ByteArrayOutputStream (Java Platform SE 8 )
Keywords: java.io.ByteArrayOutputStream class
Date: 2026-04-24

compact1, compact2, compact3
java.io

## Class ByteArrayOutputStream

- java.lang.Object
- - java.io.OutputStream
  - - java.io.ByteArrayOutputStream

- All Implemented Interfaces:
    Closeable, Flushable, AutoCloseable

      public class ByteArrayOutputStream
      extends OutputStream

  This 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. The data can be retrieved using `toByteArray()` and `toString()`.

  Closing a `ByteArrayOutputStream` has no effect. The methods in this class can be called after the stream has been closed without generating an `IOException`.

  Since:
    JDK1.0

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected byte[]` | `buf` The buffer where data is stored.
    `protected int` | `count` The number of valid bytes in the buffer.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `ByteArrayOutputStream()` Creates a new byte array output stream.
    `ByteArrayOutputStream(int size)` Creates a new byte array output stream, with a buffer capacity of the specified size, in bytes.

  - ### Method Summary

    All Methods Instance Methods Concrete Methods Deprecated Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `close()` Closing a `ByteArrayOutputStream` has no effect.
    `void` | `reset()` Resets the `count` field of this byte array output stream to zero, so that all currently accumulated output in the output stream is discarded.
    `int` | `size()` Returns the current size of the buffer.
    `byte[]` | `toByteArray()` Creates a newly allocated byte array.
    `String` | `toString()` Converts the buffer's contents into a string decoding bytes using the platform's default character set.
    `String` | `toString(int hibyte)` Deprecated. This method does not properly convert bytes into characters. As of JDK 1.1, the preferred way to do this is via the `toString(String enc)` method, which takes an encoding-name argument, or the `toString()` method, which uses the platform's default character encoding.
    `String` | `toString(String charsetName)` Converts the buffer's contents into a string by decoding the bytes using the named `charset`.
    `void` | `write(byte[] b, int off, int len)` Writes `len` bytes from the specified byte array starting at offset `off` to this byte array output stream.
    `void` | `write(int b)` Writes the specified byte to this byte array output stream.
    `void` | `writeTo(OutputStream out)` Writes the complete contents of this byte array output stream to the specified output stream argument, as if by calling the output stream's write method using `out.write(buf, 0, count)`.

    - ### Methods inherited from class java.io.OutputStream

      `flush, write`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

- - ### Field Detail

    - #### buf

          protected byte[] buf

      The buffer where data is stored.

    - #### count

          protected int count

      The number of valid bytes in the buffer.

  - ### Constructor Detail

    - #### ByteArrayOutputStream

          public ByteArrayOutputStream()

      Creates a new byte array output stream. The buffer capacity is initially 32 bytes, though its size increases if necessary.

    - #### ByteArrayOutputStream

          public ByteArrayOutputStream(int size)

      Creates a new byte array output stream, with a buffer capacity of the specified size, in bytes.
      Parameters:
        `size` - the initial size.
      Throws:
        `IllegalArgumentException` - if size is negative.

  - ### Method Detail

    - #### write

          public void write(int b)

      Writes the specified byte to this byte array output stream.
      Specified by:
        `write` in class `OutputStream`
      Parameters:
        `b` - the byte to be written.

    - #### write

          public void write(byte[] b,
                            int off,
                            int len)

      Writes `len` bytes from the specified byte array starting at offset `off` to this byte array output stream.
      Overrides:
        `write` in class `OutputStream`
      Parameters:
        `b` - the data.
        `off` - the start offset in the data.
        `len` - the number of bytes to write.

    - #### writeTo

          public void writeTo(OutputStream out)
                       throws IOException

      Writes the complete contents of this byte array output stream to the specified output stream argument, as if by calling the output stream's write method using `out.write(buf, 0, count)`.
      Parameters:
        `out` - the output stream to which to write the data.
      Throws:
        `IOException` - if an I/O error occurs.

    - #### reset

          public void reset()

      Resets the `count` field of this byte array output stream to zero, so that all currently accumulated output in the output stream is discarded. The output stream can be used again, reusing the already allocated buffer space.
      See Also:
        `ByteArrayInputStream.count`

    - #### toByteArray

          public byte[] toByteArray()

      Creates a newly allocated byte array. Its size is the current size of this output stream and the valid contents of the buffer have been copied into it.
      Returns:
        the current contents of this output stream, as a byte array.
      See Also:
        `size()`

    - #### size

          public int size()

      Returns the current size of the buffer.
      Returns:
        the value of the `count` field, which is the number of valid bytes in this output stream.
      See Also:
        `count`

    - #### toString

          public String toString()

      Converts the buffer's contents into a string decoding bytes using the platform's default character set. The length of the new `String` is a function of the character set, and hence may not be equal to the size of the buffer.

      This method always replaces malformed-input and unmappable-character sequences with the default replacement string for the platform's default character set. The CharsetDecoder class should be used when more control over the decoding process is required.

      Overrides:
        `toString` in class `Object`
      Returns:
        String decoded from the buffer's contents.
      Since:
        JDK1.1

    - #### toString

          public String toString(String charsetName)
                          throws UnsupportedEncodingException

      Converts the buffer's contents into a string by decoding the bytes using the named `charset`. The length of the new `String` is a function of the charset, and hence may not be equal to the length of the byte array.

      This method always replaces malformed-input and unmappable-character sequences with this charset's default replacement string. The `CharsetDecoder` class should be used when more control over the decoding process is required.

      Parameters:
        `charsetName` - the name of a supported `charset`
      Returns:
        String decoded from the buffer's contents.
      Throws:
        `UnsupportedEncodingException` - If the named charset is not supported
      Since:
        JDK1.1

    - #### toString

          @Deprecated
          public String toString(int hibyte)

      Deprecated. This method does not properly convert bytes into characters. As of JDK 1.1, the preferred way to do this is via the `toString(String enc)` method, which takes an encoding-name argument, or the `toString()` method, which uses the platform's default character encoding.
      Creates a newly allocated string. Its size is the current size of the output stream and the valid contents of the buffer have been copied into it. Each character c in the resulting string is constructed from the corresponding element b in the byte array such that:

      >          c == (char)(((hibyte & 0xff) << 8) | (b & 0xff))

      Parameters:
        `hibyte` - the high byte of each resulting Unicode character.
      Returns:
        the current contents of the output stream, as a string.
      See Also:
        `size()`, `toString(String)`, `toString()`

    - #### close

          public void close()
                     throws IOException

      Closing a `ByteArrayOutputStream` has no effect. The methods in this class can be called after the stream has been closed without generating an `IOException`.
      Specified by:
        `close` in interface `Closeable`
      Specified by:
        `close` in interface `AutoCloseable`
      Overrides:
        `close` in class `OutputStream`
      Throws:
        `IOException` - if an I/O error occurs.

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