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

<!-- http://repo1.maven.org/maven2/org/apache/commons/commons-lang3/3.6/commons-lang3-3.6.jar -->
<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.6</version>
</dependency>

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 customise 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)