OutputStream is an abstract class provided in the java.io package. Because it is an abstract class, in order to use its functions, we can use its subclasses. Some subclasses are FileOutputStream, ByteArrayOutputStream, ObjectOutputStream, etc. And String is just a sequence of characters, use double quotes to denote it. The java.io.ByteArrayOutputStream.toString() method uses the character set to convert the stream.

Approach 1:

1. Create a ByteArrayoutputStream object.
2. Create a String variable and initialize it.
3. Use the write method to copy the contents of the string to the object of ByteArrayoutputStream.
4. Print it.

Example:

Input:

 String = "Hello World"

Output:

 Hello World

Below is the implementation of the above approach:

// Java program to demonstrate conversion
// from outputStream to string

import java.io.*;

class GFG {

	// we know that main will throw
	// IOException so we are ducking it
	public static void main(String[] args)
		throws IOException
	{

		// declaring ByteArrayOutputStream
		ByteArrayOutputStream stream
			= new ByteArrayOutputStream();

		// Intializing string
		String st = "Hello Geek!";

		// writing the specified byte to the output stream
		stream.write(st.getBytes());

		// converting stream to byte array
		// and typecasting into string
		String finalString
			= new String(stream.toByteArray());

		// printing the final string
		System.out.println(finalString);
	}
}

Output

Hello Geek!

Approach 2:

1. Create a byte array and store ASCII value of the characters.
2. Create an object of ByteArrayoutputStream.
3. Use the write method to copy the content from the byte array to the object.
4. Print it.

Example: 

Input:

array = [71, 69, 69, 75]

Output:

GEEK

Below is the implementation of the above approach:

// Java program to demonstrate conversion
// from outputStream to string

import java.io.*;

class GFG {
	public static void main(String[] args)
		throws IOException
	{

		// Intializing empty string
		// and byte array
		String str = "";
		byte[] bs = { 71, 69, 69, 75, 83, 70, 79,
					82, 71, 69, 69, 75, 83 };

		// create new ByteArrayOutputStream
		ByteArrayOutputStream stream
			= new ByteArrayOutputStream();

		// write byte array to the output stream
		stream.write(bs);

		// converts buffers using default character set
		// toString is a method for casting into String type
		str = stream.toString();

		// print
		System.out.println(str);
	}
}

Output:

GEEKSFORGEEKS