How do I get the name of current executed method?

By Wayan in Core API, Lang Package 0 Comment 
package org.kodejava.lang;

public class GetCurrentMethodName {
    public static void main(String[] args) {
        // Get the current executing method name
        String methodName =
                Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println("methodName = " + methodName);

        GetCurrentMethodName obj = new GetCurrentMethodName();
        obj.executeAMethod();
    }

    private void executeAMethod() {
        // Get the current executing method name
        String methodName =
                Thread.currentThread().getStackTrace()[1].getMethodName();
        System.out.println("methodName = " + methodName);
    }
}

This program will print out the following string:

methodName = main
methodName = executeAMethod

How do I execute external command and obtain the result?

By Wayan in Core API, Lang Package 0 Comment

This example demonstrate how to execute an external command from Java and obtain the result of the command. Here we simply execute a Linux ls -al command on the current working directory and display the result.

package org.kodejava.lang;

import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;

public class ProcessResult {
    public static void main(String[] args) {
        try {
            Process process = Runtime.getRuntime().exec("ls -al");

            // Wait for this process to finish or terminated
            process.waitFor();

            // Get process exit value
            int exitValue = process.exitValue();
            System.out.println("exitValue = " + exitValue);

            // Read the result of the ls -al command by reading the
            // process's input stream
            InputStreamReader is = new InputStreamReader(process.getInputStream());
            BufferedReader reader = new BufferedReader(is);
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException | InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Below is our program result:

exitValue = 0
total 64
drwxr-xr-x  20 wsaryada  staff   680 Aug  7 14:13 .
drwxr-xr-x   4 wsaryada  staff   136 Jun 18 00:33 ..
-rw-r--r--@  1 wsaryada  staff  6148 Jun 29 14:04 .DS_Store
-rw-r--r--   1 wsaryada  staff   267 May 19 20:47 .editorconfig
drwxr-xr-x  16 wsaryada  staff   544 Aug 10 08:34 .git
-rw-r--r--   1 wsaryada  staff  1535 May 19 20:47 .gitignore
drwxr-xr-x  15 wsaryada  staff   510 Aug 10 08:34 .idea
-rw-r--r--   1 wsaryada  staff  1313 May 19 20:47 LICENSE
-rw-r--r--   1 wsaryada  staff   101 May 19 20:47 README.md
drwxr-xr-x   5 wsaryada  staff   170 Jul 28 23:11 kodejava-basic
drwxr-xr-x   6 wsaryada  staff   204 Jun 30 14:22 kodejava-commons
drwxr-xr-x   6 wsaryada  staff   204 Jul 27 15:32 kodejava-lang-package
drwxr-xr-x@  5 wsaryada  staff   170 Jun 16 20:49 kodejava-mail
drwxr-xr-x   6 wsaryada  staff   204 Aug  7 17:22 kodejava-mybatis
drwxr-xr-x   5 wsaryada  staff   170 Jul 20 10:36 kodejava-poi
-rw-r--r--   1 wsaryada  staff   669 Jun  2 14:29 kodejava-project.iml
drwxr-xr-x   7 wsaryada  staff   238 Jun 26 21:52 kodejava-swing
drwxr-xr-x   6 wsaryada  staff   204 Aug  3 15:06 kodejava-util-package
drwxr-xr-x   6 wsaryada  staff   204 Jun 30 15:09 maven-helloworld
-rw-r--r--   1 wsaryada  staff  1622 Aug  7 14:13 pom.xml

How do I get the path from where a class is loaded?

By Wayan in Core API, Lang Package 0 Comment

This examples shows how to get a path name or location from where our class is loaded.

package org.kodejava.lang;

public class CodeSourceLocation {
    public static void main(String[] args) {
        CodeSourceLocation csl = new CodeSourceLocation();
        csl.getCodeSourceLocation();
    }

    private void getCodeSourceLocation() {
        // The location from where the class is loaded.
        System.out.println("Code source location: " +
            getClass().getProtectionDomain().getCodeSource().getLocation());
    }
}

The code snippet print the following output:

Code source location: file:/F:/Wayan/Kodejava/kodejava-example/kodejava-lang/target/classes/

How do I validate email address using regular expression?

By Wayan in Core API, Lang Package 0 Comment

In this example we use the String‘s class matches() methods to match a string to be a valid email address based on the given regex.

This example also demonstrate the power of regular expression to validate an email address. Using regular expression makes it easier to validate data such as email address. After the code you’ll see the meaning of the regular expression used in the code below.

package org.kodejava.lang;

public class EmailAddressValidation {
    private static final String EMAIL_REGEX =
            "^[\\w-_\\.+]*[\\w-_\\.]\\@([\\w]+\\.)+[\\w]+[\\w]$";

    public static void main(String[] args) {
        EmailAddressValidation validator = new EmailAddressValidation();

        System.out.println("isValid = "
                + validator.isValidEmailAddress("[email protected]"));
        System.out.println("isValid = "
                + validator.isValidEmailAddress("[email protected]"));
        System.out.println("isValid = "
                + validator.isValidEmailAddress("[email protected]"));
        System.out.println("isValid = "
                + validator.isValidEmailAddress("user.domain.co.id"));
    }

    /**
     * Validates email address against email regular expression.
     *
     * @param email an email address to check
     * @return true if email address is valid otherwise return false.
     */
    private boolean isValidEmailAddress(String email) {
        return email.matches(EMAIL_REGEX);
    }
}

The first ^[\\w-_\\.+]. The ^ symbols means check the first character. This the regex processor that the email address should start with a word character formed of alphanumeric value (a-z 0-9) or it can also be a hyphen, underscore, dot or a plus symbol.

The second part, *[\\w-_\\.]. The * symbol means match the preceding zero or more times. As the first part, this tell the regex processor to check for another zero or more characters, and it can also contain hyphen, underscore and a dot.

The third part, \\@([\\w]+\\.)+. This check that email address should contain the @ symbol followed by one or more word separated by the dot symbol.

The last part is, [\\w]+[\\w]$, this check that after the last period there should be another word for the domain suffix such as the co.uk or co.id. And the $ ask that the email address should end by a word character.

How do I create a multithreaded program?

By Wayan in Core API, Lang Package 0 Comment

Java language which was there when the language was created. This multithreading capabilities can be said like running each program on their on CPU, although the machine only has a single CPU installed.

To create a Thread program in Java we can extend the java.lang.Thread class and override the run() method. But as we know that Java class can only extend from a single class, extending Thread class makes our class cannot be inherited from another class. To solve this problem an interface was introduced, the java.lang.Runnable.

Let see the simplified demo class below. In the program below we’ll have three separated thread executions, the main thread, thread-1 and thread-2.

package org.kodejava.lang;

public class ThreadDemo extends Thread {
    public static void main(String[] args) {
        // Creates an instance of this class.
        ThreadDemo thread1 = new ThreadDemo();

        // Creates a runnable object.
        Thread thread2 = new Thread(new Runnable() {
            public void run() {
                for (int i = 0; i < 5; i++) {
                    sayHello();
                }
            }
        });

        // Set thread priority, the normal priority of a thread is 5.
        thread1.setPriority(4);
        thread2.setPriority(6);

        // Start the execution of thread1 and thread2
        thread1.start();
        thread2.start();

        for (int i = 0; i < 5; i++) {
            sayHello();
        }
    }

    public void run() {
        for (int i = 0; i < 5; i++) {
            sayHello();
        }
    }

    /**
     * The synchronized modifier ensure that two threads cannot execute the block
     * at the same time.
     */
    private static synchronized void sayHello() {
        for (int i = 0; i < 10; i++) {
            System.out.println("Thread [" + Thread.currentThread().getName() +  "] ==> Hi...");
        }

        try {
            // Causes the currently executing thread to sleep for a random
            // milliseconds
            Thread.sleep((long) (Math.random() * 1000 + 1000));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        // Causes the currently executing thread object to temporarily pause
        // and allow other threads to execute.
        Thread.yield();
    }
}

How do I determine whether a string is a palindrome?

By Wayan in Core API, Lang Package 0 Comment

This code checks a string to determine if it is a palindrome or not. A palindrome is a word, phrase, or sequence that reads the same backward as forward.

package org.kodejava.lang;

public class PalindromeChecker {

    public static void main(String[] args) {
        String text = "Sator Arepo Tenet Opera Rotas";

        PalindromeChecker checker = new PalindromeChecker();
        System.out.println("Is palindrome = " + checker.isPalindrome(text));
    }

    /**
     * This method checks the string for palindrome. We use StringBuilder to
     * reverse the original string.
     *
     * @param text a text to be checked for palindrome.
     * @return <code>true</code> if a text is palindrome.
     */
    private boolean isPalindrome(String text) {
        System.out.println("Original text = " + text);

        String reverse = new StringBuilder(text).reverse().toString();
        System.out.println("Reverse text  = " + reverse);

        // Compare the original text with the reverse one and ignoring its case
        return text.equalsIgnoreCase(reverse);
    }
}

How do I create a thread by implementing Runnable interface?

By Wayan in Core API, Lang Package 0 Comment

Here is the second way for creating a thread. We create an object that implements the java.lang.Runnable interface. For another example see How do I create a thread by extending Thread class?.

package org.kodejava.lang;

import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.Calendar;

public class TimeThread implements Runnable {
    private final DateFormat df = new SimpleDateFormat("hh:mm:ss");

    // The run() method will be invoked when the thread of this runnable object
    // is started.
    @Override
    public void run() {
        while (true) {
            Calendar calendar = Calendar.getInstance();
            System.out.format("Now is: %s.%n", df.format(calendar.getTime()));

            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        TimeThread time = new TimeThread();

        Thread thread = new Thread(time);
        thread.start();
    }
}

An example result of this code are:

Now is: 07:18:39.
Now is: 07:18:40.
Now is: 07:18:41.
Now is: 07:18:42.
Now is: 07:18:43.

How do I create a thread by extending Thread class?

By Wayan in Core API, Lang Package 0 Comment

There are two ways that we can use tho create a thread. First is by extending the java.lang.Thread class and the second way is by creating a class that implements the java.lang.Runnable interface. See How do I create a thread by implementing Runnable interface?

In this example we’ll extend the Thread class. To run a code in a thread we need to provide the run() method in our class. Let’s see the code below.

package org.kodejava.lang;

public class NumberPrinter extends Thread {
    private final String threadName;
    private final int delay;

    public NumberPrinter(String threadName, int delay) {
        this.threadName = threadName;
        this.delay = delay;
    }

    // The run() method will be invoked when the thread is started.
    @Override
    public void run() {
        for (int i = 0; i < 10; i++) {
            System.out.println("Thread [" + threadName + "] = " + i);

            try {
                Thread.sleep(delay);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) {
        NumberPrinter printerA = new NumberPrinter("A", 1000);
        NumberPrinter printerB = new NumberPrinter("B", 750);

        printerA.start();
        printerB.start();
    }
}

The example result of our code is:

Thread [B] = 0
Thread [A] = 0
Thread [B] = 1
Thread [A] = 1
Thread [B] = 2
Thread [A] = 2
Thread [B] = 3
Thread [B] = 4
Thread [A] = 3
Thread [B] = 5
Thread [A] = 4
Thread [B] = 6
Thread [A] = 5
Thread [B] = 7
Thread [B] = 8
Thread [A] = 6
Thread [B] = 9
Thread [A] = 7
Thread [A] = 8
Thread [A] = 9

How do I get Java Home directory?

By Wayan in Core API, Lang Package 0 Comment

To get Java Home directory we can obtain it from system properties using the java.home key.

package org.kodejava.lang;

public class JavaHomeDirectory {
    public static void main(String[] args) {
        String javaHome = System.getProperty("java.home");
        System.out.println("JAVA HOME = " + javaHome);
    }
}

On my computer this code give me the following output:

JAVA HOME = C:\Program Files\Java\jdk-17

How do I get the username of operating system active user?

By Wayan in Core API, Lang Package 0 Comment

This example show you how to get operating system active user’s login name. We can obtain the username of current user by reading system properties using the user.name key.

package org.kodejava.lang;

public class GettingUserName {
    public static void main(String[] args) {
        String username = System.getProperty("user.name");
        System.out.println("username = " + username);
    }
}