Left Pad String in Java

Apr 02, 2019 Java comments

In this post, we will discuss the Java Left Pad String. If you want to attain a specific target length, you will have to learn and understand the written example codes below in order to pad a custom character to the left side of a String.

Left Pad Using Concatenation

Let's include the implementation of the int target when we utilize a simple String concatenation. Observe the process of the code below:

public class SamplePaddingTest {
   public static void main(String[] args) {
      System.out.println(leftPad("CAR", 10, '#'));
   }
   public static String leftPad(String theOriginalString, int length,
         char toPadCharacter) {
      String stringPaddedString = theOriginalString;
      while (stringPaddedString.length() < length) 
	  {
         stringPaddedString = toPadCharacter + stringPaddedString;
      }
      return stringPaddedString;
   }
}

Take a look at the variable theOriginalString, what's inside is the String "CAR", your int length is 10, and your toPadCharacter is the # sign. Now "String stringPaddedString = theOriginalString;" indicates that our new variable for the String "CAR" is replaced with the variable stringPaddedString. Let's break down the condition in the while loop. Consider the "stringPaddedString.length()" the length character of the String stringPaddedString which is 3, the "CAR" has 3 characters thereby making it 3. The condition inside the while is if 3 is less than 10 (which is the value of the variable int length), the toPadCharacter (#) will keep on adding to the left of the stringPaddedString ("CAR") until it reaches the condition of int length (10). This is because the called method leftpad in the main, will return the String in the left of theOriginalString until the condition is completed.

Check out the result below:

#######CAR

Left Pad Using StringBuilder

The same implementation applies for left padding a character to a String using StringBuilder. Check the sample code below:

public class SamplePaddingTest {
   public static void main(String[] args) {
      System.out.println(leftPad("HEY", 15, '-'));
   }
   public static String leftPad(String theOriginalString, int length,
         char toPadCharacter) {
      StringBuilder sb = new StringBuilder();
      while (sb.length() + theOriginalString.length() < 10) {
         sb.append(toPadCharacter);
      }
      sb.append(theOriginalString);
      String stringPaddedString = sb.toString();
      return stringPaddedString;
   }
}

Check out the output of the code below:

------------HEY

Left Pad Using Substring

Another way we can utilize Left padding character is with a Substing Trick. Observe the written program below:

public class SamplePaddingTest {
   public static void main(String[] args) {
      System.out.println(leftPad("NAME", 15, '*'));
   }
   public static String leftPad(String theOriginalString, int length,
         char toPadCharacter) {
      StringBuilder sb = new StringBuilder();
      for (int i = 0; i < length; i++) {
         sb.append(toPadCharacter);
      }
      String padding = sb.toString();
      String stringPaddedString = padding.substring(theOriginalString.length())
            + theOriginalString;
      return stringPaddedString;
   }
}

What the first part of the program does is it builds the length number of pad characters (15). Now to cut the excess padding of those characters, the substring method should be performed, and after this, it should be concatenated to the set String.

Check out the expected result below:

***********NAME