Here is an example of how to use the StringBuilderWriter class from the org.apache.commons.io package in Java:

import java.io.IOException;

import org.apache.commons.io.output.StringBuilderWriter;

public class Example {
    public static void main(String[] args) throws IOException {
        StringBuilderWriter stringBuilderWriter = new StringBuilderWriter();
        stringBuilderWriter.write("Hello, World!");
        stringBuilderWriter.write(" ");
        stringBuilderWriter.write("This is a test.");
        stringBuilderWriter.flush();
        stringBuilderWriter.close();
        String result = stringBuilderWriter.getBuilder().toString();
        System.out.println(result);
    }
}

This example creates a StringBuilderWriter object, which is an implementation of the Writer interface that writes to a StringBuilder. The write method is called multiple times on the stringBuilderWriter object to write different strings. The flush method is called to flush any remaining characters to the StringBuilder, and the close method is called to close the writer. The getBuilder() method is used to get the underlying StringBuilder and toString() is used to get the final string. The final string is then printed to the console.

You can also pass a StringBuilder object to the constructor of the StringBuilderWriter class to use an existing StringBuilder.