How to Use padStart and padEnd in Kotlin: Simple Guide

In Kotlin, padStart and padEnd add padding characters to the beginning or end of a string until it reaches a specified length. Use padStart(length, padChar) to add padding at the start, and padEnd(length, padChar) to add padding at the end.

Syntax

The padStart and padEnd functions have this syntax:

• fun String.padStart(length: Int, padChar: Char = ' '): String
• fun String.padEnd(length: Int, padChar: Char = ' '): String

Explanation:

• length: The total length of the resulting string after padding.
• padChar: The character used for padding (default is space).
• Returns a new string padded to the specified length.

kotlin

val original = "42"
val paddedStart = original.padStart(5, '0')
val paddedEnd = original.padEnd(5, '*')

Example

This example shows how to use padStart and padEnd to add zeros at the start and stars at the end of a string.

kotlin

fun main() {
    val number = "42"
    val paddedStart = number.padStart(5, '0')
    val paddedEnd = number.padEnd(5, '*')
    println("Original: '$number'")
    println("padStart(5, '0'): '$paddedStart'")
    println("padEnd(5, '*'): '$paddedEnd'")
}

Output

Original: '42'
padStart(5, '0'): '00042'
padEnd(5, '*'): '42***'

Common Pitfalls

Common mistakes include:

• Using a length smaller than the original string length, which returns the original string unchanged.
• Forgetting that padChar must be a single character.
• Expecting padStart or padEnd to modify the original string (they return a new string).

kotlin

fun main() {
    val text = "hello"
    // Wrong: length less than original, no padding added
    println(text.padStart(3, '*')) // prints 'hello'

    // Wrong: padChar must be a Char, not a String
    // println(text.padEnd(8, "**")) // This will not compile

    // Right usage
    println(text.padEnd(8, '*')) // prints 'hello***'
}

Output

hello
hello***

Quick Reference

Function	Purpose	Parameters	Returns
padStart(length, padChar)	Adds padding at the start	length: Int, padChar: Char (default ' ')	New padded string
padEnd(length, padChar)	Adds padding at the end	length: Int, padChar: Char (default ' ')	New padded string

Key Takeaways

Use padStart to add characters at the beginning of a string to reach a desired length.

Use padEnd to add characters at the end of a string to reach a desired length.

If the specified length is less than or equal to the string length, the original string is returned unchanged.

The pad character must be a single character, not a string.

padStart and padEnd return new strings; they do not change the original string.