Builder: StringBuilder.insert splices text without rebuilding

insert(index, text) shifts the existing characters aside and drops new text in at a position, all within the same buffer. It avoids slicing the string into pieces and concatenating them back together.

Code

StringBuilder sb = new StringBuilder("Hello world");
sb.insert(5, ",");
sb.insert(sb.length(), "!");
System.out.println(sb);

Output

Hello, world!