Java StringBuilder.insert() is a method in the Java StringBuilder class that inserts a string or character sequence at a specified index in the builder. It allows for efficient string manipulation without creating new string objects, which is especially useful for building complex strings dynamically. Introduced in Java 1997 with Java 1.1, insert() is a key method in scenarios that require frequent string modifications, such as in generating output for reports, file content, or user interfaces.

https://docs.oracle.com/javase/8/docs/api/java/lang/StringBuilder.html