMind provides a free GeoLite database (IP Address to Location). Get a free GeoLite free Databases – here Get a GeoIP client Java APIs – here Start code it. 2. GeoLite Java […]
In this article, we show you 2 examples to join two lists in Java. JDK – List.addAll() Apache Common – ListUtils.union() 1. List.addAll() example Just combine two lists with List.addAll(). JoinListsExample.java package com.mkyong.example; import java.util.ArrayList; import java.util.List; public class JoinListsExample { public static void main(String[] args) { List<String> listA = new ArrayList<String>(); listA.add("A"); List<String> listB […]
A short guide to show you how to convert a String with commas to a long type. 1. For a normal String, you can use Long.valueOf to convert it directly. String bigNumber = "1234567899"; long result = Long.valueOf(bigNumber); 2. For a String with commas, you can use java.text.NumberFormat to convert it. String bigNumber = "1,234,567,899"; […]
In this article, we will show you a few examples to make HTTP GET/POST requests via the following APIs Apache HttpClient 4.5.10 OkHttp 4.2.2 Java 11 HttpClient Java 1.1 HttpURLConnection (Not recommend) 1. Apache HttpClient In the old days, this Apache HttpClient is the de facto standard to send an HTTP GET/POST request in Java. […]
This article shows you how to use Apache HttpClient to send an HTTP GET/POST requests, JSON, authentication, timeout, redirection and some frequent used examples. P.S Tested with HttpClient 4.5.10 pom.xml <dependency> <groupId>org.apache.httpcomponents</groupId> <artifactId>httpclient</artifactId> <version>4.5.10</version> </dependency> 1. Send GET Request 1.1 Close manually. HttpClientExample1_1.java package com.mkyong.http; import org.apache.http.HttpEntity; import org.apache.http.HttpHeaders; import org.apache.http.client.methods.CloseableHttpResponse; import org.apache.http.client.methods.HttpGet; import org.apache.http.impl.client.CloseableHttpClient; […]
A funny Java example to create an ASCII art graphic. The concept is simple, get the image’s rgb color in “integer mode”, later, replace the color’s integer with ascii text. P.S This example is credited for this post ASCIIArtService.java package com.mkyong.service; import java.awt.*; import java.awt.image.BufferedImage; import java.io.IOException; public class ASCIIArtService { public static void main(String[] […]
This article shows how to read and parse a comma-separated values (CSV) file. Topics: CSV file and RFC 4180 OpenCSV – Read and Parse CSV file OpenCSV – Convert CSV file to Object Parse CSV file Challenges – Embedded commas, double quotes, and line breaks Single class to read and parse a CSV file String#split […]
In this article, we show you how to use the Locale class to play around the list of countries. P.S Tested with JDK 1.6 1. List of Countries The Locale.getISOCountries() will return a list of all 2-letter country codes defined in ISO 3166. ListCountry.java package com.webmitta.model; import java.util.Locale; public class ListCountry { public static void […]
In Java, you can use HttpServletRequest.getRemoteAddr() to get the client’s IP address that’s accessing your Java web application. import javax.servlet.http.HttpServletRequest; String ipAddress = request.getRemoteAddr(); 1. Proxy Server or Cloudflare For web application which is behind a proxy server, load balancer or the popular Cloudflare solution, you should get the client IP address via the HTTP […]
In this tutorial, we show you 2 examples to calculate date / time difference in Java : Manual time calculation. Joda time library. 1. Manual time calculation Converts Date in milliseconds (ms) and calculate the differences between two dates, with following rules : 1000 milliseconds = 1 second 60 seconds = 1 minute 60 minutes […]
In this example, we will show you how to get Google PageRank (PR) in Java. To request a PageRank for “mkyong.com”, you just need to send following HTTP request : http://toolbarqueries.google.com/tbr?client=navclient-auto&hl=en&ch=6236440745 &ie=UTF-8&oe=UTF-8&features=Rank&q=info:mkyong.com P.S Above URL is used by Google toolbar plugin. The tricky part is following hashing value : ch=6236440745 Google is using Bob Jenkins […]
Here’s an example to show you how to search a file named “post.php” from directory “/Users/mkyong/websites” and all its subdirectories recursively. FileSearch.java package com.mkyong; import java.io.File; import java.util.ArrayList; import java.util.List; public class FileSearch { private String fileNameToSearch; private List<String> result = new ArrayList<String>(); public String getFileNameToSearch() { return fileNameToSearch; } public void setFileNameToSearch(String fileNameToSearch) { […]
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 […]
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 […]
