How to Replace Character in String at Index in Java

1. Replace a Character in a String at Index in Java Using substring()
2. Replace a Character in a String at Index in Java Using StringBuilder()
3. Convert the String to an Array of chars to Replace a Character in a String at Index in Java

This article will introduce how we can replace a character in a string at a specific index in Java. We can use various ways to achieve our goal, which are mentioned in the following examples.

Replace a Character in a String at Index in Java Using substring()

In our first example, we have a string - ab that has the character A, which is an uppercase letter that does not fit the sentence, and we want to replace it with a lowercase character a.

To replace it, we use the substring() function of the String class that takes a range or the string’s beginning index as an argument. Our target character is located at the position of index 8.

ab.substring(0, index) returns the part of the string from 0 to 8th position. We concatenate this part of the string with our new character a, and join the remaining string using ab.substring(index + 1).

public class ReplaceCharAtIndex {
  public static void main(String[] args) {
    String ab = "This is A String";
    int index = 8;

    String newString = ab.substring(0, index) + 'a' + ab.substring(index + 1);

    System.out.println(newString);
  }
}

Output:

This is a String

Replace a Character in a String at Index in Java Using StringBuilder()

We have the same string that we use in our previous example but will use StringBuilder() to create a new string that is mutable because a normal string in Java is immutable. As the newString is now modifiable, we can use its setChartAt() method to set a new char to a position or index.

newString.setCharAt(8, 'a') sets the character a at the 8th position.

public class ReplaceCharAtIndex {
  public static void main(String[] args) {
    String ab = "This is A String";

    StringBuilder newString = new StringBuilder(ab);

    newString.setCharAt(8, 'a');

    System.out.println(newString);
  }
}

Output:

This is a String

Convert the String to an Array of chars to Replace a Character in a String at Index in Java

The last method converts the string oldString to an array of char using toCharArray(). We can replace any value in an array by specifying its position.

As we can see that there is a typo in oldString where the word is misspelled, and we need to replace the character ( m ) with the character ( n ). We can replace our character at an index using charArray[index] = 'n'.

At last, we have to convert the char[] to a string by using String.valueOf(). The output shows that the character has been replaced.

public class ReplaceCharAtIndex {
  public static void main(String[] args) {
    String oldString = "This is an example strimg";
    int index = 23;

    char[] charArray = oldString.toCharArray();
    charArray[index] = 'n';

    String newString = String.valueOf(charArray);

    System.out.println(newString);
  }
}

Output:

This is an example string