Here is a Java program to convert an OutputStream to a String:

import java.io.ByteArrayOutputStream;
import java.io.OutputStream;

public class ConvertOutputStreamToString {
    public static void main(String[] args) {
        try {
            // Create an OutputStream
            OutputStream outputStream = new ByteArrayOutputStream();
            
            // Write some data to the OutputStream
            String data = "Hello, world!";
            outputStream.write(data.getBytes());
            
            // Convert the OutputStream to a String
            String outputString = outputStream.toString();
            
            // Print the output String
            System.out.println(outputString);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

This program first creates an OutputStream object, outputStream, and writes some data to it using the OutputStream.write(byte[] b) method. The data is in the form of a string, which is converted to a byte array using the String.getBytes() method.

Next, the program converts the OutputStream to a String using the OutputStream.toString() method. This method returns a String containing the characters that were written to the OutputStream.

Note that the toString() method of an OutputStream returns a string representation of the object, and not the data that was written to the stream. To get the data that was written to the stream, you need to use a ByteArrayOutputStream instead of a plain OutputStream. The ByteArrayOutputStream writes the data to an internal byte array, which can then be converted to a String using the String(byte[] bytes) constructor, like this:

import java.io.ByteArrayOutputStream;
import java.io.OutputStream;

public class ConvertByteArrayOutputStreamToString {
    public static void main(String[] args) {
        try {
            // Create a ByteArrayOutputStream
            ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
            
            // Write some data to the ByteArrayOutputStream
            String data = "Hello, world!";
            byteArrayOutputStream.write(data.getBytes());
            
            // Convert the ByteArrayOutputStream to a String
            String outputString = new String(byteArrayOutputStream.toByteArray());
            
            // Print the output String
            System.out.println(outputString);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

This program uses a ByteArrayOutputStream instead of a plain OutputStream, and converts the internal byte array of the ByteArrayOutputStream to a String using the String(byte[] bytes) constructor.