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Replace a character at a specific index in a String in Java

Last Updated : 11 Jul, 2025

 In Java, here we are given a string, the task is to replace a character at a specific index in this string.

Input: String = "Geeks Gor Geeks", index = 6, ch = 'F'
Output: "Geeks For Geeks."
Input: String = "Geeks", index = 0, ch = 'g'
Output: "geeks"

There are certain methods to replace characters in String are mentioned below:

1. Using String Class
2. Using StringBuilder
3. Using StringBuffer

There is no predefined method in String Class to replace a specific character in a String, as of now. However, this can be achieved indirectly by constructing a new String with 2 different substrings, one from the beginning till the specific index - 1, the new character at the specific index, and the other from the index + 1 till the end.

Below is the implementation of the above approach: 

// Driver Class
public class GFG {
    // main function
    public static void main(String args[])
    {
        // Get the String
        String str = "Geeks Gor Geeks";

        // Get the index
        int index = 6;

        // Get the character
        char ch = 'F';

        // Print the original string
        System.out.println("Original String = " + str);

        str = str.substring(0, index) + ch
              + str.substring(index + 1);

        // Print the modified string
        System.out.println("Modified String = " + str);
    }
}

Original String = Geeks Gor Geeks
Modified String = Geeks For Geeks

Unlike String Class, the StringBuilder class is used to represent a mutable string of characters and has a predefined method for change a character at a specific index - setCharAt(). Replace the character at the specific index by calling this method and passing the character and the index as the parameter.

Below is the implementation of the above approach: 

// Java Program to replace character in
// a String using StringBuffer

// Driver Class
public class GFG {
    // main function
    public static void main(String args[])
    {
        // Get the String
        String str = "Geeks Gor Geeks";

        // Get the index
        int index = 6;

        // Get the character
        char ch = 'F';

        // Print the original string
        System.out.println("Original String = " + str);

        StringBuilder string = new StringBuilder(str);
        string.setCharAt(index, ch);

        // Print the modified string
        System.out.println("Modified String = " + string);
    }
}

Original String = Geeks Gor Geeks
Modified String = Geeks For Geeks

Like StringBuilder, the StringBuffer class has a predefined method for this purpose - setCharAt(). Replace the character at the specific index by calling this method and passing the character and the index as the parameter. StringBuffer is thread-safe and can be used in a multi-threaded environment. StringBuilder is faster when compared to StringBuffer, but is not thread-safe.

Below is the implementation of the above approach:

// Java Program to replace character in
// a String using StringBuffer

// Driver Class
public class GFG {
    // main function
    public static void main(String args[])
    {
        // Get the String
        String str = "Geeks Gor Geeks";

        // Get the index
        int index = 6;

        // Get the character
        char ch = 'F';

        // Print the original string
        System.out.println("Original String = " + str);

        StringBuffer string = new StringBuffer(str);
        string.setCharAt(index, ch);

        // Print the modified string
        System.out.println("Modified String = " + string);
    }
}

Original String = Geeks Gor Geeks
Modified String = Geeks For Geeks

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