How do I read java.util.Vector object using java.util.Enumeration?

By Wayan in Core API, Util Package 0 Comment

The following code snippets show you how to iterate and read java.util.Vector elements using the java.util.Enumeration.

package org.kodejava.util;

import java.util.Enumeration;
import java.util.Vector;

public class VectorEnumeration {

    public static void main(String[] args) {
        Vector<String> data = new Vector<>();
        data.add("one");
        data.add("two");
        data.add("three");

        StringBuilder sb = new StringBuilder("Data: ");

        // Iterates vector object to read it elements
        for (Enumeration<String> enumeration = data.elements();
             enumeration.hasMoreElements(); ) {
            sb.append(enumeration.nextElement()).append(",");
        }

        // Delete the last ","
        sb.deleteCharAt(sb.length() - 1);
        System.out.println(sb);
    }
}

How do I remove the first or the last found element from Deque?

By Wayan in Core API, Util Package 0 Comment

This example demonstrates the use of Deque.removeFirstOccurrence() method call to remove the first occurrences of an element in the Deque object and Deque.removeLastOccurrence() method call to remove the last occurrences of an element in the Deque object.

package org.kodejava.util;

import java.util.Deque;
import java.util.Iterator;
import java.util.LinkedList;

public class DequeRemoveDemo {
    public static void main(String[] args) {
        Deque<String> deque1 = new LinkedList<>();
        deque1.offer("a");
        deque1.offer("b");
        deque1.offer("c");
        deque1.offer("d");
        deque1.offer("e");
        deque1.offer("d");

        Deque<String> deque2 = new LinkedList<>(deque1);

        // Removes the first occurrence of letter "d" from deque1
        deque1.removeFirstOccurrence("d");

        Iterator<String> first = deque1.iterator();
        System.out.println("After removeFirstOccurrence(Object o): ");
        System.out.println("=======================================");
        while (first.hasNext()) {
            System.out.println(first.next());
        }

        // Removes the last occurrence of letter "d" from deque2
        deque2.removeLastOccurrence("d");
        Iterator<String> last = deque2.iterator();
        System.out.println("After removeLastOccurrence(Object o): ");
        System.out.println("========================================");
        while (last.hasNext()) {
            System.out.println(last.next());
        }
    }
}

The output of the code snippet above is:

After removeFirstOccurrence(Object o): 
=======================================
a
b
c
e
d
After removeLastOccurrence(Object o): 
========================================
a
b
c
d
e

How do I create a Last-In-First-Out Deque?

By Wayan in Core API, Util Package 0 Comment

This example show you how to create a LIFO (Last In First Out) Deque. We call the Deque.peekLast() method to get the last element from the Deque, poll the last element and repeat until all the elements read.

package org.kodejava.util;

import java.util.Deque;
import java.util.LinkedList;

public class DequeLifoDemo {

    public static void main(String[] args) {
        // Create an instance of a Deque, here we use the LinkedList
        // class which implements the Deque interface.
        Deque<String> deque = new LinkedList<>();
        deque.add("one");
        deque.add("two");
        deque.add("three");
        deque.add("four");

        StringBuilder in = new StringBuilder("Items IN in order : ");

        // Returns an iterator over the elements in this deque in
        // proper sequence
        for (String s : deque) {
            in.append(s).append(",");
        }

        in.deleteCharAt(in.length() - 1);
        System.out.println(in);

        StringBuilder out = new StringBuilder("Items OUT in order: ");
        for (int i = 0; i < deque.size(); ) {
            out.append(deque.peekLast()).append(",");

            // Retrieves and removes the last element of this deque,
            // or returns null if this deque is empty.
            deque.pollLast();
        }

        out.deleteCharAt(out.length() - 1);
        System.out.println(out);
    }
}

The output of our code snippet is:

Items IN in order : one,two,three,four
Items OUT in order: four,three,two,one

How do I get timezone ids by their milliseconds offset?

By Wayan in Core API, Util Package 0 Comment 
package org.kodejava.util;

import java.util.TimeZone;

public class TimeZoneByOffset {
    public static void main(String[] args) {
        // Gets the available IDs according to the given time zone
        // offset in milliseconds.
        int offset = 8 * 60 * 60 * 1000;
        String[] timezones = TimeZone.getAvailableIDs(offset);

        System.out.println("List of available IDs for GMT:+08:00");
        System.out.println("====================================");
        for (String id : timezones) {
            System.out.println(id);
        }
    }
}

Here are the timezone ids in the GMT+8 offset:

List of available IDs for GMT:+08:00
====================================
Asia/Brunei
Asia/Choibalsan
Asia/Chongqing
Asia/Chungking
Asia/Harbin
Asia/Hong_Kong
Asia/Irkutsk
Asia/Kuala_Lumpur
Asia/Kuching
Asia/Macao
Asia/Macau
Asia/Makassar
Asia/Manila
Asia/Shanghai
Asia/Singapore
Asia/Taipei
Asia/Ujung_Pandang
Asia/Ulaanbaatar
Asia/Ulan_Bator
Australia/Perth
Australia/West
CTT
Etc/GMT-8
Hongkong
PRC
Singapore

How do I retrieve available catalogs in database?

By Wayan in JDBC 0 Comment

The examples below show you how to get the available catalog names in MySQL database. The available catalog names can be obtained from the DatabaseMetaData object by calling the getCatalogs() method. This method returns a ResultSet object. We iterate it and get the schema name by calling the getString() method and pass TABLE_CAT as the column name.

package org.kodejava.jdbc;

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;

public class MetadataGetCatalog {
    private static final String URL = "jdbc:mysql://localhost";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection connection =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD)) {

            // Gets DatabaseMetaData
            DatabaseMetaData metadata = connection.getMetaData();

            // Retrieves the schema names available in this database
            ResultSet rs = metadata.getCatalogs();

            while (rs.next()) {
                String catalog = rs.getString("TABLE_CAT");
                System.out.println("Catalog: " + catalog);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Here is the list of catalog names available in the MySQL database:

Catalog: kodejava
Catalog: musicdb
Catalog: mysql
Catalog: performance_schema
Catalog: sys

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.1.0</version>
</dependency>

Maven Central

How do I create port scanner program?

By Wayan in Core API, Networking 0 Comment

In this example you’ll see how to create a simple port scanner program to check the open ports for the specified host name.

package org.kodejava.net;

import java.io.IOException;
import java.net.InetAddress;
import java.net.Socket;

public class PortScanner {
    public static void main(String[] args) throws Exception {
        String host = "localhost";
        InetAddress inetAddress = InetAddress.getByName(host);

        String hostName = inetAddress.getHostName();
        for (int port = 0; port <= 65535; port++) {
            try {
                Socket socket = new Socket(hostName, port);
                String text = hostName + " is listening on port " + port;
                System.out.println(text);
                socket.close();
            } catch (IOException e) {
                // empty catch block
            }
        }
    }
}

The result of the code snippet above are:

localhost is listening on port 21
localhost is listening on port 80
localhost is listening on port 135
localhost is listening on port 445
localhost is listening on port 3000
...
...
localhost is listening on port 63342
localhost is listening on port 63467
localhost is listening on port 64891
localhost is listening on port 64921
localhost is listening on port 65001

How do I get the maximum number of concurrent connections?

By Wayan in JDBC 0 Comment

This example shows you how to get the maximum number of concurrent connections to a database that are possible.
To get this information, we use the DatabaseMetaData.getMaxConnections() method call. If the return value is zero, it means that there is no limit or the limit is unknown.

package org.kodejava.jdbc;

import java.sql.Connection;
import java.sql.DatabaseMetaData;
import java.sql.SQLException;
import java.sql.DriverManager;

public class MaxConnections {
    private static final String URL = "jdbc:mysql://localhost/kodejava";
    private static final String USERNAME = "kodejava";
    private static final String PASSWORD = "s3cr*t";

    public static void main(String[] args) {
        try (Connection connection =
                     DriverManager.getConnection(URL, USERNAME, PASSWORD)) {

            // Get database meta data.
            DatabaseMetaData metaData = connection.getMetaData();

            // Retrieves the maximum number of concurrent
            // connections to this database that are possible.
            // A result of zero means that there is no limit or
            // the limit is not known.
            int max = metaData.getMaxConnections();
            System.out.println("Max concurrent connections: " + max);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>

Maven Central

How do I check whether a driver is JDBC compliant?

By Wayan in JDBC 0 Comment 
package org.kodejava.jdbc;

import java.sql.Driver;
import java.sql.DriverManager;
import java.sql.SQLException;

public class JDBCCompliant {
    private static final String URL = "jdbc:mysql://localhost/kodejava";

    public static void main(String[] args) {
        try {
            Driver driver = DriverManager.getDriver(URL);

            // Check if the driver is a genuine JDBC compliant driver.
            if (driver.jdbcCompliant()) {
                System.out.println("A genuine JDBC compliant driver");
            } else {
                System.out.println("Not a genuine JDBC compliant driver");
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>

Maven Central

How do I get JDBC driver property information?

By Wayan in JDBC 0 Comment

The example below shows you how to get the information about the possible properties for the corresponding JDBC driver.

package org.kodejava.jdbc;

import java.sql.*;

public class DriverPropertyInfoDemo {
    private static final String URL = "jdbc:mysql://localhost/kodejava";

    public static void main(String[] args) {
        try {
            // Gets information about the possible properties for this
            // driver.
            Driver driver = DriverManager.getDriver(URL);
            DriverPropertyInfo[] props = driver.getPropertyInfo(URL, null);

            for (DriverPropertyInfo info : props) {
                System.out.println("Name       : " + info.name);
                System.out.println("Description: " + info.description);
                System.out.println("Value      : " + info.value);
                System.out.println("-----------------------------------");

                String[] choices = info.choices;
                if (choices != null) {
                    StringBuilder sb = new StringBuilder("Choices    : ");
                    for (String choice : choices) {
                        sb.append(choice).append(",");
                    }

                    System.out.println(sb);
                }
            }

        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

The example result of the code snippet above:

Name       : host
Description: Hostname of MySQL Server
Value      : localhost
-----------------------------------
Name       : port
Description: Port number of MySQL Server
Value      : 3306
-----------------------------------
Name       : dbname
Description: Database name;
Value      : kodejava
-----------------------------------
Name       : user
Description: Username to authenticate as
Value      : null
-----------------------------------
Name       : password
Description: Password to use for authentication
Value      : null
-----------------------------------
Name       : clientCertificateKeyStorePassword
Description: Password for the client certificates key store.
Value      : null
-----------------------------------
Name       : serverRSAPublicKeyFile
Description: File path to the server RSA public key file for sha256_password authentication. If not specified, the public key will be retrieved from the server.
Value      : null
-----------------------------------
Name       : cacheDefaultTimeZone
Description: Caches clients default time zone. This results in better performance when dealing with time zone conversions in Date and Time data types, however it wont be aware of time zone changes if they happen at runtime.
Value      : true
-----------------------------------
Choices    : TRUE,FALSE,YES,NO,
...
...
...

Maven Dependencies

<dependency>
    <groupId>com.mysql</groupId>
    <artifactId>mysql-connector-j</artifactId>
    <version>8.4.0</version>
</dependency>

Maven Central