Java – Left pad a string with spaces or zeros

Java programs to add left padding to a string in such a way that total string length should be a fixed predefined number. For example, if we have a string of length 10, and we want to increase it’s length to 15 – by adding left padding then use the example given this post.

1. How left padding is added

When you add left padding, you essentially add a character repeatedly until string length reaches to defined length.

For example –

howtodoinjava.com			//no padding
    howtodoinjava.com		//left padding of 4 spaces
....howtodoinjava.com		//left padding of 4 dots
0000howtodoinjava.com		//left padding of 4 zeros

Read More: Java remove leading whitespaces from String

2. Java left padding with spaces

To add left padding, most useful and easy way is to use StringUtils.leftPad() method.

2.1. Maven dependency

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.8</version>
</dependency>

2.2. Method Syntax (overloaded)

/**
 * @param str  		- the String to pad out, may be null
 * @param size 		- the size to pad to i.e. Total length of result string
 * @param padChar  	- the character or string to pad with
 * 
 * @return left padded String or original String if no padding is necessary or if null String input
 */

public static String leftPad(final String str, final int size) {...}
public static String leftPad(final String str, final int size, String padStr) {...}
public static String leftPad(final String str, final int size, final char padChar) { ... }

2.3. Left padding example

System.out.println( StringUtils.leftPad("howtodoinjava", 20, " ") );
System.out.println( StringUtils.leftPad("howtodoinjava", 30, " ") );
System.out.println( StringUtils.leftPad("howtodoinjava", 15, " ") );

Program output:

       howtodoinjava
                 howtodoinjava
  howtodoinjava

3. Java left pad a string with zeros

Java program to use StringUtils.leftPad() method to left pad a string with zeros, by adding leading zeros to string.

System.out.println( StringUtils.leftPad("0123456789", 10, "0") );
System.out.println( StringUtils.leftPad("789", 10, "0") );
System.out.println( StringUtils.leftPad("56789", 10, "0") );

Program output:

0123456789
0000000789
0000056789

4. Summary

In above examples, we learned to left pad string with spaces to fixed length. We also saw to left pad a number with zeros.

Use this left padding to format strings to fixed length – to display in UI.

Happy Learning !!

Reference:

StringUtils Java Doc

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HowToDoInJava

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