Programming

basic programming

eBook Apa yang Cocok bagi Pemula yang Baru Belajar Java?

Kebetulan lagi rame di kaskus programmer forum, tentang ebook yang cocok untuk pemula. Supaya ga jawab berulang-ulang, mending gw tulis disini aja deh.

Untuk melatih pondasi, coba baca bukunya donald knuth
Donald Knuth

lalu lanjut ke buku-nya deitel deitel, java how to program
Deitel Deitel

klo dah faseh naik ke bukunya gang of four, bruce eckel dan kathy sierra
Gang of Four

Bruce Eckel

kathy sierra

klo ada waktu iseng, buknya Joshua Bloch seru juga
Joshua Bloch

kemudian lanjut ke bukunya craig walls dan gavin king
Craig Walls

Gavin King

kalo dah faseh abis itu langsung ke bukunya Debu Panda dan Reza Rahman
EJB3

Semoga membantu untuk para calon-calon programmer diluar sana, Maju Terus Ilmu Pengetahuan Indonesia…!!

How To Check Window’s Opened Comm Port using Java

On my latest project, i need to connect my java application to a GSM Modem, in order to sending and receiving SMS text. But before my application connected to opened comm port, i need to check which port number is opened. So this is a small java class to check my opened comm port. Btw, im using rtxt libraries to help me communicate with the opened port.

import gnu.io.CommPortIdentifier;
import java.util.Enumeration;

public class Main {
    
    private void doSomething() {
        Enumeration portList = CommPortIdentifier.getPortIdentifiers();
        System.out.println(portList.hasMoreElements());
        while (portList.hasMoreElements()) {
            CommPortIdentifier portId = (CommPortIdentifier) portList.nextElement();
            System.out.println(portId.getName());
        }
    }
    
    public static void main(String[] args) {
        Main main = new Main();
        main.doSomething();
    }
}

Dont forget to add rxtx libraries your java’s path. You can find the Window’s installation path, here.

Hope it helps other, have fun 🙂

A Weird Exception When Using Java’s BigDecimal, java.lang.ArithmeticException: Rounding necessary

I found this weird exception when im trying to convert double values to BigDecimal,

java.lang.ArithmeticException: Rounding necessary
	at java.math.BigDecimal.divideAndRound(BigDecimal.java:1439)
	at java.math.BigDecimal.setScale(BigDecimal.java:2394)
	at java.math.BigDecimal.setScale(BigDecimal.java:2437)

This is my java code

BigDecimal bgAvg = new BigDecimal(0);
bgAvg = bgAvg.add(new BigDecimal(myDoubleValue).setScale(2));

Somehow the error is gone after i changed my code into this

BigDecimal bgAvg = new BigDecimal(0);
bgAvg = bgAvg.add(BigDecimal.valueOf(myDoubleValue).setScale(2));

Hope it helped others, have fun with Java 😉

How to Set %JAVA_HOME% Variable on Apache Tomcat’s Catalina Script

Usually im using Apache Tomcat’s catalina script to launch and start Apache Tomcat, but sometimes the script wont run because of not having JAVA_HOME variables on your OS environment variables or you cant add new environment variables due to lack of privileges.

So instead of adding JAVA_HOME variable into my OS, i set the JAVA_HOME on my catalina scripts. This is how i do it, i add this script on the top of catalina.bat file.

set JAVA_HOME=C:\Program Files (x86)\Java\jdk1.7.0_07

Note, there is no space before and after “=” sign.

Easy isnt it 😉

Integrating BCrypt Hashing With Hibernate Framework

In this example, im trying to simulate a simple login to MySQL database. Usually i hash password value using MD5, but now im trying to do hashing using BCrypt Algorithm. Im using a simple java BCrypt class downloaded from here.

First as always, a simple table and row.

CREATE TABLE `users` (
  `username` varchar(20) NOT NULL DEFAULT '',
  `pwd` varchar(80) DEFAULT NULL,
  PRIMARY KEY (`username`)
)

insert into `users`(`username`,`pwd`) values ('edwin','$2a$12$bUwElzXYO116G6x.fLm5FOAJNB46R0974sAh2TQumJei4ia.x0YPy');

Next is creating a simple java class and xml to represent database tables.

package com.edw.bean;

public class Users  implements java.io.Serializable {

     private String username;
     private String pwd;

    public Users() {
    }

	// other setter and getter
}
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.edw.bean.Users" table="users" catalog="test">
        <id name="username" type="string">
            <column name="username" length="20" />
            <generator class="assigned" />
        </id>
        <property name="pwd" type="string">
            <column name="pwd" length="80" />
        </property>
    </class>
</hibernate-mapping>

Next is my hibernate.cfg.xml configuration

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
  <session-factory>
    <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/test</property>
    <property name="hibernate.connection.username">root</property>
    <property name="hibernate.connection.password">****</property>
    <mapping resource="com/edw/bean/Users.hbm.xml"/>
  </session-factory>
</hibernate-configuration>

And my java class to load hibernate.cfg.xml

package com.edw.util;

import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.SessionFactory;

public class HibernateUtil {

    private static final SessionFactory sessionFactory;
    
    static {
        try {           
            sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
        } catch (Throwable ex) {            
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }
    
    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
}

And this is my Main java class,

package com.edw.main;

import com.edw.bean.Users;
import com.edw.util.BCrypt;
import com.edw.util.HibernateUtil;
import org.apache.log4j.Logger;
import org.hibernate.Session;

public class Main {
    
    private static Logger logger = Logger.getLogger(Main.class );
    
    private Boolean startApp(String username, String password) {
        Session session = HibernateUtil.getSessionFactory().openSession();
        try {
            Users user = (Users)session.createQuery("from Users where username = :username")
                    .setString("username", username)
                    .uniqueResult();      
            // compare password with database's encrypted password
            if(BCrypt.checkpw(password, user.getPwd()))
                return true;
            return false;
        } catch (Exception e) {
            logger.error(e,e);
        } finally {            
            session.close();
        }
        return false;
    }
    
    private String hashPassword(String password) {
        return BCrypt.hashpw(password, BCrypt.gensalt(12));
    }
    
    public static void main(String[] args) {
        Main main = new Main();
        boolean success = main.startApp("edwin", "12345");
        if(success)
            logger.debug("Password is Right");
        else
            logger.debug("Password is Wrong");
        
        // simulate 10 hashed string password
        for (int i = 0; i < 10; i++) {
            logger.debug(main.hashPassword("12345"));
        }        
    }
}

This is my Netbeans project structure,

And this is my Netbean’s console