StringBuilder setCharAt() in Java with Examples

The setCharAt(int index, char ch) method of StringBuilder class is used to set the character at the position index passed as ch. This method changes the old sequence to represents a new sequence which is identical to old sequence only difference is a new character ch is present at position index. The index argument must be greater than or equal to 0, and less than the length of the String contained by StringBUilder object.
Syntax:

public void setCharAt(int index, char ch)

Parameters:
This method accepts two 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:
This method returns nothing.
Exception:
If the index is negative, greater than length() then IndexOutOfBoundsException.

Below programs illustrate the java.lang.StringBuilder.setCharAt() method:
Example 1:

// Java program to demonstrate 
// the setCharAt() Method. 
  
class GFG { 
    public static void main(String[] args) 
    { 
  
        // create a StringBuilder object 
        // with a String pass as parameter 
        StringBuilder str 
            = new StringBuilder("WelcomeGeeks"); 
  
        // print string 
        System.out.println("String = "
                           + str.toString()); 
  
        // set char at index 2 to 'L' 
        str.setCharAt(2, 'L'); 
  
        // print string 
        System.out.println("After setCharAt() String = "
                           + str.toString()); 
    } 
} 

Output:

String = WelcomeGeeks
After setCharAt() String = WeLcomeGeeks

Example 2:

// Java program to demonstrate 
// the setCharAt() Method. 
  
class GFG { 
    public static void main(String[] args) 
    { 
  
        // create a StringBuilder object 
        // with a String pass as parameter 
        StringBuilder str 
            = new StringBuilder("Tony Stark will die"); 
  
        // print string 
        System.out.println("String = "
                           + str.toString()); 
  
        // set char at index 9 to '1' 
        str.setCharAt(9, '1'); 
  
        // print string 
        System.out.println("After setCharAt() String = "
                           + str.toString()); 
    } 
} 

Output:

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

Example 3: When negative index is passed:

// Java program to demonstrate 
// Exception thrown by the setCharAt() Method. 
  
class GFG { 
    public static void main(String[] args) 
    { 
  
        // create a StringBuilder object 
        // with a String pass as parameter 
        StringBuilder str 
            = new StringBuilder("Tony Stark"); 
  
        try { 
            // pass  index -15 
            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)

References:
https://docs.oracle.com/javase/10/docs/api/java/lang/StringBuilder.html#setCharAt(int, char)

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