Here's a Java program to convert a stack trace to a string:

import java.io.PrintWriter;
import java.io.StringWriter;

public class StackTraceToString {
    public static void main(String[] args) {
        try {
            // Throw an exception to generate a stack trace
            throw new RuntimeException("Test exception");
        } catch (Exception e) {
            // Convert the stack trace to a string
            StringWriter sw = new StringWriter();
            PrintWriter pw = new PrintWriter(sw);
            e.printStackTrace(pw);
            String stackTrace = sw.toString();
            
            // Print the stack trace string
            System.out.println(stackTrace);
        }
    }
}

This program uses a StringWriter and a PrintWriter to capture the stack trace generated by the Exception thrown in the try block. The StringWriter provides a way to write characters to a string, and the PrintWriter provides a way to write formatted text to a character output stream. By passing the StringWriter to the PrintWriter constructor, we can write the stack trace to the string.

After the stack trace is written to the string, it can be printed or used as needed.