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The method used for inserting a string into the existing contents of a StringBuilder object is the `insert()` method.

To use the `insert()` method, we need to define a StringBuilder object and make use of the method with appropriate parameters. The first parameter is the index at which the string should be inserted in the StringBuilder, and the second parameter is the string to be inserted. "strings." Example: ```java public class StringInserter { public static void main(String[] args) { StringBuilder sb = new StringBuilder("Hello World"); sb.insert(5, ", Beautiful"); // Insert ', Beautiful' at index 5 System.out.println(sb); // Output "Hello, Beautiful World." } } ``` In this example, we create a StringBuilder object `sb` with the initial content "Hello World". Then, we call the `insert()` method to insert the string ", Beautiful" at index 5. Finally, we print the modified content of the StringBuilder object, which is "Hello, Beautiful World".