StringBuilder setCharAt() in Java with Examples

The setCharAt() method of the StringBuilder class is used to replace a character at a specified index with a new character. This method modifies the existing StringBuilder object and does not create a new string.It does not return any value.

class GFG {
    public static void main(String[] args)
    {
        StringBuilder sb = new StringBuilder("Java");
        sb.setCharAt(1, 'O');
        System.out.println(sb);
    }
}

JOva

Explanation: The character at index 1 is replaced with 'O' & original StringBuilder object is modified directly.

Syntax

public void setCharAt(int index, char ch)

Parameters:

• index: Integer type value which refers to the index of character you want to set.
• ch: Character type value which refers to the new char.

Returns: returns nothing.

Exception: If the index is negative, greater than length() then IndexOutOfBoundsException.

Examples of setCharAt()

Example 1: This code shows how setCharAt() replaces a character at a specific index in a StringBuilder.

class GFG{
    public static void main(String[] args){
        
       StringBuilder str = new StringBuilder("WelcomeGeeks");
        System.out.println("String = " + str.toString());
        str.setCharAt(2, 'L');
        System.out.println("After setCharAt() String = " + str.toString());
    }
}

String = WelcomeGeeks
After setCharAt() String = WeLcomeGeeks

Explanation:

• str.setCharAt(2, 'L'): targets index 2 of the StringBuilder object str.
• The character at that position is replaced with 'L', modifying "WelcomeGeeks" to "WeLcomeGeeks" in place.

Example 2: This replaces a character in the middle of a sentence using setCharAt().

class GFG{
    public static void main(String[] args){
        
        StringBuilder str = new StringBuilder("Tony Stark will die");
        System.out.println("String = "+ str.toString());
        str.setCharAt(9, '1');
        System.out.println("After setCharAt() String = " + str.toString());
    }
}

String = Tony Stark will die
After setCharAt() String = Tony Star1 will die

Explanation:

• str.setCharAt(9, '1'): updates the character at index 9 in the StringBuilder object str.
• The original sequence is modified in place, changing "Tony Stark will die" to "Tony Star1 will die".

Example 3: When an invalid (negative) index is provided, behavior of setCharAt() changes.

class GFG{
    public static void main(String[] args){
        StringBuilder str= new StringBuilder("Tony Stark");
        try {
            str.setCharAt(-15, 'A');
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Output:

java.lang.StringIndexOutOfBoundsException: String index out of range: -15
    at java.lang.AbstractStringBuilder.setCharAt(AbstractStringBuilder.java:407)
    at java.lang.StringBuilder.setCharAt(StringBuilder.java:76)
    at GFG.main(File.java:16)

Explanation:

• str.setCharAt(-15, 'A'): attempts to access an index outside the valid range of the StringBuilder.
• Since negative indexes are not allowed, the JVM throws a StringIndexOutOfBoundsException at runtime.