How do I generate random alphanumeric strings?

The following code snippet demonstrates how to use RandomStringGenerator class from the Apache Commons Text library to generate random strings. To create an instance of the generator we can use the RandomStringGenerator.Builder() class build() method. The builder class also helps us to configure the properties of the generator. Before calling the build() method we can set the properties of the builder using the following methods:

  • withinRange() to specifies the minimum and maximum code points allowed in the generated string.
  • filteredBy() to limits the characters in the generated string to those that match at least one of the predicates supplied. Some enum for the predicates: CharacterPredicates.DIGITS, CharacterPredicates.LETTERS.
  • selectFrom() to limits the characters in the generated string to those who match at supplied list of Character.
  • usingRandom() to overrides the default source of randomness.

After configuring and building the generator based the properties defined, we can generate the random strings using the generate() methods of the RandomStringGenerator. There are two methods available:

  • generate(int length) generates a random string, containing the specified number of code points.
  • generate(int minLengthInclusive, int maxLengthInclusive) generates a random string, containing between the minimum (inclusive) and the maximum (inclusive) number of code points.

And here is your code snippet:

package org.kodejava.commons.text;

import org.apache.commons.text.CharacterPredicates;
import org.apache.commons.text.RandomStringGenerator;

public class RandomStringDemo {
    public static void main(String[] args) {
        RandomStringGenerator generator = new RandomStringGenerator.Builder()
                .withinRange('0', 'z')
                .filteredBy(CharacterPredicates.DIGITS, CharacterPredicates.LETTERS)

        for (int i = 0; i < 10; i++) {
            System.out.println(generator.generate(10, 20));

Below are examples of generated random alphanumeric strings:


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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 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.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 final 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', '-');

      "Name", 22),
      "Birth Date", 16),
      "Age", 6));

        for (Object[] data : people) {
                    data[0], data[1],
                    ChronoUnit.YEARS.between((LocalDate) data[1],;


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 |

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How do I generate random string?


import java.util.Random;

public class RandomString {
    public static final String SOURCES =

    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.
    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:


How to truncate a string after n number of words?

package org.kodejava.regex;

public class GetNumberOfWordsFromString {
    public static void main(String[] args) {
        String text = "The quick brown fox jumps over the lazy dog.";

        String one = truncateAfterWords(1, text);
        System.out.println("1 = " + one);

        String two = truncateAfterWords(2, text);
        System.out.println("2 = " + two);

        String four = truncateAfterWords(4, text);
        System.out.println("4 = " + four);

        String six = truncateAfterWords(6, text);
        System.out.println("6 = " + six);

     * Truncate a string after n number of words.
     * @param words number of words to truncate after.
     * @param text  the text.
     * @return truncated text.
    public static String truncateAfterWords(int words, String text) {
        String regex = String.format("^((?:\\W*\\w+){%s}).*$", words);
        return text.replaceAll(regex, "$1");

The result of the snippet:

1 = The
2 = The quick
4 = The quick brown fox
6 = The quick brown fox jumps over

How to remove non ASCII characters from a string?

The code snippet below remove the characters from a string that is not inside the range of x20 and x7E ASCII code. The regex below strips non-printable and control characters. But it also keeps the linefeed character n (x0A) and the carriage return r (x0D) characters.

package org.kodejava.regex;

public class ReplaceNonAscii {
    public static void main(String[] args) {
        String str = "Thè quïck brøwn føx jumps over the lãzy dôg.";
        System.out.println("str = " + str);

        // Replace all non ascii chars in the string.
        str = str.replaceAll("[^\\x0A\\x0D\\x20-\\x7E]", "");
        System.out.println("str = " + str);

Snippet output:

str = Thè quïck brøwn føx jumps over the lãzy dôg.
str = Th quck brwn fx jumps over the lzy dg.

How to split a string by a number of characters?

The following code snippet will show you how to split a string by numbers of characters. We create a method called splitToNChars() that takes two arguments. The first arguments is the string to be split and the second arguments is the split size.

This splitToNChars() method will split the string in a for loop. First we’ll create a List object that will store parts of the split string. Next we do a loop and get the substring for the defined size from the text and store it into the List. After the entire string is read we convert the List object into an array of String by using the List‘s toArray() method.

Let’s see the code snippet below:

package org.kodejava.lang;

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

public class SplitStringForEveryNChar {
    public static void main(String[] args) {

        System.out.println(Arrays.toString(splitToNChar(text, 3)));
        System.out.println(Arrays.toString(splitToNChar(text, 4)));
        System.out.println(Arrays.toString(splitToNChar(text, 5)));

     * Split text into n number of characters.
     * @param text the text to be split.
     * @param size the split size.
     * @return an array of the split text.
    private static String[] splitToNChar(String text, int size) {
        List<String> parts = new ArrayList<>();

        int length = text.length();
        for (int i = 0; i < length; i += size) {
            parts.add(text.substring(i, Math.min(length, i + size)));
        return parts.toArray(new String[0]);

When run the code snippet will output:


How do I split a string with multiple spaces?

This code snippet show you how to split string with multiple white-space characters. To split the string this way we use the "\s+" regular expression. The white-space characters include space, tab, line-feed, carriage-return, new line, form-feed.

Let’s see the code snippet below:

package org.kodejava.lang;

import java.util.Arrays;

public class SplitStringMultiSpaces {
    public static void main(String[] args) {
        String text = "04/11/2021    SHOES      RUNNING RED   99.9 USD";

        // Split the string using the \s+ regex to split multi spaces
        // line of text.
        String[] items = text.split("\\s+");
        System.out.println("Length = " + items.length);
        System.out.println("Items  = " + Arrays.toString(items));

The result of the code snippet is:

Length = 6
Items  = [04/11/2021, SHOES, RUNNING, RED, 99.9, USD]

How do I use string in switch statement?

Starting from Java 7 release you can now use a String in the switch statement. On the previous version we can only use constants type of byte, char, short, int (and their corresponding reference / wrapper type) or enum constants in the switch statement.

The code below give you a simple example on how the Java 7 extended to allow the use of String in switch statement.

package org.kodejava.basic;

public class StringInSwitchExample {
    public static void main(String[] args) {
        String day = "Sunday";
        switch (day) {
            case "Sunday":
            case "Monday":
            case "Tuesday":
            case "Wednesday":

How do I break a text or sentence into words?

At first, it might look simple. We can just split the text using the String.split(), the word is split using space. But what if a word ends with questions marks (?) or exclamation marks (!) instead? There might be some other rules that we also need to care.

Using the java.text.BreakIterator makes it much simpler. The class’s getWordInstance() factory method creates a BreakIterator instance for words break. Instantiating a BreakIterator and passing a locale information makes the iterator to breaks the text or sentence according the rule of the locale. This is really helpful when we are working with a complex language such as Japanese or Chinese.

Let us see an example of using the BreakIterator below.

package org.kodejava.text;

import java.text.BreakIterator;
import java.util.Locale;

public class BreakIteratorExample {
    public static void main(String[] args) {
        String data = "The quick brown fox jumps over the lazy dog.";
        String search = "dog";

        // Gets an instance of BreakIterator for word break for the
        // given locale. We can instantiate a BreakIterator without
        // specifying the locale. The locale is important when we
        // are working with languages like Japanese or Chinese where
        // the breaks standard may be different compared to English.
        BreakIterator bi = BreakIterator.getWordInstance(Locale.US);

        // Set the text string to be scanned.

        // Iterates the boundary / breaks
        System.out.println("Iterates each word: ");
        int count = 0;
        int lastIndex = bi.first();
        while (lastIndex != BreakIterator.DONE) {
            int firstIndex = lastIndex;
            lastIndex =;

            if (lastIndex != BreakIterator.DONE
                    && Character.isLetterOrDigit(data.charAt(firstIndex))) {
                String word = data.substring(firstIndex, lastIndex);
                System.out.printf("'%s' found at (%s, %s)%n",
                        word, firstIndex, lastIndex);

                // Counts how many times the word dog occurs.
                if (word.equalsIgnoreCase(search)) {

        System.out.println("Number of word '" + search + "' found = " + count);

Here are the program output:

Iterates each word: 
'The' found at (0, 3)
'quick' found at (4, 9)
'brown' found at (10, 15)
'fox' found at (16, 19)
'jumps' found at (20, 25)
'over' found at (26, 30)
'the' found at (31, 34)
'lazy' found at (35, 39)
'dog' found at (40, 43)
Number of word 'dog' found = 1

How do I get char value of a string at a specified position?

The chartAt() method of the String class returns the char value at the specified index. An index ranges from 0 to length() - 1. If we specified an index beyond of this range a StringIndexOutOfBoundsException exception will be thrown.

These method use zero based index which means the first char value of the sequence is at index 0, the next at index 1, and so on, as for array indexing.

package org.kodejava.lang;

public class CharAtExample {
    public static void main(String[] args) {
        String[] colors = {"black", "white", "brown", "green", "yellow", "blue"};

        for (String color : colors) {
            // Get char value of a string at index number 3. We'll get the 
            // fourth character of each color in the array because the 
            // index is zero based.
            char value = color.charAt(3);
            System.out.printf("The fourth char of %s is '%s'.%n", color, value);


Here is the program output:

The fourth char of black is 'c'
The fourth char of white is 't'
The fourth char of brown is 'w'
The fourth char of green is 'e'
The fourth char of yellow is 'l'
The fourth char of blue is 'e'