Java - ByteArrayOutputStream Class

---

---

Introduction

The Java ByteArrayOutputStream class implements an output stream in which the data is written into a byte array. The buffer automatically grows as data is written to it.Following are the important points about ByteArrayOutputStream −

• Closing a ByteArrayOutputStream has no effect.

• The methods in this class can be called after the stream has been closed without generating an IOException.

Class declaration

Following is the declaration for Java.io.ByteArrayOutputStream class −

public class ByteArrayOutputStream
   extends OutputStream

Field

Following are the fields for Java.io.ByteArrayOutputStream class −

• protected byte[] buf − This is the buffer where data is stored.

• protected int count − This is the number of valid bytes in the buffer.

Class constructors

Class methods

Methods inherited

This class inherits methods from the following classes −

• Java.io.OutputStream
• Java.io.Object

Example - Using close() in a Try-With-Resources Block

The following example shows the usage of Java ByteArrayInputStream close() method.

ByteArrayOutputStreamDemo.java

package com.tutorialspoint;

import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class ByteArrayOutputStreamDemo {
   public static void main(String[] args) {
      // Using try-with-resources to handle the stream
      try (ByteArrayOutputStream outputStream = new ByteArrayOutputStream()) {
         // Write data to the stream
         outputStream.write("Java Programming".getBytes());

         // Print the stream's content
         System.out.println("Data in stream: " + outputStream.toString());

         // No need to explicitly call close() because try-with-resources handles it
      } catch (IOException e) {
         System.err.println("IOException occurred: " + e.getMessage());
      }
      System.out.println("Stream closed automatically by try-with-resources.");
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Data in stream: Java Programming
Stream closed automatically by try-with-resources.

Explanation

• A ByteArrayOutputStream is used inside a try-with-resources block.

• Data is written to the stream and then printed.

• The close() method is called automatically when the try block exits, thanks to the try-with-resources construct.

• This approach ensures proper handling of resources without requiring explicit close() calls, making the code cleaner and less error-prone.

Example - Using reset() to Reuse the Stream

The following example shows the usage of Java ByteArrayInputStream reset() method.

ByteArrayOutputStreamDemo.java

package com.tutorialspoint;

import java.io.ByteArrayOutputStream;
import java.io.IOException;

public class ByteArrayOutputStreamDemo {
   public static void main(String[] args) {
      // Create a ByteArrayOutputStream
      ByteArrayOutputStream outputStream = new ByteArrayOutputStream();

      try {
         // Write some data to the stream
         outputStream.write("Hello, World!".getBytes());
         System.out.println("First output: " + outputStream.toString());

         // Reset the stream to clear the buffer
         outputStream.reset();
         System.out.println("Stream reset.");

         // Write new data to the same stream
         outputStream.write("Java Programming".getBytes());
         System.out.println("Second output: " + outputStream.toString());

         // Close the stream (optional for ByteArrayOutputStream)
         outputStream.close();
      } catch (IOException e) {
         System.err.println("IOException occurred: " + e.getMessage());
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

First output: Hello, World!
Stream reset.
Second output: Java Programming

Explanation

• A ByteArrayOutputStream is created.

• The first string, "Hello, World!", is written to the stream and printed.

• The reset() method is called, which clears the buffer and makes the stream empty again.

• The second string, "Java Programming", is written to the same stream after resetting it.

• The new content of the stream is printed, showing that the previous data has been discarded.

Example - Using toByteArray() to Retrieve Data from the Stream

The following example shows the usage of Java ByteArrayInputStream toByteArray() method.

ByteArrayOutputStreamDemo.java

package com.tutorialspoint;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.util.Arrays;

public class ByteArrayOutputStreamDemo {
   public static void main(String[] args) {
      // Create a ByteArrayOutputStream
      ByteArrayOutputStream outputStream = new ByteArrayOutputStream();

      try {
         // Write some data to the stream
         outputStream.write("Hello, World!".getBytes());

         // Convert the stream content to a byte array
         byte[] byteArray = outputStream.toByteArray();

         // Print the byte array
         System.out.println("Byte array: " + Arrays.toString(byteArray));

         // Convert the byte array back to a string and print it
         String content = new String(byteArray);
         System.out.println("String content: " + content);

         // Close the stream (optional for ByteArrayOutputStream)
         outputStream.close();
      } catch (IOException e) {
         System.err.println("IOException occurred: " + e.getMessage());
      }
   }
}

Output

Let us compile and run the above program, this will produce the following result −

Byte array: [72, 101, 108, 108, 111, 44, 32, 87, 111, 114, 108, 100, 33]
String content: Hello, World!

Explanation

• The string "Hello, World!" is written to the ByteArrayOutputStream.

• The toByteArray() method is called to retrieve the content of the stream as a new byte array.

• The byte array is printed using Arrays.toString() for better visualization of the raw byte data.

• The byte array is converted back to a string using the String constructor to demonstrate how the data can be reused.

• Although calling close() on a ByteArrayOutputStream has no real effect, it is included for consistency.