## How do I convert number into Roman Numerals?

You want to convert numbers into their Roman numerals representation and vice versa. The solution here is to tackle the problem as a unary problem where the Roman numerals represented as a single element, the “I” character. We start by representing the number as a repeated sequence of the “I” characters. And then replace the characters according to next bigger symbol in roman numeral.

To convert from the Roman numerals to numbers we reverse the process. By the end of the process we will get a sequence of repeated “I” characters. The length of the final string returned by this process is the result of the roman numerals conversion to number.

In the code snippet below we create two methods. The `toRoman(int number)` method for converting number to roman numerals and the `toNumber(String roman)` method for converting from roman numerals to number. Both of this method utilize the `String.replace()` method for calculating the conversion result.

Let’s see the code in action.

``````package org.kodejava.example.lang;

public class RomanNumber {
public static void main(String[] args) {
for (int n = 1; n <= 4999; n++) {
String roman = RomanNumber.toRoman(n);
int number = RomanNumber.toNumber(roman);

System.out.println(number + " = " + roman);
}
}

private static String toRoman(int number) {
return String.valueOf(new char[number]).replace('\0', 'I')
.replace("IIIII", "V")
.replace("IIII", "IV")
.replace("VV", "X")
.replace("VIV", "IX")
.replace("XXXXX", "L")
.replace("XXXX", "XL")
.replace("LL", "C")
.replace("LXL", "XC")
.replace("CCCCC", "D")
.replace("CCCC", "CD")
.replace("DD", "M")
.replace("DCD", "CM");
}

private static Integer toNumber(String roman) {
return roman.replace("CM", "DCD")
.replace("M", "DD")
.replace("CD", "CCCC")
.replace("D", "CCCCC")
.replace("XC", "LXL")
.replace("C", "LL")
.replace("XL", "XXXX")
.replace("L", "XXXXX")
.replace("IX", "VIV")
.replace("X", "VV")
.replace("IV", "IIII")
.replace("V", "IIIII").length();
}
}
``````

The 10 randoms result of the conversion listed below:

```18 = XVIII
208 = CCVIII
843 = DCCCXLIII
1995 = MCMXCV
2000 = MM
2017 = MMXVII
2562 = MMDLXII
3276 = MMMCCLXXVI
4067 = MMMMLXVII
4994 = MMMMCMXCIV
```

## How do I align string print out in left, right, center alignment?

The following code snippet will teach you how to align string in left, right or center alignment when you want to print out string to a console. We will print the string using the `printf(String format, Object... args)` method. The `format` specifier / parameter defines how the string will be formatted for output and the `args` is the value that will be formatted.

The `format` parameter / specifier include flags, width, precision and conversion-characters in the order shown below. The square brackets in the notation means the part is an optional parameter.

``````% [flags] [width] [.precision] conversion-character
``````
Flags Description
`-` left-align the output, when not specified the default is to right-align
`+` print (`+`) or (`-`) sign for numeric value
`0` zero padded a numeric value
`,` comma grouping separator for number greater that 1000
space will output a (`-`) symbol for negative value and a space if positive
Conversion Description
`s` string, use capital `S` to uppercase the strings
`c` character, use capital `C` to uppercase the characters
`d` integer: `byte`, `short`, `integer`, `long`
`f` floating point number: `float`, `double`
`n` new line

Width: Defines the field width for printing out the value of argument. It also represents the minimum number of characters to
be printed out to the output.

Precision: For floating-point conversion the precision define the number of digits of precision in a floating point value. For string value this will extract the substring.

To center the string for output we use the `StringUtils.center()` method from the Apache Commons Lang library. This method will center-align the string `str` in a larger string of `size` using the default space character (‘ ‘). You can supply the third parameter to define your own space character / string.

``````package org.kodejava.example.lang;

import org.apache.commons.lang3.StringUtils;

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

public class StringAlignment {
private static Object[][] people = {
{"Alice", LocalDate.of(2000, Month.JANUARY, 1)},
{"Bob", LocalDate.of(1989, Month.DECEMBER, 15)},
{"Carol", LocalDate.of(1992, Month.JULY, 24)},
{"Ted", LocalDate.of(2006, Month.MARCH, 13)},
};

public static void main(String[] args) {
String nameFormat = "| %1\$-20s | ";
String dateFormat = " %2\$tb %2\$td, %2\$tY  | ";
String ageFormat = " %3\$3s |%n";
String format = nameFormat.concat(dateFormat).concat(ageFormat);
String line = new String(new char[48]).replace('\0', '-');

System.out.println(line);
System.out.printf("|%s|%s|%s|%n",
StringUtils.center("Name", 22),
StringUtils.center("Birth Date", 16),
StringUtils.center("Age", 6));
System.out.println(line);

for (Object[] data : people) {
System.out.printf(format,
data[0], data[1],
ChronoUnit.YEARS.between((LocalDate) data[1], LocalDate.now()));
}

System.out.println(line);
}
}
``````

Here is the output of our code snippet above:

```------------------------------------------------
|         Name         |   Birth Date   | Age  |
------------------------------------------------
| Alice                |  Jan 01, 2000  |   17 |
| Bob                  |  Dec 15, 1989  |   27 |
| Carol                |  Jul 24, 1992  |   24 |
| Ted                  |  Mar 13, 2006  |   10 |
------------------------------------------------
```

Maven Dependencies

``````<dependency>
<groupId>org.apache.commons</groupId>
<artifactId>commons-lang3</artifactId>
<version>3.5</version>
</dependency>
``````

## 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;

@Override
int field = temporal.get(ChronoField.DAY_OF_WEEK);
DayOfWeek dayOfWeek = DayOfWeek.of(field);

if (DayOfWeek.FRIDAY.equals(dayOfWeek)) {
} else if (DayOfWeek.SATURDAY.equals(dayOfWeek)) {
}
}
}
``````

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;

public static void main(String[] args) {

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;

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

System.out.println("First day of month = " + date1);

System.out.println("Last day of month  = " + date2);

System.out.println("Next Monday        = " + date3);

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 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 load file from resource directory?

In the following code snippet we will learn how to load files from resource directory or folder. Resource files can be in a form of image, audio, text, etc. Text resource file for example can be use to store application configurations, such as database configuration.

To load this resource file you can use a couple methods utilizing the `java.lang.Class` methods or the `java.lang.ClassLoader` methods. Both `Class` and `ClassLoader` provides `getResource()` and `getResourceAsStream()` methods to load resource file. The first method return a `URL` object while the second method return an `InputStream`.

When using the `Class` method, if the resource name started with “`/`” that identifies it is an absolute name. Absolute name means that it will load from the specified directory name or package name. While if it is not started with “`/`” then it is identified as a relative name. This means that it will look in the same package as the class that tries to load the resource.

``````App.class.getResource("database.conf");
``````

The snippet will attempt to load the resource file from the same package as the `App` class. If the `App` class package is `org.kodejava` then the `database.conf` file must be located at `/org/kodejava/`. This is the relative resource name.

``````App.class.getResource("/org/kodejava/conf/database.conf"):
``````

The snippet will attempt to load the resource file from the given package name. You should place the configuration file under `/org/kodejava/conf/` to enable the application to load it. This is the absolute resource name. Below is a snippet that use the `Class` method to load resources.

``````private void loadUsingClassMethod() throws IOException {
Properties properties = new Properties();

// This actually load resource from
// "/org/kodejava/example/lang/database.conf".
URL resource = getClass().getResource("database.conf");
System.out.println("JDBC Driver: " + properties.get("jdbc.driver"));

// from the root of the package. This will load "/database.conf".
InputStream is = getClass().getResourceAsStream("/database.conf");
System.out.println("JDBC Driver: " + properties.get("jdbc.driver"));
}
``````

When we use the `ClassLoader` method the resource name should not begins with a “`/`“. This method will not apply any absolute / relative transformation to the resource name like the `Class` method. Here a snippet of a method that use the `ClassLoader` method.

``````private void loadUsingClassLoaderMethod() throws IOException {
Properties properties = new Properties();

// When using the ClassLoader method the resource name should
// not started with "/". This method will not apply any
// absolute/relative transformation to the resource name.
System.out.println("JDBC URL: " + properties.get("jdbc.url"));

System.out.println("JDBC URL: " + properties.get("jdbc.url"));
}
``````

Below is the main program that calls the methods above.

``````package org.kodejava.example.lang;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.Properties;

public static void main(String[] args) throws Exception {
}
}
``````

In the snippet above we load two difference resources. One contains Oracle database configuration and the other is MySQL database configuration.

`/resources/org/kodejava/example/lang/database.conf`

```jdbc.driver=oracle.jdbc.driver.OracleDriver
jdbc.url=jdbc:oracle:thin:@localhost:1521:xe
```

`/resources/database.conf`

```jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost/kodejava
```

The result of this code snippet are:

```LoadResourceFile.loadUsingClassMethod
JDBC Driver: oracle.jdbc.driver.OracleDriver
JDBC Driver: com.mysql.jdbc.Driver

JDBC URL: jdbc:mysql://localhost/kodejava
JDBC URL: jdbc:mysql://localhost/kodejava
```

## How do I fill array with non-default value?

This code snippet will show you how to create array variable and initialized it with a non-default value. By default, when we create an array of something in Java all entries will have its default value. For primitive types like `int`, `long`, `float` the default value are zero (`0` or `0.0`). For reference types (anything that holds an object in it) will have `null` as the default value. For boolean variable it will be `false`.

If you want to initialize the array to different value you can use the `Arrays.fill()` method. This method will help you to set the value for every elements of the array.

Let see the following code snippet as an example:

``````package org.kodejava.example.util;

import java.util.Arrays;

public class ArraysFillExample {
public static void main(String[] args) {
// Assign -1 to each elements of numbers array
int[] numbers = new int[5];
Arrays.fill(numbers, -1);
System.out.println("Numbers: " + Arrays.toString(numbers));

// Assign 1.0f to each elements of prices array
float[] prices = new float[5];
Arrays.fill(prices, 1.0f);
System.out.println("Prices : " + Arrays.toString(prices));

// Assign empty string to each elements of words array
String[] words = new String[5];
Arrays.fill(words, "");
System.out.println("Words  : " + Arrays.toString(words));

// Assign 9 to each elements of the multi array
int[][] multi = new int[3][3];
for (int[] array : multi) {
Arrays.fill(array, 9);
}
System.out.println("Multi  : " + Arrays.deepToString(multi));
}
}
``````

In the code snippet above we utilize the `Arrays.fill()` utility method to assign value for each elements of the `int`, `float` and `String` array. To change the default value of multi dimensional array we can’t directly call the `Arrays.fill()` method. In the example we use for-loop to set each elements of the sub-array using the `Arrays.fill()` method.

The output of the code snippet above are:

```Numbers: [-1, -1, -1, -1, -1]
Prices : [1.0, 1.0, 1.0, 1.0, 1.0]
Words  : [, , , , ]
Multi  : [[9, 9, 9], [9, 9, 9], [9, 9, 9]]
```