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 search collection elements?

By Wayan in Core API, Util Package 0 Comment

This code example use the Collections.binarySearch() to search an specified object inside a specified collections. Prior to calling the binarySearch() method we need to sort the elements of the collection. If the object is not sorted according to their natural order the search result will be undefined.

package org.kodejava.util;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.Collections;
import java.text.DateFormatSymbols;

public class CollectionSearch {
    public static void main(String[] args) {
        DateFormatSymbols dfs = new DateFormatSymbols();

        LinkedList<String> monthList =
                new LinkedList<>(Arrays.asList(dfs.getMonths()));

        // Sort the collection elements
        Collections.sort(monthList);
        System.out.println("Months = " + monthList);

        // Get the position of November inside the monthList. It returns a positive
        // value if the item found in the monthList.
        int index = Collections.binarySearch(monthList, "November");
        if (index > 0) {
            System.out.println("Found at index = " + index);
            System.out.println("Month = " + monthList.get(index));
        }
    }
}

The output of the code snippet above is below.

Months = [, April, August, December, February, January, July, June, March, May, November, October, September]
Found at index = 10
Month = November

How do I rotate elements of a collection?

By Wayan in Core API, Util Package 0 Comment

This example demonstrate how to rotate the elements of a collection object. We can use the java.util.Collections class and call the rotate() method with the collection to rotate and the distance as the parameters.

package org.kodejava.util;

import java.util.List;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Arrays;

public class CollectionRotate {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>();

        // Add some items into the collection
        for (int i = 0; i < 25; i++) {
            numbers.add(i);
        }

        // Print the collection items        
        System.out.println(Arrays.toString(numbers.toArray()));

        // Rotates the elements in the collection by the 10.
        Collections.rotate(numbers, 10);

        // Print the rotated collection items
        System.out.println(Arrays.toString(numbers.toArray()));
    }
}

Here is the program result:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24]
[15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]

How do I display file contents in hexadecimal?

By Wayan in Core API, IO 0 Comment

In this program we read the file contents byte by byte and then print the value in hexadecimal format. As an alternative to read a single byte we can read the file contents into array of bytes at once to process the file faster.

package org.kodejava.io;

import java.io.FileInputStream;

public class HexDumpDemo {
    public static void main(String[] args) throws Exception {
        // Open the file using FileInputStream
        String fileName = "F:/Wayan/Kodejava/kodejava-example/data.txt";
        try (FileInputStream fis = new FileInputStream(fileName)) {
            // A variable to hold a single byte of the file data
            int i = 0;

            // A counter to print a new line every 16 bytes read.
            int count = 0;

            // Read till the end of the file and print the byte in hexadecimal
            // valueS.
            while ((i = fis.read()) != -1) {
                System.out.printf("%02X ", i);
                count++;

                if (count == 16) {
                    System.out.println();
                    count = 0;
                }
            }
        }
    }
}

And here are some result from the file read by the above program.

31 32 33 34 35 36 37 38 39 30 31 32 33 34 35 36 
37 38 39 30 31 32 33 34 35 36 37 38 39 30 31 32 
33 34 35 36 37 38 39 30 31 32 33 34 35 36 37 38 
39 30 31 32 33 34 35 36 37 38 39 30 31 32 33 34 
35 36 37 38 39 30 0A

How do I know if a date is after another date?

By Wayan in Core API, Util Package 0 Comment

This example demonstrate Date‘s class after() method to check if a date is later than another date.

package org.kodejava.util;

import java.util.Date;
import java.util.Calendar;

public class DateCompareAfter {
    public static void main(String[] args) {
        // Get current date
        Date today = new Date();

        // Add 1 day from the current date.
        Calendar calendar = Calendar.getInstance();
        calendar.add(Calendar.DATE, 1);
        Date tomorrow = calendar.getTime();

        // Tests if this date is after the specified date. This method will
        // return true if the value time represented by the tomorrow object
        // is later than today.
        if (tomorrow.after(today)) {
            System.out.println(tomorrow + " is after " + today);
        }
    }
}

The result of the code snippet above is:

Tue Oct 05 20:52:34 CST 2021 is after Mon Oct 04 20:52:33 CST 2021

How do I know if a date is before another date?

By Wayan in Core API, Util Package 0 Comment

This example demonstrate Date‘s class before() method to check if a date is earlier than another date.

package org.kodejava.util;

import java.util.Date;
import java.util.Calendar;

public class DateCompareBefore {
    public static void main(String[] args) {
        // Get current date
        Date today = new Date();

        // Subtract 1 day from the current date.
        Calendar calendar = Calendar.getInstance();
        calendar.add(Calendar.DATE, -1);
        Date yesterday = calendar.getTime();

        // Tests if this date is before the specified date. This method will
        // return true if the value time represented by the yesterday object
        // is earlier than today.
        if (yesterday.before(today)) {
            System.out.println(yesterday + " is before " + today);
        }
    }
}

The result of the code snippet above is:

Sun Oct 03 20:49:36 CST 2021 is before Mon Oct 04 20:49:36 CST 2021

How do I create an inner class?

By Wayan in Basic, Core API 0 Comment

An inner class is a class defined inside another class. Inner classes can, in fact, be constructed in several contexts. An inner class defined as a member of a class can be instantiated anywhere in that class. An inner class defined inside a method can only be referred to later in the same method. Inner classes can also be named or anonymous.

package org.kodejava.basic;

public class InnerClassDemo {
    private Bean bean;

    /**
     * Inner class, the compiled class will be named InnerClassDemo$Bean.class
     */
    class Bean {
        public int width;
        public int height;

        @Override
        public String toString() {
            return width + " x " + height;
        }
    }

    public InnerClassDemo() {
        Bean bean = new Bean();
        bean.width = 100;
        bean.height = 200;

        this.bean = bean;
    }

    public Bean getBean() {
        return this.bean;
    }

    public static void main(String[] args) {
        InnerClassDemo inner = new InnerClassDemo();
        System.out.println("inner.getBean() = " + inner.getBean());
    }
}

How do I clone an object?

By Wayan in Basic, Core API 0 Comment

To enable our object to be cloned we need to override Object class clone method. We can also add a java.lang.Cloneable interface to our class, this interface is an empty interface. When we call the clone() method we need the add a try-catch block to catch the CloneNotSupportedException. This exception will be thrown if we tried to clone an object that doesn’t suppose to be cloned.

Calling the clone() method does a stateful, shallow copy down inside the Java Virtual Machine (JVM). It creates a new object and copies all the fields from the old object into the newly created object.

package org.kodejava.basic;

public class CloneDemo implements Cloneable {
    private int number;
    private transient int data;

    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    public static void main(String[] args) {
        CloneDemo clone = new CloneDemo();
        clone.number = 5;
        clone.data = 1000;

        try {
            // Create a clone of CloneDemo object. When we change the value of
            // number and data field in the cloned object it won't affect the
            // original object.
            CloneDemo objectClone = (CloneDemo) clone.clone();
            objectClone.number = 10;
            objectClone.data = 5000;

            System.out.println("cloned object = " + objectClone);
            System.out.println("origin object = " + clone);
        } catch (CloneNotSupportedException e) {
            e.printStackTrace();
        }
    }

    public String toString() {
        return "[number = " + number + "; data = " + data + "]";
    }
}

How do I convert java.util.Set into Array?

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

import java.util.*;

public class SetToArray {
    public static void main(String[] args) {
        // Create a java.util.Set object and add some integers into the Set.
        Set<Integer> numberSet = new HashSet<>();
        numberSet.add(1);
        numberSet.add(2);
        numberSet.add(3);
        numberSet.add(5);
        numberSet.add(8);

        // Converting a java.util.Set into an array can be done by creating a
        // java.util.List object from the Set and then convert it into an array
        // by calling the toArray() method on the list object.
        List<Integer> numberList = new ArrayList<>(numberSet);
        Integer[] numbers = numberList.toArray(new Integer[0]);

        // Display the content of numbers array.
        for (int i = 0; i < numbers.length; i++) {
            Integer number = numbers[i];
            System.out.print(number + (i < numbers.length - 1 ? ", " : "\n"));
        }

        // Display the content of numbers array using the for-each loop.
        for (Integer number : numbers) {
            System.out.print(number + ", ");
        }
    }
}

How do I convert Set into List?

By Wayan in Core API, Util Package 0 Comment

The code below gives you an example of converting a java.util.Set into a java.util.List. It has simply done by creating a new instance of List and pass the Set as the argument of the constructor.

package org.kodejava.util;

import java.util.*;

public class SetToList {
    public static void main(String[] args) {
        // Create a Set and add some objects into the Set.
        Set<Object> set = new HashSet<>();
        set.add("A");
        set.add(10L);
        set.add(new Date());

        // Convert the Set to a List can be done by passing the Set instance into
        // the constructor of a List implementation class such as ArrayList.
        List<Object> list = new ArrayList<>(set);
        for (Object o : list) {
            System.out.println("Object = " + o);
        }
    }
}

The output of the code snippet are:

Object = A
Object = 10
Object = Mon Oct 04 20:20:26 CST 2021