How do I use Kotlin data types like String, Int, and Boolean?

In Kotlin, data types like String, Int, and Boolean are fundamental types that are used to work with text, numbers, and logical values respectively. Kotlin provides these types as part of its standard library, and they are straightforward to use. Below is an explanation of how to work with them:

1. String

A String in Kotlin represents a sequence of characters. You can create and manipulate strings easily.

Example:

fun main() {
    val name: String = "Kotlin"
    val greeting = "Hello, $name!" // String interpolation
    val length = name.length       // Access string property

    println(greeting)  // Output: Hello, Kotlin!
    println("Length of name: $length")  // Output: Length of name: 6

    // Multi-line string
    val multiLineString = """
        This is a
        multi-line string.
    """.trimIndent()

    println(multiLineString)
}

Key Features of Strings:

  • String interpolation ($) to include variables or expressions within a string.
  • Supports multi-line strings using triple quotes (""").
  • String manipulation methods, e.g., .length, .substring(), .toUpperCase(), etc.

2. Int

An Int is a basic data type representing a 32-bit integer in Kotlin. It is used for whole numbers.

Example:

fun main() {
    val number: Int = 42
    val doubled = number * 2
    val isEven = number % 2 == 0

    println("Number: $number")  // Output: Number: 42
    println("Doubled: $doubled")  // Output: Doubled: 84
    println("Is even? $isEven")  // Output: Is even? true
}

Key Points:

  • Int is one of the numeric data types, which also include Long, Short, Byte, Double, and Float.
  • Kotlin handles mathematical operations with Int and other numeric types directly.
  • Reading and writing numbers are simple, and type conversion can be performed using .toInt(), .toDouble(), etc.

3. Boolean

A Boolean in Kotlin represents a value that is either true or false.

Example:

fun main() {
    val isKotlinFun: Boolean = true
    val isJavaFun = false

    println("Is Kotlin fun? $isKotlinFun")  // Output: Is Kotlin fun? true
    println("Is Java fun? $isJavaFun")      // Output: Is Java fun? false

    // Logical operations
    val bothFun = isKotlinFun && isJavaFun  // AND operation
    val eitherFun = isKotlinFun || isJavaFun // OR operation

    println("Both fun? $bothFun")  // Output: Both fun? false
    println("Either fun? $eitherFun") // Output: Either fun? true
}

Key Points:

  • Booleans are used for logical operations and for controlling conditions in if statements, loops, etc.
  • Supports logical operators: && (AND), || (OR), ! (NOT).

Type Inference

In Kotlin, thanks to type inference, you don’t always need to explicitly specify the type of a variable. The compiler can infer the type based on the assigned value.

Example:

fun main() {
    val name = "Kotlin"  // Automatically inferred as String
    val age = 25         // Automatically inferred as Int
    val isActive = true  // Automatically inferred as Boolean

    println(name)
    println(age)
    println(isActive)
}

Additional Tips

  • Type Conversion: When converting between data types (e.g., String to Int), use methods like toInt(), toDouble(), etc.
    val number = "123".toInt()  // Converts String to Int
    val decimal = 3.14.toString()  // Converts Double to String
    
  • Null Safety: These types are non-null by default. If you want a variable to hold a null value, use the nullable types (e.g., String?, Int?, Boolean?).
    val nullableString: String? = null
    

With those basics, you are ready to work with String, Int, and Boolean in Kotlin!

What is the purpose of String.strip() method of Java 11?

The purpose of the String.strip() method in Java 11 is to remove whitespaces from both the beginning and end of a string. This is very similar to the String.trim() method available in earlier versions of Java, but there is a key difference between them.

Here’s the difference:

  • String.strip(): Introduced in Java 11, strip() uses the unicode definition of whitespace. It removes not only space characters but also all other types of unicode-defined spaces, such as the thin space \u2009, etc.
  • String.trim(): Available from Java 1.0, trim() is more limited. It considers a whitespace to be any character whose ASCII value is less than or equal to 32 (a space, tab, newline, and a few other control characters).

Here are examples of how they work:

package org.kodejava.lang;

public class StringStripExample {
    public static void main(String[] args) {
        // String.strip()
        String first = " \u2009Hello  ";
        System.out.println(first.strip()); // Outputs "Hello"

        // String.trim()
        String second = " \u2009Hello  ";
        System.out.println(second.trim()); // Outputs "\u2009Hello"
    }
}

Output:

Hello
 Hello

Thus, strip() method is more comprehensive in removing different types of whitespace defined in Unicode, while trim() only removes ASCII control characters and spaces.

There are also String.stripLeading() and String.stripTrailing() methods that were introduced in Java 11, and they are similar to the strip() method, but they only remove the whitespace characters from either the beginning or the end of the string, respectively.

Here is what they do:

  • String.stripLeading(): This method removes any leading whitespace from the string. “Leading” in this context means any whitespace characters at the beginning of the string.
  • String.stripTrailing(): This method removes any trailing whitespace from the string. “Trailing” in this context means any whitespace characters at the end of the string.

Both stripLeading() and stripTrailing() use the Unicode definition of whitespace, the same as strip() method.

Here are examples of how they work:

package org.kodejava.lang;

public class StringStripLeadingTrailingExample {
    public static void main(String[] args) {
        // Strip leading whitespace
        String first = " \u2009Hello World  ";
        System.out.println(first.stripLeading());  // Outputs "Hello World  "

        // Strip trailing whitespace
        String second = " \u2009Hello World  ";
        System.out.println(second.stripTrailing()); // Outputs " \u2009Hello World"
    }
}

Output:

Hello World  
  Hello World

As demonstrated, stripLeading() removed the whitespace characters from the front of the string, and stripTrailing() removed the whitespace characters from the end of the string.

While \u00A0 is technically a type of whitespace (specifically, a non-breaking space or NBSP), it isn’t considered as such by the strip(), stripLeading(), and stripTrailing() methods, which follow the Character.isWhitespace(char) method’s definition of what constitutes a whitespace character.

According to the Java documentation, the Character.isWhitespace(char) method, which the strip() methods use, considers the following characters as whitespace:

  • ‘\t’ U+0009 HORIZONTAL TABULATION
  • ‘\n’ U+000A LINE FEED
  • ‘\u000B’ U+000B VERTICAL TABULATION
  • ‘\f’ U+000C FORM FEED
  • ‘\r’ U+000D CARRIAGE RETURN
  • ‘\u001C’ U+001C FILE SEPARATOR
  • ‘\u001D’ U+001D GROUP SEPARATOR
  • ‘\u001E’ U+001E RECORD SEPARATOR
  • ‘\u001F’ U+001F UNIT SEPARATOR
  • SPACE_SEPARATOR category types

The \u2009 (thin space) and \u0020 (space) are part of SPACE_SEPARATOR category according to Unicode standard and will be correctly stripped.

The \u00A0 (non-breaking space) is actually part of a different category called the NO-BREAK_SPACE and is not considered whitespace by Character.isWhitespace(char), so it won’t be stripped.

How do I use String.join() method in Java?

The String.join() method in Java is a static method added in Java 8 to the java.lang.String class. The String.join() is a static utility method used to concatenate multiple strings, arrays or collections (like lists and sets) of strings. This method makes it easier to join multiple strings with a specific delimiter. A delimiter is a sequence of characters used to separate strings.

This method returns a new String composed of copies of the CharSequence elements joined together with a copy of the specified delimiter. This method saves us from writing boilerplate loop code just for concatenating strings with a delimiter.

Here is an example of how you can use it:

package org.kodejava.lang;

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

public class StringJoinList {
    public static void main(String[] args) {
        List<String> list = Arrays.asList("Java", "is", "cool");
        String result = String.join(" ", list);
        System.out.println(result);
    }
}

Output:

Java is cool

In this example, String.join() takes two parameters:

  1. A delimiter that is a CharSequence (like a String) that is placed between each joined String.
  2. An Iterable object like a List or a Set, over which the method iterates and joins all elements into a single String.

You can also use String.join() with an array of elements:

package org.kodejava.lang;

public class StringJoinArray {
    public static void main(String[] args) {
        String[] array = new String[]{"Java", "is", "cool"};
        String result = String.join(" ", array);
        System.out.println(result);
    }
}

Output:

Java is cool

In this case, String.join() still takes a delimiter as the first argument, but the second argument is an Array of elements to be joined.

How do I create a string of repeated characters?

The following code demonstrates how to create a string of repeated characters. We use the String.repeat(int count) method introduced in Java 11. This method takes one parameter of type int which is the number of times to repeat the string. The count must be a positive number, a negative number will cause this method to throw java.lang.IllegalArgumentException.

In the snippet below, we use the method to repeat characters and draw some triangles. We combine the repeat() method with a for loop to draw the triangles.

package org.kodejava.basic;

public class StringRepeatDemo {
    public static void main(String[] args) {
        String star = "*";
        String fiveStars = star.repeat(5);
        System.out.println("fiveStars = " + fiveStars);

        String arrow = "-->";
        String arrows = arrow.repeat(10);
        System.out.println("arrows    = " + arrows);
    }
}

The outputs of the code snippet above are:

fiveStars = *****
arrows    = -->-->-->-->-->-->-->-->-->-->
package org.kodejava.basic;

public class StringRepeatDemo {
    public static void main(String[] args) {
        String asterisk = "#";
        for (int i = 1; i <= 10; i++) {
            System.out.println(asterisk.repeat(i));
        }
}

The outputs of the code snippet above are:

#
##
###
####
#####
######
#######
########
#########
##########
package org.kodejava.basic;

public class StringRepeatDemo {
    public static void main(String[] args) {
        int height = 10;
        for (int i = 1, j = 1; i <= height; i++, j += 2) {
            System.out.println(" ".repeat(height - i) + "*".repeat(j));
        }
    }
}

The outputs of the code snippet above are:

         *
        ***
       *****
      *******
     *********
    ***********
   *************
  ***************
 *****************
*******************

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)
                .build();

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

Below are examples of generated random alphanumeric strings:

weJDtVARLIFS96WXje
FYrNzTR3Q3dUrLT3Xsc
4F1fu8nSsA
nIQi3a4Oyv9
l6QcsP9bejdbaLd2jd
Cc9YgTfgwo
2B8un8YCcxn9m2
RAN2dZAWalUIWeZeoS
jPQspicyaKfAzS14twH
GTurc0lWkSid03rG0JZ

Apache Logo

Maven Dependencies

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-text</artifactId>
    <version>1.12.0</version>
</dependency>

Maven Central