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

In Java, a String is immutable, so its characters cannot be changed directly after the string is created. To replace a character at a specific index, we can either create a new String using substring() or use mutable classes such as StringBuilder and StringBuffer.

• Java String objects are immutable.
• A String cannot be modified directly; replacement creates a new String.

Example

Input: String = "Geeks Gor Geeks", index = 6, ch = 'F'
Output: Geeks For Geeks

Input: String = "Geeks", index = 0, ch = 'g'
Output: geeks

Methods to Replace Character in a String at Specific Index

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

1. Using String with substring()

Since String is immutable, we cannot directly modify a character. We can create a new string by combining:

• Characters before the specified index
• Characters after the specified index

public class GFG {
    public static void main(String[] args) {

        String str = "Geeks Gor Geeks";
        int index = 6;
        char ch = 'F';

        System.out.println("Original String = " + str);

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

        System.out.println("Modified String = " + str);
    }
}

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

2. Using StringBuilder

StringBuilder is a mutable sequence of characters, so a character can be directly replaced using the setCharAt() method.

public class GFG {
    public static void main(String[] args) {

        String str = "Geeks Gor Geeks";
        int index = 6;
        char ch = 'F';

        System.out.println("Original String = " + str);

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

        System.out.println("Modified String = " + sb);
    }
}

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

3. Using StringBuffer

StringBuffer is also mutable and provides the setCharAt() method. Unlike StringBuilder, its methods are synchronized, making it suitable when thread-safe mutable string operations are required.

public class GFG {
    public static void main(String[] args) {

        String str = "Geeks Gor Geeks";
        int index = 6;
        char ch = 'F';

        System.out.println("Original String = " + str);

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

        System.out.println("Modified String = " + sb);
    }
}v