Java – How to Fix java.lang.UnsupportedClassVersionError Error?
This article represents tips on how to fix Java error such as UnsupportedClassVersionError. Please feel free to comment/suggest if I missed to mention one or more important points. Also, sorry for the typos. How to Reproduce the Error? Set the JAVA_HOME with one version of Java. And, include “bin” of different Java version in PATH variable. Compile the class, say, HelloWorld.java with “javac” command Start the JVM with following class: “java HelloWorld” With above, you could see error such as following: java.lang.UnsupportedClassVersionError: HelloWorld : Unsupported major.minor ver sion 52.0 at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(Unknown Source) at java.security.SecureClassLoader.defineClass(Unknown Source) at java.net.URLClassLoader.defineClass(Unknown Source) at java.net.URLClassLoader.access$100(Unknown Source) at java.net.URLClassLoader$1.run(Unknown Source) at java.net.URLClassLoader$1.run(Unknown Source) …
Java – How to Calculate Size of Objects & Arrays
This article represents a list of web pages which can help one understand the memory usage of Java objects and arrays along with examples. Please feel free to comment/suggest any other cool pages. Also, sorry for the typos. The in-memory size of the object depends on the architecture, mainly on whether the VM is 32 or 64-bit. The actual VM implementation also matters. How to calculate memory usage of a Java object?: Very simplified explanation of how one could calculate a memory of any Java object. For example, lets say, you want to calculate the memory of a Java object which holds two int variables, one boolean variable, one Long object, …
How to Dockerize Springboot Web App
This blog represents instructions and code samples on how to Dockerize a Springboot Web app. The source code be found from this git repository. Following are key aspects of this blog: Dockerfile & image Install the image Run the container and access the app Dockerfile & Image Download the code from this page and put them all in a folder. Before we go one to create the image, lets know a little more about the Dockerfile. Following points need to be noted in the Dockerfile: The image created is a Centos6 image The Java is installed in a predefined path “/opt/jdk” Then, Maven is installed and JAVA_HOME is set. The source …
Java Code Sample to Access Firebase Data
This article represents Java code sample which can be used to access Firebase database. Please feel free to comment/suggest if I missed to mention one or more important points. Also, sorry for the typos. Pre-requisite for Firebase Database Access It is recommended to not allow anyone to access Firebase database without any kind of authentication. The minimum that could be done is enable “Anonymous Authentication” by logging in Firebase console. Java Code Sample for Firebase Database Access In the code below, note some of the following: Code for anonymous authentication Code for accessing a user object based on a given userId Firebase firebase = new Firebase(“https://dbname.firebaseio.com/”); firebase.authAnonymously(new Firebase.AuthResultHandler() { @Override …
Liskov Substitution Principle with Java Code Examples
This article describes what is Liskov Substitution Principle along with code examples in Java. Please feel free to comment/suggest if I missed mentioning one or more important points. Also, sorry for the typos. In this post, you will learn some of the following: What is Liskov Substitution Principle (LSP)? Code Samples to Illustrate LSP What is code quality characteristic that represents by LSP? You may also want to check another code quality principle such as Single Responsibility Principle explained with Java Examples. What is Liskov Substitution Principle (LSP)? Take a look at this paper on Liskov Substitution Principle which provides great details on Liskov Substitution Principle. As per LSP, functions …
Dockers Containers only to shine more with Kubernetes
A year back, I got started with Dockers Containers. 2-3 months down the road and I realized that thankfully I learnt a revolutionary technology because of some of the following reasons: Quick Learning: Docker containers were used to spin up my exploratory learning environment, on-demand demand, for any tool/ framework/Programming language I wanted to learn On-demand Self-service Environments: Docker containers were used to setup on-demand self-service dev/test environments. This was a huge productivity booster for developers and test engineers. Automated Deployments: With Docker containers, Jenkins, Repository etc, I saw the automated deployments getting created in dev/testing/UAT servers. Following are some of the areas where I felt the need for some …
Springboot Web Hello World with a Java & Pom.xml file
This blog presents instructions on how to quickly get started with Springboot Hello World with just one Java file and a Pom.xml. Before getting set up with files, make sure you have installed and configured following Java; Setup Java_Home as it is required by maven Maven; Maven would be used to package our app. Java Code Sample Following is the code for Java file. Make sure to create hello/HelloController.java file within src/main/java. This is required for maven to package it correctly. package hello; import org.springframework.web.bind.annotation.RestController; import org.springframework.web.bind.annotation.RequestMapping; import org.springframework.boot.*; import org.springframework.boot.autoconfigure.*; import org.springframework.web.bind.annotation.*; @RestController @SpringBootApplication public class HelloController { @RequestMapping(“/”) String home() { return “Hello World!”; } public static …
How to Upload CSV Files using Apache FileUpload/CSV
This article represents code samples on how to use Apache Commons FileUpload and Apache Commons CSV libraries to upload CSV file using Java Servlet. Please feel free to comment/suggest if I missed to mention one or more important points. Also, sorry for the typos. Code Samples – Java Code Following is the code for Java Servlet. package com.vitalflux.core; import java.io.IOException; import java.io.StringReader; import java.util.List; import javax.servlet.RequestDispatcher; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import org.apache.commons.csv.CSVFormat; import org.apache.commons.csv.CSVRecord; import org.apache.commons.fileupload.FileItem; import org.apache.commons.fileupload.disk.DiskFileItemFactory; import org.apache.commons.fileupload.servlet.ServletFileUpload; public class FileIOController extends HttpServlet { private static final long serialVersionUID = 1L; protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { if (ServletFileUpload.isMultipartContent(request)) …
5 Reasons to Choose Ionic Framework over Xamarin
Are you one of them wondering about which framework to use out of Xamarin or Ionic Framework for developing your next mobile app? Well, rightfully so. You could get some opinions on this stackoverflow page. For a person like me who does not have much knowledge on C# although I could get started with it fast, I would choose Ionic framework as all it requires is a good knowledge of HTML/CSS & Javascript (AngularJS). Checking out quick google trends, here is what it looked like: From above picture, one could quickly figure out that Xamarin (blue) is the clear winner. However, here are the 5 reasons why you would want …
Ethereum Installation Issue – Missing MSVCR120.dll
This blog represents tips to fix the “missing MSVCR120.dll from your computer” when one tries to start the just installed Ethereum MIX IDE from this page. All that one requires to do is download and install Visual C++ redistributable packages for Visual Studio 2013 from this link: https://www.microsoft.com/en-us/download/details.aspx?id=40784 and you should be all set. After installation, try starting MIX by clicking on AlethZero and you should be all set.
Docker – Install & Access Docker Compose from Putty Client
If you are one of them who access the docker machine using putty client, and you want to work with docker-compose, here is what you need to do: Open a putty client accessing the docker machine. Log in as root using command “sudo -i” Execute the following commands to download and install the docker-compose curl -L https://github.com/docker/compose/releases/download/1.6.2/docker-compose-`uname -s`-`uname -m` > /usr/local/bin/docker-compose chmod +x /usr/local/bin/docker-compose Once done, execute the command “docker-compose –version” to check whether version number is printed. You could read about Docker-compose on this page.
Ionic – How to do Check All with Ion-Checkbox
This blog represents code sample to achieve “Check all” or “Select all” using ION-Checkbox. AngularJS/Ionic Controller Code to Check All Say, you have a list of contacts (name) with check boxes. What you want is to achieve “check all” when you check one checkbox. $scope.allcontacts is a temporary object to capture the checkbox checked value. $scope.allcontacts = {}; $scope.allcontacts.checked = false; $scope.checkAll = function() { if ($scope.allcontacts.checked) { $scope.allcontacts.checked = true; } else { $scope.allcontacts.checked = false; } for (var i=0; i < $scope.contacts.length; i++) { $scope.contacts[i].checked = $scope.allcontacts.checked; }; }; HTML Code including Ion-Checkbox to Achieve “Check All” <ion-list> <ion-checkbox class=”item-checkbox-right” ng-model=”allcontacts.checked” ng-checked=”allcontacts.checked” ng-click=”checkAll()”> All Contacts </ion-checkbox> <ion-checkbox ng-repeat=”contact …
Android – ADB Shell Batch Scripting – Automate SQLite Database Copy
This is a quick blog to represent tips on how one could run multiple adb shell commands as part of one windows batch script or automate SQLite database copy to local filesystem. Say, you have got to copy the SQLite database from within your app to your local filesystem. The commands could be found on one of my earlier blog: http://vitalflux.com/ionic-how-to-copy-sqlite-database/ Copy following commands in a text file, say, commands.txt run-as package-name-of-the-app chmod 666 databases/database_name exit cp /data/data/package-name-of-app/databases/database_name /sdcard/ run-as package-name-of-the-app chmod 600 databases/database_name exit exit Write following set of commands in another batch script (.bat file) and name it as, say, adb.bat. del database_name cmd /c adb shell < commands.txt cmd …
Ionic – How to Access or Browse SQLite Database
The blog represents instructions on how could you access or browse SQLite database embedded in your android app. The way that works for me is to copy the database on to my system and access the same. Make sure you connected your mobile phone to your laptop/desktop. Check with command “adb devices” to see that your device (phone) is listed. Following commands can be used to get the Cordova SQLite database on your hard-disk filesystem: adb shell run-as package-name-of-the-app chmod 666 databases/database_name exit cp /data/data/package-name-of-app/databases/database_name /sdcard/ run-as package-name-of-the-app chmod 600 databases/database_name exit; exit Once you are on system command prompt, execute following command to copy the database file to current directory …
Blockchain – Ethereum Hello World with Visual Studio
This article represents tips on how to get started with Ethereum Hello World program with Visual Studio. Please feel free to comment/suggest if I missed to mention one or more important points. Also, sorry for the typos. Following are the key steps: Download Visual Studio community version from Visual Studio Download page. This is a very small file. However, once you click on this file, it starts downloading and installing the Visual Studio IDE which is 8 GB large file. So, have patience. Once completed, it will be installed. Start the Visual studio. For the first time, it takes considerable file to start. So, have patience. Download the extension from …
10 Definitions to Understand BlockChain Better
This article represents 10 definitions of BlockChain that I gathered from several pages. The idea is to understand BlockChain in different words. Please feel free to comment/suggest if I missed to mention one or more important points. Also, sorry for the typos. Following are the 10 definitions of BlockChain: The block chain consists of blocks that hold timestamped batches of recent valid transactions. Each block includes the hash of the prior block, linking the blocks together. The linked blocks form a chain, with each additional block reinforcing those before it, thus giving the database type its name. The original definition was written by Satoshi Nakamoto and found in the original …
I found it very helpful. However the differences are not too understandable for me