setCharAt

void setCharAt(int index, char ch)
The character at the specified index is set to ch.

public class Main {
  public static void main(String[] argv) {
    StringBuilder sb = new StringBuilder();
    sb.append("java2s.com");

   
    sb.setCharAt(0, 'J');
    
    System.out.println(sb.toString());

  }
}

The output: 


Java2s.com
Home 
  Java Book 
    Essential Classes  

StringBuilder:
  1. StringBuilder
  2. StringBuilder Constructor
  3. append(char c)
  4. StringBuilder capacity()
  5. charAt()
  6. delete(int start, int end)
  7. deleteCharAt
  8. StringBuilder length()
  9. StringBuilder reverse()
  10. setCharAt
  11. Sub string from StringBuilder
  12. toString():Convert StringBuilder to String