How do I generate random string?

package org.kodejava.example.security;

import java.security.SecureRandom;
import java.util.Random;

public class RandomString {
    public static final String SOURCES =
            "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";

    public static void main(String[] args) {
        RandomString rs = new RandomString();
        System.out.println(rs.generateString(new Random(), SOURCES, 10));
        System.out.println(rs.generateString(new Random(), SOURCES, 10));
        System.out.println(rs.generateString(new SecureRandom(), SOURCES, 15));
        System.out.println(rs.generateString(new SecureRandom(), SOURCES, 15));
    }

    /**
     * Generate a random string.
     *
     * @param random the random number generator.
     * @param characters the characters for generating string.
     * @param length the length of the generated string.
     * @return
     */
    public String generateString(Random random, String characters, int length) {
        char[] text = new char[length];
        for (int i = 0; i < length; i++) {
            text[i] = characters.charAt(random.nextInt(characters.length()));
        }
        return new String(text);
    }
}

Example string produced by the code snippets are:

uxEUFqTqS0
vr89vdF4gh
ysYF9XEHhO5FtDf
aBANhrLObZ1XLJi

How to check if an object reference is not null?

Usually, if not always, we use the if statement combined with == or != operators to check if an object reference is null or not. We do this to validate arguments passed to constructors or methods doesn’t contain a null value. These null check can be seen as clutter in our code.

The solution is to use the java.util.Objects class. This static utility class provides methods like requireNonNull(T) and requireNonNull(T, String) to check if the specified object reference is not null. If null these method will throw a NullPointerException. Using the second method variant we can customize the exception message.

The example below shows how we use these methods.

package org.kodejava.example.util;

import java.util.Objects;

public class ObjectsNullCheckDemo {
    private String firstName;
    private String lastName;

    /**
     * Validate constructor arguments. The firstName and lastName 
     * arguments can't be null. A NullPointerException with the 
     * specified message will be thrown.
     */
    public ObjectsNullCheckDemo(String firstName, String lastName) {
        this.firstName = Objects.requireNonNull(firstName, 
                "First name can't be null.");
        this.lastName = Objects.requireNonNull(lastName, 
                "Last name can't be null.");
    }

    public void setFirstName(String firstName) {
        // First name can't be null.
        this.firstName = Objects.requireNonNull(firstName,
                "First name can't be null.");
    }

    public void setLastName(String lastName) {
        // Last name can't be null.
        this.lastName = Objects.requireNonNull(lastName,
                "Last name can't be null.");
    }

    public static void main(String[] args) {
        // This line is fine.
        ObjectsNullCheckDemo demo = new ObjectsNullCheckDemo("John", "Doe");
        System.out.println("demo = " + demo);

        try {
            // This line produce a NullPointerException
            ObjectsNullCheckDemo demo1 = new ObjectsNullCheckDemo("Alice", null);
        } catch (Exception e) {
            e.printStackTrace();
        }

        String name = null;
        try {
            // The line below will throw java.lang.NullPointerException.
            Objects.requireNonNull(name);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public String toString() {
        return "ObjectsNullCheckDemo{" +
                "firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                '}';
    }
}

Running the code above will print the following result:

demo = ObjectsNullCheckDemo{firstName='John', lastName='Doe'}
java.lang.NullPointerException: Last name can't be null.
    at java.util.Objects.requireNonNull(Objects.java:228)
    at org.kodejava.example.util.ObjectsNullCheckDemo.(ObjectsNullCheckDemo.java:14)
    at org.kodejava.example.util.ObjectsNullCheckDemo.main(ObjectsNullCheckDemo.java:34)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
java.lang.NullPointerException
    at java.util.Objects.requireNonNull(Objects.java:203)
    at org.kodejava.example.util.ObjectsNullCheckDemo.main(ObjectsNullCheckDemo.java:42)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:483)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

How do I create a custom TemporalAdjuster?

In this example we are going to learn how to implement a custom TemporalAdjuster. We are going to create TemporalAdjuster to find the next working day from a specified date. We will use 5 working days, from Monday to Friday.

The custom temporal adjuster class should implement the TemporalAdjuster interface, which define a single method that we must implement, the adjustInto(Temporal) method.

package org.kodejava.example.datetime;

import java.time.DayOfWeek;
import java.time.temporal.ChronoField;
import java.time.temporal.ChronoUnit;
import java.time.temporal.Temporal;
import java.time.temporal.TemporalAdjuster;

public class NextWorkingDayAdjuster implements TemporalAdjuster {
    @Override
    public Temporal adjustInto(Temporal temporal) {
        int field = temporal.get(ChronoField.DAY_OF_WEEK);
        DayOfWeek dayOfWeek = DayOfWeek.of(field);

        int daysToAdd = 1;
        if (DayOfWeek.FRIDAY.equals(dayOfWeek)) {
            daysToAdd = 3;
        } else if (DayOfWeek.SATURDAY.equals(dayOfWeek)) {
            daysToAdd = 2;
        }
        return temporal.plus(daysToAdd, ChronoUnit.DAYS);
    }
}

The NextWorkingDayAdjuster move the temporal object a day forward. Except if it is on Friday or Saturday, which will move the temporal object three days or two days forward respectively. This will make it return Monday as the next working day.

After creating the custom adjuster, now let’s create an example that use the NextWorkingDayAdjuster class.

package org.kodejava.example.datetime;

import java.time.LocalDate;
import java.time.Month;
import java.time.temporal.TemporalAdjuster;

public class NextWorkingDayAdjusterDemo {
    public static void main(String[] args) {
        TemporalAdjuster nextWorkingDay = new NextWorkingDayAdjuster();

        LocalDate now = LocalDate.now();
        LocalDate nextDay = now.with(nextWorkingDay);
        System.out.println("now            = " + now);
        System.out.println("nextWorkingDay = " + nextDay);

        LocalDate friday = LocalDate.of(2016, Month.MARCH, 11);
        nextDay = friday.with(nextWorkingDay);
        System.out.println("friday         = " + friday);
        System.out.println("nextWorkingDay = " + nextDay);

        LocalDate saturday = LocalDate.of(2016, Month.MARCH, 12);
        nextDay = saturday.with(nextWorkingDay);
        System.out.println("saturday       = " + saturday);
        System.out.println("nextWorkingDay = " + nextDay);
    }
}

And here are the results of our code:

now            = 2016-03-10
nextWorkingDay = 2016-03-11
friday         = 2016-03-11
nextWorkingDay = 2016-03-14
saturday       = 2016-03-12
nextWorkingDay = 2016-03-14

Using format flags to format negative number in parentheses

In this example we are going to learn to use a java.util.Formatter to format negative number in parentheses. The Formatter can use a format flags to format a value. To display a negative number in parentheses we can user the ( flag. This flag display negative number inside parentheses instead of using the - symbol.

The following code snippet below will show you how to do it. We start the example by using the Formatter object and simplified using the format() method of the String class.

package org.kodejava.example.util;

import java.util.Formatter;
import java.util.Locale;

public class FormatNegativeNumber {
    public static void main(String[] args) {
        // Creates an instance of Formatter, format the number using the
        // format and print out the result.
        Formatter formatter = new Formatter();
        formatter.format("%(,.2f", -199.99f);
        System.out.println("number1 = " + formatter);

        // Use String.format() method instead of creating an instance of
        // Formatter. Format a negative number using Germany locale.
        String number2 = String.format(Locale.GERMANY, "%(,8.2f", -49.99);
        System.out.println("number2 = " + number2);

        // Format number using Indonesian locale. The thousand separator is "."
        // in Indonesian number.
        String number3 = String.format(new Locale("id", "ID"), "%(,d", -10000);
        System.out.println("number3 = " + number3);
    }
}

The result of this code snippet:

number1 = (199.99)
number2 =  (49,99)
number3 = (10.000)

How do I manipulate LocalDate object using TemporalAdjuster?

In the previous example we manipulate the value of LocalDate by adding or subtracting the value of date object by days, months, years using methods like plusMonths() or minusDays(). Or by changing the year or the month of the date object using methods like withYear() or withMonth().

But there are times that we want to manipulate the date object so that we can get the first day of the month or the last day of the month. We want to manipulate the date value to advance the date to the first Monday after the current day or the last the of the year.

To manipulate the date object in this way we can use the with() method and pass a TemporalAdjuster object as an argument. Fortunately, the Date and Time API already provide some commonly used TemporalAdjuster. These TemporalAdjuster are provided as a static factory methods that we can find in the java.time.temporal.TemporalAdjusters class.

The following example is a code snippet to manipulate the date object using TemporalAdjuster / TemporalAdjusters class.

package org.kodejava.example.datetime;

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.temporal.TemporalAdjusters;

public class DateManipulationWithTemporalAdjuster {
    public static void main(String[] args) {
        LocalDate date = LocalDate.now();
        System.out.println("Current date       = " + date);

        LocalDate date1 = date.with(TemporalAdjusters.firstDayOfMonth());
        System.out.println("First day of month = " + date1);

        LocalDate date2 = date.with(TemporalAdjusters.lastDayOfMonth());
        System.out.println("Last day of month  = " + date2);

        LocalDate date3 = date.with(TemporalAdjusters.next(DayOfWeek.MONDAY));
        System.out.println("Next Monday        = " + date3);

        LocalDate date4 = date.with(TemporalAdjusters.lastDayOfYear());
        System.out.println("Last day of year   = " + date4);
    }
}

The result of the code snippet are:

Current date       = 2016-03-06
First day of month = 2016-03-01
Last day of month  = 2016-03-31
Next Monday        = 2016-03-07
Last day of year   = 2016-12-31

The table below shows the complete of static factory method provided by the TemporalAdjusters class.

Method Name Method Description
dayOfWeekInMonth Returns a new date in the same month with the ordinal day-of-week.
firstDayOfMonth Returns a new date set to the first day of the current month.
firstDayOfNextMonth Returns a new date set to the first day of the next month.
firstDayOfNextYear Returns a new date set to the first day of the next year.
firstDayOfYear Returns a new date set to the first day of the current year.
firstInMonth Returns a new date in the same month with the first matching day-of-week.
lastDayOfMonth Returns a new date set to the last day of the current month.
lastDayOfYear Returns a new date set to the last day of the current year.
lastInMonth Returns a new date in the same month with the last matching day-of-week.
next Returns the next day-of-week adjuster.
nextOrSame Returns the next-or-same day-of-week adjuster.
ofDateAdjuster Returns user-written adjuster.
previous Returns the previous day-of-week adjuster.
previousOrSame Returns the previous-or-same day-of-week adjuster.

How do I install third-party libraries in Maven repository?

Sometimes when the required libraries / dependencies is not available in the Maven Central Repository we need to manually install it to our local repository. This library must be placed in the correct directory in our local repository to enable Maven to find it. The default location is under the ${user.home}/.m2/repository.

To make this job easier Maven provide a maven-install-plugin that will help us to install the third-party library in the correct place. The following command shows how to do it.

The long command 🙂

mvn install:install-file -Dfile=<path-to-file> -DgroupId=<group-id> \
        -DartifactId=<artifact-id> -Dversion=<version> -Dpackaging=<packaging>

Where:

  • -Dfile = path to the third-party library file
  • -DgroupId = the groupId of the library
  • -DartifactId = the artifactId of the library
  • -Dversion = the version number of the library
  • -Dpackaging = the library packaging

An example to install an Oracle JDBC library to your local repository is:

mvn install:install-file -Dfile=ojdbc7.jar -DgroupId=com.oracle \
        -DartifactId=ojdbc7 -Dversion=12.1.0.2 -Dpackaging=jar

The simple command 🙂

If you have the pom.xml file, you can install it with the following command:

mvn install:install-file \
        -Dfile=<path-to-file> \
        -DpomFile=<path-to-pomfile>

Where:

  • -Dfile = path to the third-party library file
  • -DpomFile = the location to the library pom.xml file

Starting with the Maven version 2.5 you can use even a simpler command. When the library is build by Maven, a pom.xml file will be placed under the META-INF directory. This pom.xml file will be used by default when we install the library. To install a library all you need is the following command:

mvn install:install-file -Dfile=<path-to-file>

How do I manipulate the value of LocalDate object?

In the following example we will learn how to manipulate a LocalDate object. There are many methods available for us to change the value of a LocalDate object. For example we can change the year, month and day of LocalDate object. We can use methods like withYear(), withDayOfMonth(), plusYears(), minusMonths(), etc. All these methods will return a new LocalDate object, the original LocalDate will stay unchanged.

Let’s see the following code example for demonstration on how to manipulate the value of LocalDate object.

package org.kodejava.example.datetime;

import java.time.LocalDate;
import java.time.Month;
import java.time.temporal.ChronoField;
import java.time.temporal.ChronoUnit;

public class LocalDateManipulation {
    public static void main(String[] args) {
        absoluteAttributeManipulations();
        relativeAttributeManipulations();
    }

    private static void relativeAttributeManipulations() {
        System.out.println("LocalDateManipulation.relativeAttributeManipulations");
        LocalDate date1 = LocalDate.of(2015, Month.AUGUST, 17);
        LocalDate date2 = date1.minusYears(70);
        LocalDate date3 = date2.plusMonths(10);
        LocalDate date4 = date3.minusDays(15);
        LocalDate date5 = date4.plusWeeks(52);
        LocalDate date6 = date5.minus(52, ChronoUnit.WEEKS);

        System.out.println("of(2015, Month.AUGUST, 17)        => " + date1);
        System.out.println("date1.minusYears(70)              => " + date2);
        System.out.println("date1.plusMonths(10)              => " + date3);
        System.out.println("date3.minusDays(15)               => " + date4);
        System.out.println("date4.plusWeeks(52)               => " + date5);
        System.out.println("date5.minus(52, ChronoUnit.WEEKS) => " + date6);

    }

    private static void absoluteAttributeManipulations() {
        System.out.println("LocalDateManipulation.absoluteAttributeManipulations");
        LocalDate date1 = LocalDate.of(2016, Month.JANUARY, 1);
        LocalDate date2 = date1.withYear(2010);
        LocalDate date3 = date2.withMonth(Month.DECEMBER.getValue());
        LocalDate date4 = date3.withDayOfMonth(15);
        LocalDate date5 = date4.with(ChronoField.DAY_OF_YEAR, 100);

        System.out.println("of(2016, Month.JANUARY, 1)                 => " + date1);
        System.out.println("date1.withYear(2010)                       => " + date2);
        System.out.println("date2.withMonth(Month.DECEMBER.getValue()) => " + date3);
        System.out.println("date3.withDayOfMonth(15)                   => " + date4);
        System.out.println("date4.with(ChronoField.DAY_OF_YEAR, 100)   => " + date5);
    }
}

The results of this code snippet are:

LocalDateManipulation.absoluteAttributeManipulations
of(2016, Month.JANUARY, 1)                 => 2016-01-01
date1.withYear(2010)                       => 2010-01-01
date2.withMonth(Month.DECEMBER.getValue()) => 2010-12-01
date3.withDayOfMonth(15)                   => 2010-12-15
date4.with(ChronoField.DAY_OF_YEAR, 100)   => 2010-04-10

LocalDateManipulation.relativeAttributeManipulations
of(2015, Month.AUGUST, 17)        => 2015-08-17
date1.minusYears(70)              => 1945-08-17
date1.plusMonths(10)              => 1946-06-17
date3.minusDays(15)               => 1946-06-02
date4.plusWeeks(52)               => 1947-06-01
date5.minus(52, ChronoUnit.WEEKS) => 1946-06-02

How do I run FTP server in OS X?

I need to test FTP client codes, so I need to find an FTP server for testing my codes. After searching for a while I find out that OS X already equipped FTP server. I am currently using OS X El Capitan 10.11.*.

Let’s now test the FTP server on OS X with the following steps:

  • Launch the Terminal.app
  • Type the following command to start the FTP server.
sudo -s launchctl load -w /System/Library/LaunchDaemons/ftp.plist
  • Connect to the FTP server by running ftp localhost command.
  • We’ll be asked to enter the username and password.
$ ftp localhost
Trying ::1...
Connected to localhost.
220 ::1 FTP server (tnftpd 20100324+GSSAPI) ready.
Name (localhost:wsaryada): wsaryada
331 User wsaryada accepted, provide password.
Password: 
230 User wsaryada logged in.
Remote system type is UNIX.
Using binary mode to transfer files.
ftp>
  • If we see the messages above and the ftp> prompt means that the FTP server works and ready to accept our command.
  • We can also try to access the FTP server using a browser. In the URL box type ftp://localhost to connect. We need to supply username and password to login.
  • To exit or close the connection to FTP server we can run the exit command.
  • Finally, to shutdown the FTP server we run:
sudo -s launchctl unload -w /System/Library/LaunchDaemons/ftp.plist

After the FTP server ready, I can now continue to create some test program to access the FTP server. There are already some examples you can find in the Apache Commons Net category that use the FTPClient library to access FTP server.

How to use Google’s Maven Central mirror?

The following configuration will use Google’s mirror of the Maven Central repository. Alter your ${M2_HOME}/conf/settings.xml or ${user.home}/.m2/settings.xml to add the mirror as seen in the following configuration file.

<?xml version="1.0" encoding="UTF-8"?>
<settings>
    .
    .
    <mirrors>
        <mirror>
            <id>google-maven-central</id>
            <name>Google Maven Central</name>
            <url>//maven-central.storage.googleapis.com</url>
            <mirrorOf>central</mirrorOf>
        </mirror>
    </mirrors>
    .
    .
</settings>

How to configure a proxy in Maven settings?

When we work behind a proxy server we need to configure Maven to be able to connect to the internet. To enable proxy we can configure Maven settings.xml file, either in ${M2_HOME}/conf/settings.xml or ${user.home}/.m2/settings.xml file.

<?xml version="1.0" encoding="UTF-8"?>
<settings>
    .
    .
    <proxies>
        <proxy>
            <id>my-proxy</id>
            <active>true</active>
            <protocol>http</protocol>
            <host>proxy.example.org</host>
            <port>8080</port>
            <username>username</username>
            <password>password</password>
            <nonProxyHosts>*.example.org|*.example.com</nonProxyHosts>
        </proxy>
    </proxies>
    .
    .
</settings>

The <proxy> element in the configuration above contains the information about the proxy server. These include information about the host, port, username and password. Set these elements to match your proxy server configuration.