Maven error Annotations are not supported in -source 1.3

The Error

Are you using annotations in your project?

If so, you will get this error if you try to do mvn compile or mvn install.

You may have also seen this if you are building your project on eclipse as well.

The solution

The default maven relies on JDK 1.3 for its building the project and you might be using a JDK above 1.3. Also, JDK 1.3 does not implement annotations.

The solution is to tell the maven the current non-1.3 JDK you are using on you pom.xml file.



For the source and target you will provide your current JDK you are using.

Running maven project in eclipse step by step

If you have java web project that is maven based, and you want to run the project in eclipse here are the steps.

Mind you this is for the project which is already a maven project, say you did it through terminal or you got it from other repository and you want to run it locally through eclipse.

Assumed -> you have setup the webserver like tomcat on eclipse.

1. Go to Eclipse Run->Run Configuration

2. Double click on M2 Maven Build

3. On the base directory: insert ${project_loc}

4. On the goals: insert tomcat:run and the select and apply

5. Right click on the project and select configure->Convert to Maven Project

configure build path

configure build path

6. Again right click on the project and select Maven->Update Project


7. For further pulling the dependencies from maven to eclipse, right click on it->properties->Deployment Assembly

8 Click on Add button and select Java build path entries and select Maven Dependencies

9. Now, your dependencies should be pulled all is ready for testing. Go to project menu and click on build project. This will be inactive if Build Automatically is selected.

10. Then to test, right click on one of your servlet and select run as and click on the server option. If all went good, you should be able to see the output from your servlet on the browser.

The above simple steps would allow you to convert the Java web project into maven project in eclipse.

See: How to change directory structure for maven

See: Resolving classpath problem in eclipse

mysql command not found on mac after installation

After downloading mysql dmg on mac if you go to terminal and try to issue mysql command and get mysql command not found there keep reading.

This, in most cases, is the terminal not knowing the mysql command unless you go to the actual installation folder.

To check that out, go to

/user/local/mysql/bin/mysql --version

If this is giving you the right mysql information, then it is a matter of telling the terminal what the mysql command is.

Temporary fix

export PATH=$PATH:/usr/local/mysql/bin

The above command would allow the command files to be recognized by the current issuing terminal. But you won’t be able to access those on new terminal

Permanent fix

echo "export PATH=$PATH:/usr/local/mysql/bin" >>~/.bash_profile

The above command would simply append the path command you exported to the .bash_profile file.

Importing packages to eclipse added by maven to java web project

If eclipse is having a hard time acknowledging the packages you got through maven, here are the step by step fix for it..

1. Go to your projects root, where the pom.xml resides

2. issue mvn eclipse:clean

3. issue mvn eclipse:eclipse

4. this could be optional, on eclipse go to the project, right click on it and select build project

resources can not be added

eclipse j2ee error There are No resources that can be added or removed from the server

Eclipse showing There are No resources that can be added or removed from the server when adding project

If you are trying to test the java web application (j2ee) on your local machine with built in eclipse support and getting error, then it is related to project facet.

Here is how you can solve the error There are No resources that can be added or removed from the server coming from eclipse

1. right click on your web project in eclispe

2. Select properties

3. Select Project facets

4. Select Dynamic Web Module

5. Click on apply and then click on OK

This will solve the problem.

See also: How to resolve class pass error when working with maven

testing k complementary pairs algorithm with junit

This is the test class for the java solution of the k-complementary problem listed here

package algorithm;

import static org.junit.Assert.assertArrayEquals;

import org.junit.Before;
import org.junit.Test;

 * Testing KComplementary algorithm
 * @author Kaleb Woldearegay
public class KComplementaryTest {
	private KComplementary kComplementary;
	public void initiate() {
		this.kComplementary = new KComplementary();
	public void test1() {
		Integer[][] expectedResult = new Integer[][]{{1,9},{5,5},{9,1}};
		int[] test = new int[]{1,5,9};
		assertArrayEquals(this.kComplementary.getKComplementaryPairs(10,  test), expectedResult);
	public void test2() {
		Integer[][] expectedResult = new Integer[][]{{5,7},{7,5}};
		int[] test = new int[]{3,5,7};
		assertArrayEquals(this.kComplementary.getKComplementaryPairs(12,  test), expectedResult);
	public void test3() {
		Integer[][] expectedResult = new Integer[][]{{-1,1},{0,0},{1,-1}};
		int[] test = new int[]{5,-1,0,-2,3, 1};
		assertArrayEquals(this.kComplementary.getKComplementaryPairs(0,  test), expectedResult);


See more algorithm solutions by clicking Here

Find the pairs that makes K Complementary in the given array java solution

Find K complementary pairs from the given array

So the problem is, given an array of numbers, you are going to find the pairs whose sum is K.

Say, if you are given 7, 1, 5, 6, 9, 3, 11, -1 and K = 10 then the pairs would be

=> 1 and 9
=> 3 and 7
=> 11 and -1

Here is the O(n) solution for the K complementary problem in java

It turns out, this is not a pure O(n) solution. Can you spot why?

package algorithm;

import java.util.HashMap;
//import java.util.List;
import java.util.Map;

 * Algorithm to find the pairs making the K complementary in O(n) complexity
 * @author
 * @see - a JUnit test for this class
public class KComplementary {
	private Map pairs;
	public static void main(String[] args) {
		KComplementary kcomp = new KComplementary();
		int[] numbers = new int[]{7, 1, 5, 6, 9, 3, 11, -1};
		Integer[][] pairs = kcomp.getKComplementaryPairs(10, numbers);
		for (Integer[] thePairs : pairs) {
			System.out.println(" Pairs are "+thePairs[0] + " and " + thePairs[1]);
	public KComplementary() {
		this.pairs = new HashMap();
	 * An algorithm to find the pair from the given array that would sum up the given K
	 * @note - the algorithm would be done in o(n). First it will run through the whole 
	 * numbers and creates a temporary list of pairs in HashMap with 
	 * (value, sum-value). 
	 * @param sum
	 * @param listOfIntegers
	 * @return
	public Integer[][] getKComplementaryPairs(int sum, int[] listOfIntegers) {
		 * I could have used the ArrayList assuming the number of pairs we are getting 
		 * is not known, but giving it a little bit of thought, the number of pairs 
		 * is known in advance, at least the maximum we can get is known.
		 * By not using the Array list, the algorithm would run O(n) rather than
		 * O(n**2) since ArrayList.add would would be O(n) by itself 
		//List complementaryPairs = new ArrayList();
		Integer[][] complementaryPairs = new Integer[listOfIntegers.length][2];
		//First fill up the pairs with the complementary numbers
		for (int number : listOfIntegers) { //O(n) complexity
			this.pairs.put(number, sum-number);
		//then filter out the pairs that don't have corresponding complementary number
		int index = 0;
		for (int number : listOfIntegers) { //O(n) complexity 
			int complementary = sum - number;
			//check if this key exists in the pairs
			if ( this.pairs.containsKey(complementary) ) {
				//Had I used array List this would have been used
				//Integer[] comps = {number, complementary};
				//complementaryPairs.add(comps); //this is O(n)
				complementaryPairs[index][0] = number;
				complementaryPairs[index][1] = complementary;
				index ++;
		//Now trim the array since we know the exact record
		Integer[][] trimmed = new Integer[index][2];
		index = 0;
		for (Integer[] item : complementaryPairs) { //O(n) complexity
			if (item[0] != null) {
				trimmed[index][0] = item[0];
				trimmed[index][1] = item[1];
		//Total complexity O(n)+O(n)+O(n) ==> O(n)
		//had I used arrayList this would have been used
		//return complementaryPairs.toArray(new Integer[0][0]);
		return trimmed;

The above solution runs and would do the job without a problem. But it is not linear solution as it claims to be.

I have got another solution that would explain why and comes up with a solution for it. Check it out here

You can see the jUnit for this class also at here

using mongodb with nodejs and angular js

This is a continuation of the tutorial started on node js angular and mongo db shown on
node js console tutorial

This is a final part of the node angular and mongodb tutorial.

So far each part of the whole process has been shown by far. This is a part showing how the data would be pulled and shown to the angular based page.

Mind you, in this tutorial, the page was getting the list from the hard coded array and that is simply changed to the array that is read from mongodb. That part can be hooked to other datasources like MsSQL server or MySQL or anything.

Using MongoDB in Node JS

1. Create a folder named node-mongo

2. install mongodb and express packages there

sudo npm install express
sudo npm install mongodb

*I will have another tutorial showing how to use the package manager using package.json. But for now just install the packages like the one I showed above.

3. create server.js in node-mongo folder and add the following content in it

var express = require('express');
var app = express();

//connection string for mongo
var mongoPath = "mongodb://localhost:27017/nodemongo";
//require mongo and get the client.
var mongoClient = require('mongodb').MongoClient;

//a router for the home page. "/" being the home page
app.get("/", function(request, response){
    response.sendFile(__dirname + "/views/home.html");

//a router for listening to /listcountries 
app.get("/listcountries", function(request, response){

    // mongo connection comes from here
    mongoClient.connect(mongoPath, function(error, database){
        var result = {};
        //once connected, then get the row from mongo database. find() would list all
        if (error) {
            result.message = "Connection error";
        } else {
            var collection = database.collection("countries");
            collection.find().toArray((err, listOfCountries)=>{

console.log("serving web on port 8080");

By now, most of the usages on this file would be easy to understand if you have followed the previous tutorials.

How to install mongo in the computer and how to add some collections to it and the like is beyond the scope of this tutorial and I assume you have already installed mongo on your system and have created a database and collection with it.

In the example above, I have used mongo database named nodemongo and collection countries

4. create a folder views in node-mongo

5. create a file home.html and add the following code in in it

<body ng-app="NodeMongoApp">
<h2>here you would unleash the science of listing countries</h2>
<div ng-controller="MongoController">
    <a href="#" ng-click="listCountries()">list countries</a>

    <ul ng-repeat="country in countries">

<script src=""></script>
    var app = angular.module("NodeMongoApp", []);
    app.controller("MongoController", function($scope, $http){
        $scope.listCountries = function() {
                $scope.countries = countries.list;    

That is it.

Now you will want to be in the node-mongo folder and issue

node server.js

and from your browser, go to http://localhost:8080

You can download the whole project here

using angular js with node js application example

This is a continuation of the tutorial started on node js angular and mongo db shown on
node js console tutorial

Take a look on this tutorial and I will use the same setup with it

Follow until step 5 from the above example.

If you see the server.js, it will populate the hardcoded countries the template file countries.ejs file.

But this time, we dont need the ejs file at all, rather all what is needed from the node js server is to provide the json formatted response of the countries.

In the server.js file, add the following code

var express = require("express");
var app = express();

app.set("views", __dirname + '/views');

app.get("/", function(request, response){
   response.sendFile(__dirname + '/views/index.html'); 

app.get("/listcountries", function(request, response){
    var title = "List of Countries";
    var hardCodedCountries = [{"name":"America","population":300}, {"name":"Britain","population":53},{"name":"Canada","population":35},{"name": "Democratic Congo","population":67},{"name": "Ethiopia","population":90}, {"name":"Finland", "population":5}, {"name":"Gabon", "population": 2}];

console.log("Hit http://localhost:8080");

On this one, the view engine is removed and also the response.render is replaced by response.send

In short, the node has created an endpoint named /listcountries that would list countries and send that to the requester.

6. Now, the next task would be to update the page that would list the countries using angular

update the index.html file in views folder as follows

<head><title>Node with template</title></head>
<body ng-app="AngularNodeApp">
<div ng-controller="CountryController">
	<h2>Listing countries has never been easy</h2>
	<a href="#" ng-click="listCountries()">List Countries</a>

	<ul ng-repeat="country in countries">
		<li>{{}} | {{country.population}}</li>
<script src=""></script>
var app = angular.module("AngularNodeApp", []);
app.controller("CountryController", function($scope, $http){
	//get the list of countries
	$scope.listCountries = function() {
                //call the API to listcountries and get the list of countries as json response
                //assign countries variable that is accessed through $scope
		$scope.countries = countries_from_node;

Here it is clear that, the front and backend are separated in that the front end would act in its own by requesting what it needs from the backend only. As long as the node js is responding the list through its listcountries

Next: Using mongo db in the angular node js application

introduction to Angular js for usage of node js

This is a continuation of the tutorial started on node js angular and mongo db shown on
node js console tutorial

I might not go over the basics of angular js here except what is needed for the tutorial.

The main idea in using Angular js would be to make sure the front end and back end are completely separated and they would be talking each other through end points that are designated through node js.

Angular basics

Angular is an MVC architecture oriented javascript framework. I will have some example that would illustrate that as a basic introduction.

	<head><title>Testing angular</title></head>
	<body ng-app="HelloTest">
		<div ng-controller="HelloController">
            <div> <input type="text" ng-model="color" ng-keyup="updateColor()" name="color" /></div>
            <div> The selected color is {{current_color}} </div>

	    <script src=""></script>  
		    angular.module("HelloTest", []).controller("HelloController", function($scope){
                $scope.concat = "Whatever goes here";    
                $scope.updateColor = function () {
                    $scope.current_color = $scope.color;

Save the above code in a file angular.html and just double click on it to see it running.

The above simple example shows a basic usage of events in Angular js. All it does is printing whatever you put in the text box as color.

Basic points in the Angular application

ng-app="HelloApp" : this would be assigned usually on the root of the html dom. It tells that pages is under the given ng-app. See how that name aligns with the name angular.module("HelloApp")

ng-controller The app can contain a number of controllers under it. A controller is the one that sits between the view and logic that would fill up the view. in this example, the ng-controller="HelloController" will be used for the directive it has been defined. In this example for the whole element of the div it has been declared for.

You can have as many controller as you want in the given app

//create the application here
var app = angular.module("HelloTest", []);

//create the controller.
app.controller("HelloController", function($scope){
                $scope.concat = "Whatever goes here";    
                $scope.updateColor = function () {
                    $scope.current_color = $scope.color;

//create another controller.
app.controller("AnotherController", function($scope){
                $scope.concat = "In another controller";    

The above shows how you can have more than one controller in the app.

$scope is the special variable passed around that would give the whole scope that the controller has authority of.

the {{ }} is the place holder that would be assigned the value from the controller. In the example above, concat and current_color are the ones to be assigned from the controller and get their values being assigned where ever they are.

Angular has lots and lots of directives for the condition, loop and the like

Next: get the list of countries from node and showing them using Angular example