Module java.base
Package java.nio

Class ByteBuffer

java.lang.Object
java.nio.Buffer
java.nio.ByteBuffer
All Implemented Interfaces:
Comparable<ByteBuffer>
Direct Known Subclasses:
MappedByteBuffer
public abstract class ByteBuffer
extends Buffer
implements Comparable<ByteBuffer>
A byte buffer.

This class defines six categories of operations upon byte buffers:

  • Absolute and relative get and put methods that read and write single bytes;

  • Absolute and relative bulk get methods that transfer contiguous sequences of bytes from this buffer into an array;

  • Absolute and relative bulk put methods that transfer contiguous sequences of bytes from a byte array or some other byte buffer into this buffer;

  • Absolute and relative get and put methods that read and write values of other primitive types, translating them to and from sequences of bytes in a particular byte order;

  • Methods for creating view buffers, which allow a byte buffer to be viewed as a buffer containing values of some other primitive type; and

  • A method for compacting a byte buffer.

Byte buffers can be created either by allocation, which allocates space for the buffer's content, or by wrapping an existing byte array into a buffer.

Direct vs. non-direct buffers

A byte buffer is either direct or non-direct. Given a direct byte buffer, the Java virtual machine will make a best effort to perform native I/O operations directly upon it. That is, it will attempt to avoid copying the buffer's content to (or from) an intermediate buffer before (or after) each invocation of one of the underlying operating system's native I/O operations.

A direct byte buffer may be created by invoking the allocateDirect factory method of this class. The buffers returned by this method typically have somewhat higher allocation and deallocation costs than non-direct buffers. The contents of direct buffers may reside outside of the normal garbage-collected heap, and so their impact upon the memory footprint of an application might not be obvious. It is therefore recommended that direct buffers be allocated primarily for large, long-lived buffers that are subject to the underlying system's native I/O operations. In general it is best to allocate direct buffers only when they yield a measureable gain in program performance.

A direct byte buffer may also be created by mapping a region of a file directly into memory. An implementation of the Java platform may optionally support the creation of direct byte buffers from native code via JNI. If an instance of one of these kinds of buffers refers to an inaccessible region of memory then an attempt to access that region will not change the buffer's content and will cause an unspecified exception to be thrown either at the time of the access or at some later time.

Whether a byte buffer is direct or non-direct may be determined by invoking its isDirect method. This method is provided so that explicit buffer management can be done in performance-critical code.

Access to binary data

This class defines methods for reading and writing values of all other primitive types, except boolean. Primitive values are translated to (or from) sequences of bytes according to the buffer's current byte order, which may be retrieved and modified via the order methods. Specific byte orders are represented by instances of the ByteOrder class. The initial order of a byte buffer is always BIG_ENDIAN.

For access to heterogeneous binary data, that is, sequences of values of different types, this class defines a family of absolute and relative get and put methods for each type. For 32-bit floating-point values, for example, this class defines: 

 float  getFloat()
 float  getFloat(int index)
  void  putFloat(float f)
  void  putFloat(int index, float f)

Corresponding methods are defined for the types char, short, int, long, and double. The index parameters of the absolute get and put methods are in terms of bytes rather than of the type being read or written.

For access to homogeneous binary data, that is, sequences of values of the same type, this class defines methods that can create views of a given byte buffer. A view buffer is simply another buffer whose content is backed by the byte buffer. Changes to the byte buffer's content will be visible in the view buffer, and vice versa; the two buffers' position, limit, and mark values are independent. The asFloatBuffer method, for example, creates an instance of the FloatBuffer class that is backed by the byte buffer upon which the method is invoked. Corresponding view-creation methods are defined for the types char, short, int, long, and double.

View buffers have three important advantages over the families of type-specific get and put methods described above:

  • A view buffer is indexed not in terms of bytes but rather in terms of the type-specific size of its values;

  • A view buffer provides relative bulk get and put methods that can transfer contiguous sequences of values between a buffer and an array or some other buffer of the same type; and

  • A view buffer is potentially much more efficient because it will be direct if, and only if, its backing byte buffer is direct.

The byte order of a view buffer is fixed to be that of its byte buffer at the time that the view is created.

Invocation chaining

Methods in this class that do not otherwise have a value to return are specified to return the buffer upon which they are invoked. This allows method invocations to be chained. The sequence of statements 

 bb.putInt(0xCAFEBABE);
 bb.putShort(3);
 bb.putShort(45);
can, for example, be replaced by the single statement 
 bb.putInt(0xCAFEBABE).putShort(3).putShort(45);

Since:
1.4

  • Method Summary

    Modifier and Type Method Description
    ByteBuffer alignedSlice​(int unitSize)
    Creates a new byte buffer whose content is a shared and aligned subsequence of this buffer's content.
    int alignmentOffset​(int index, int unitSize)
    Returns the memory address, pointing to the byte at the given index, modulo the given unit size.
    static ByteBuffer allocate​(int capacity)
    Allocates a new byte buffer.
    static ByteBuffer allocateDirect​(int capacity)
    Allocates a new direct byte buffer.
    byte[] array()
    Returns the byte array that backs this buffer  (optional operation).
    int arrayOffset()
    Returns the offset within this buffer's backing array of the first element of the buffer  (optional operation).
    abstract CharBuffer asCharBuffer()
    Creates a view of this byte buffer as a char buffer.
    abstract DoubleBuffer asDoubleBuffer()
    Creates a view of this byte buffer as a double buffer.
    abstract FloatBuffer asFloatBuffer()
    Creates a view of this byte buffer as a float buffer.
    abstract IntBuffer asIntBuffer()
    Creates a view of this byte buffer as an int buffer.
    abstract LongBuffer asLongBuffer()
    Creates a view of this byte buffer as a long buffer.
    abstract ByteBuffer asReadOnlyBuffer()
    Creates a new, read-only byte buffer that shares this buffer's content.
    abstract ShortBuffer asShortBuffer()
    Creates a view of this byte buffer as a short buffer.
    ByteBuffer clear()
    Clears this buffer.
    abstract ByteBuffer compact()
    Compacts this buffer  (optional operation).
    int compareTo​(ByteBuffer that)
    Compares this buffer to another.
    abstract ByteBuffer duplicate()
    Creates a new byte buffer that shares this buffer's content.
    boolean equals​(Object ob)
    Tells whether or not this buffer is equal to another object.
    ByteBuffer flip()
    Flips this buffer.
    abstract byte get()
    Relative get method.
    ByteBuffer get​(byte[] dst)
    Relative bulk get method.
    ByteBuffer get​(byte[] dst, int offset, int length)
    Relative bulk get method.
    abstract byte get​(int index)
    Absolute get method.
    ByteBuffer get​(int index, byte[] dst)
    Absolute bulk get method.
    ByteBuffer get​(int index, byte[] dst, int offset, int length)
    Absolute bulk get method.
    abstract char getChar()
    Relative get method for reading a char value.
    abstract char getChar​(int index)
    Absolute get method for reading a char value.
    abstract double getDouble()
    Relative get method for reading a double value.
    abstract double getDouble​(int index)
    Absolute get method for reading a double value.
    abstract float getFloat()
    Relative get method for reading a float value.
    abstract float getFloat​(int index)
    Absolute get method for reading a float value.
    abstract int getInt()
    Relative get method for reading an int value.
    abstract int getInt​(int index)
    Absolute get method for reading an int value.
    abstract long getLong()
    Relative get method for reading a long value.
    abstract long getLong​(int index)
    Absolute get method for reading a long value.
    abstract short getShort()
    Relative get method for reading a short value.
    abstract short getShort​(int index)
    Absolute get method for reading a short value.
    boolean hasArray()
    Tells whether or not this buffer is backed by an accessible byte array.
    int hashCode()
    Returns the current hash code of this buffer.
    abstract boolean isDirect()
    Tells whether or not this byte buffer is direct.
    ByteBuffer limit​(int newLimit)
    Sets this buffer's limit.
    ByteBuffer mark()
    Sets this buffer's mark at its position.
    int mismatch​(ByteBuffer that)
    Finds and returns the relative index of the first mismatch between this buffer and a given buffer.
    ByteOrder order()
    Retrieves this buffer's byte order.
    ByteBuffer order​(ByteOrder bo)
    Modifies this buffer's byte order.
    ByteBuffer position​(int newPosition)
    Sets this buffer's position.
    abstract ByteBuffer put​(byte b)
    Relative put method  (optional operation).
    ByteBuffer put​(byte[] src)
    Relative bulk put method  (optional operation).
    ByteBuffer put​(byte[] src, int offset, int length)
    Relative bulk put method  (optional operation).
    abstract ByteBuffer put​(int index, byte b)
    Absolute put method  (optional operation).
    ByteBuffer put​(int index, byte[] src)
    Absolute bulk put method  (optional operation).
    ByteBuffer put​(int index, byte[] src, int offset, int length)
    Absolute bulk put method  (optional operation).
    ByteBuffer put​(ByteBuffer src)
    Relative bulk put method  (optional operation).
    abstract ByteBuffer putChar​(char value)
    Relative put method for writing a char value  (optional operation).
    abstract ByteBuffer putChar​(int index, char value)
    Absolute put method for writing a char value  (optional operation).
    abstract ByteBuffer putDouble​(double value)
    Relative put method for writing a double value  (optional operation).
    abstract ByteBuffer putDouble​(int index, double value)
    Absolute put method for writing a double value  (optional operation).
    abstract ByteBuffer putFloat​(float value)
    Relative put method for writing a float value  (optional operation).
    abstract ByteBuffer putFloat​(int index, float value)
    Absolute put method for writing a float value  (optional operation).
    abstract ByteBuffer putInt​(int value)
    Relative put method for writing an int value  (optional operation).
    abstract ByteBuffer putInt​(int index, int value)
    Absolute put method for writing an int value  (optional operation).
    abstract ByteBuffer putLong​(int index, long value)
    Absolute put method for writing a long value  (optional operation).
    abstract ByteBuffer putLong​(long value)
    Relative put method for writing a long value  (optional operation).
    abstract ByteBuffer putShort​(int index, short value)
    Absolute put method for writing a short value  (optional operation).
    abstract ByteBuffer putShort​(short value)
    Relative put method for writing a short value  (optional operation).
    ByteBuffer reset()
    Resets this buffer's position to the previously-marked position.
    ByteBuffer rewind()
    Rewinds this buffer.
    abstract ByteBuffer slice()
    Creates a new byte buffer whose content is a shared subsequence of this buffer's content.
    abstract ByteBuffer slice​(int index, int length)
    Creates a new byte buffer whose content is a shared subsequence of this buffer's content.
    String toString()
    Returns a string summarizing the state of this buffer.
    static ByteBuffer wrap​(byte[] array)
    Wraps a byte array into a buffer.
    static ByteBuffer wrap​(byte[] array, int offset, int length)
    Wraps a byte array into a buffer.

    Methods declared in class java.nio.Buffer

    capacity, hasRemaining, isReadOnly, limit, position, remaining

    Methods declared in class java.lang.Object

    clone, finalize, getClass, notify, notifyAll, wait, wait, wait

  • Method Details

    • allocateDirect

      public static ByteBuffer allocateDirect​(int capacity)
      Allocates a new direct byte buffer.

      The new buffer's position will be zero, its limit will be its capacity, its mark will be undefined, each of its elements will be initialized to zero, and its byte order will be BIG_ENDIAN. Whether or not it has a backing array is unspecified.

      Parameters:
      capacity - The new buffer's capacity, in bytes
      Returns:
      The new byte buffer
      Throws:
      IllegalArgumentException - If the capacity is a negative integer

    • allocate

      public static ByteBuffer allocate​(int capacity)
      Allocates a new byte buffer.

      The new buffer's position will be zero, its limit will be its capacity, its mark will be undefined, each of its elements will be initialized to zero, and its byte order will be BIG_ENDIAN. It will have a backing array, and its array offset will be zero.

      Parameters:
      capacity - The new buffer's capacity, in bytes
      Returns:
      The new byte buffer
      Throws:
      IllegalArgumentException - If the capacity is a negative integer

    • wrap

      public static ByteBuffer wrap​(byte[] array, int offset, int length)
      Wraps a byte array into a buffer.

      The new buffer will be backed by the given byte array; that is, modifications to the buffer will cause the array to be modified and vice versa. The new buffer's capacity will be array.length, its position will be offset, its limit will be offset + length, its mark will be undefined, and its byte order will be BIG_ENDIAN. Its backing array will be the given array, and its array offset will be zero.

      Parameters:
      array - The array that will back the new buffer
      offset - The offset of the subarray to be used; must be non-negative and no larger than array.length. The new buffer's position will be set to this value.
      length - The length of the subarray to be used; must be non-negative and no larger than array.length - offset. The new buffer's limit will be set to offset + length.
      Returns:
      The new byte buffer
      Throws:
      IndexOutOfBoundsException - If the preconditions on the offset and length parameters do not hold

    • wrap

      public static ByteBuffer wrap​(byte[] array)
      Wraps a byte array into a buffer.

      The new buffer will be backed by the given byte array; that is, modifications to the buffer will cause the array to be modified and vice versa. The new buffer's capacity and limit will be array.length, its position will be zero, its mark will be undefined, and its byte order will be BIG_ENDIAN. Its backing array will be the given array, and its array offset will be zero.

      Parameters:
      array - The array that will back this buffer
      Returns:
      The new byte buffer

    • slice

      public abstract ByteBuffer slice()
      Creates a new byte buffer whose content is a shared subsequence of this buffer's content.

      The content of the new buffer will start at this buffer's current position. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer, its mark will be undefined, and its byte order will be BIG_ENDIAN. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Specified by:
      slice in class Buffer
      Returns:
      The new byte buffer
      See Also:
      alignedSlice(int)

    • slice

      public abstract ByteBuffer slice​(int index, int length)
      Creates a new byte buffer whose content is a shared subsequence of this buffer's content.

      The content of the new buffer will start at position index in this buffer, and will contain length elements. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be length, its mark will be undefined, and its byte order will be BIG_ENDIAN. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Specified by:
      slice in class Buffer
      Parameters:
      index - The position in this buffer at which the content of the new buffer will start; must be non-negative and no larger than limit()
      length - The number of elements the new buffer will contain; must be non-negative and no larger than limit() - index
      Returns:
      The new buffer
      Throws:
      IndexOutOfBoundsException - If index is negative or greater than limit(), length is negative, or length > limit() - index
      Since:
      13

    • duplicate

      public abstract ByteBuffer duplicate()
      Creates a new byte buffer that shares this buffer's content.

      The content of the new buffer will be that of this buffer. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's capacity, limit, position, and mark values will be identical to those of this buffer, and its byte order will be BIG_ENDIAN. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Specified by:
      duplicate in class Buffer
      Returns:
      The new byte buffer

    • asReadOnlyBuffer

      public abstract ByteBuffer asReadOnlyBuffer()
      Creates a new, read-only byte buffer that shares this buffer's content.

      The content of the new buffer will be that of this buffer. Changes to this buffer's content will be visible in the new buffer; the new buffer itself, however, will be read-only and will not allow the shared content to be modified. The two buffers' position, limit, and mark values will be independent.

      The new buffer's capacity, limit, position, and mark values will be identical to those of this buffer, and its byte order will be BIG_ENDIAN.

      If this buffer is itself read-only then this method behaves in exactly the same way as the duplicate method.

      Returns:
      The new, read-only byte buffer

    • get

      public abstract byte get()
      Relative get method. Reads the byte at this buffer's current position, and then increments the position.
      Returns:
      The byte at the buffer's current position
      Throws:
      BufferUnderflowException - If the buffer's current position is not smaller than its limit

    • put

      public abstract ByteBuffer put​(byte b)
      Relative put method  (optional operation).

      Writes the given byte into this buffer at the current position, and then increments the position.

      Parameters:
      b - The byte to be written
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If this buffer's current position is not smaller than its limit
      ReadOnlyBufferException - If this buffer is read-only

    • get

      public abstract byte get​(int index)
      Absolute get method. Reads the byte at the given index.
      Parameters:
      index - The index from which the byte will be read
      Returns:
      The byte at the given index
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit

    • put

      public abstract ByteBuffer put​(int index, byte b)
      Absolute put method  (optional operation).

      Writes the given byte into this buffer at the given index.

      Parameters:
      index - The index at which the byte will be written
      b - The byte value to be written
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit
      ReadOnlyBufferException - If this buffer is read-only

    • get

      public ByteBuffer get​(byte[] dst, int offset, int length)
      Relative bulk get method.

      This method transfers bytes from this buffer into the given destination array. If there are fewer bytes remaining in the buffer than are required to satisfy the request, that is, if length > remaining(), then no bytes are transferred and a BufferUnderflowException is thrown.

      Otherwise, this method copies length bytes from this buffer into the given array, starting at the current position of this buffer and at the given offset in the array. The position of this buffer is then incremented by length.

      In other words, an invocation of this method of the form src.get(dst, off, len) has exactly the same effect as the loop 

      
     for (int i = off; i < off + len; i++)
         dst[i] = src.get();
      except that it first checks that there are sufficient bytes in this buffer and it is potentially much more efficient.

      Parameters:
      dst - The array into which bytes are to be written
      offset - The offset within the array of the first byte to be written; must be non-negative and no larger than dst.length
      length - The maximum number of bytes to be written to the given array; must be non-negative and no larger than dst.length - offset
      Returns:
      This buffer
      Throws:
      BufferUnderflowException - If there are fewer than length bytes remaining in this buffer
      IndexOutOfBoundsException - If the preconditions on the offset and length parameters do not hold

    • get

      public ByteBuffer get​(byte[] dst)
      Relative bulk get method.

      This method transfers bytes from this buffer into the given destination array. An invocation of this method of the form src.get(a) behaves in exactly the same way as the invocation 

           src.get(a, 0, a.length)

      Parameters:
      dst - The destination array
      Returns:
      This buffer
      Throws:
      BufferUnderflowException - If there are fewer than length bytes remaining in this buffer

    • get

      public ByteBuffer get​(int index, byte[] dst, int offset, int length)
      Absolute bulk get method.

      This method transfers length bytes from this buffer into the given array, starting at the given index in this buffer and at the given offset in the array. The position of this buffer is unchanged.

      An invocation of this method of the form src.get(index, dst, offset, length) has exactly the same effect as the following loop except that it first checks the consistency of the supplied parameters and it is potentially much more efficient: 

      
     for (int i = offset, j = index; i < offset + length; i++, j++)
         dst[i] = src.get(j);

      Parameters:
      index - The index in this buffer from which the first byte will be read; must be non-negative and less than limit()
      dst - The destination array
      offset - The offset within the array of the first byte to be written; must be non-negative and less than dst.length
      length - The number of bytes to be written to the given array; must be non-negative and no larger than the smaller of limit() - index and dst.length - offset
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If the preconditions on the index, offset, and length parameters do not hold
      Since:
      13

    • get

      public ByteBuffer get​(int index, byte[] dst)
      Absolute bulk get method.

      This method transfers bytes from this buffer into the given destination array. The position of this buffer is unchanged. An invocation of this method of the form src.get(index, dst) behaves in exactly the same way as the invocation: 

           src.get(index, dst, 0, dst.length)

      Parameters:
      index - The index in this buffer from which the first byte will be read; must be non-negative and less than limit()
      dst - The destination array
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative, not smaller than limit(), or limit() - index < dst.length
      Since:
      13

    • put

      public ByteBuffer put​(ByteBuffer src)
      Relative bulk put method  (optional operation).

      This method transfers the bytes remaining in the given source buffer into this buffer. If there are more bytes remaining in the source buffer than in this buffer, that is, if src.remaining() > remaining(), then no bytes are transferred and a BufferOverflowException is thrown.

      Otherwise, this method copies n = src.remaining() bytes from the given buffer into this buffer, starting at each buffer's current position. The positions of both buffers are then incremented by n.

      In other words, an invocation of this method of the form dst.put(src) has exactly the same effect as the loop 

           while (src.hasRemaining())
         dst.put(src.get());
      except that it first checks that there is sufficient space in this buffer and it is potentially much more efficient.

      Parameters:
      src - The source buffer from which bytes are to be read; must not be this buffer
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there is insufficient space in this buffer for the remaining bytes in the source buffer
      IllegalArgumentException - If the source buffer is this buffer
      ReadOnlyBufferException - If this buffer is read-only

    • put

      public ByteBuffer put​(byte[] src, int offset, int length)
      Relative bulk put method  (optional operation).

      This method transfers bytes into this buffer from the given source array. If there are more bytes to be copied from the array than remain in this buffer, that is, if length > remaining(), then no bytes are transferred and a BufferOverflowException is thrown.

      Otherwise, this method copies length bytes from the given array into this buffer, starting at the given offset in the array and at the current position of this buffer. The position of this buffer is then incremented by length.

      In other words, an invocation of this method of the form dst.put(src, off, len) has exactly the same effect as the loop 

      
     for (int i = off; i < off + len; i++)
         dst.put(src[i]);
      except that it first checks that there is sufficient space in this buffer and it is potentially much more efficient.

      Parameters:
      src - The array from which bytes are to be read
      offset - The offset within the array of the first byte to be read; must be non-negative and no larger than src.length
      length - The number of bytes to be read from the given array; must be non-negative and no larger than src.length - offset
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there is insufficient space in this buffer
      IndexOutOfBoundsException - If the preconditions on the offset and length parameters do not hold
      ReadOnlyBufferException - If this buffer is read-only

    • put

      public final ByteBuffer put​(byte[] src)
      Relative bulk put method  (optional operation).

      This method transfers the entire content of the given source byte array into this buffer. An invocation of this method of the form dst.put(a) behaves in exactly the same way as the invocation 

           dst.put(a, 0, a.length)

      Parameters:
      src - The source array
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there is insufficient space in this buffer
      ReadOnlyBufferException - If this buffer is read-only

    • put

      public ByteBuffer put​(int index, byte[] src, int offset, int length)
      Absolute bulk put method  (optional operation).

      This method transfers length bytes from the given array, starting at the given offset in the array and at the given index in this buffer. The position of this buffer is unchanged.

      An invocation of this method of the form dst.put(index, src, offset, length) has exactly the same effect as the following loop except that it first checks the consistency of the supplied parameters and it is potentially much more efficient: 

      
     for (int i = offset, j = index; i < offset + length; i++, j++)
         dst.put(j, src[i]);

      Parameters:
      index - The index in this buffer at which the first byte will be written; must be non-negative and less than limit()
      src - The array from which bytes are to be read
      offset - The offset within the array of the first byte to be read; must be non-negative and less than src.length
      length - The number of bytes to be read from the given array; must be non-negative and no larger than the smaller of limit() - index and src.length - offset
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If the preconditions on the index, offset, and length parameters do not hold
      ReadOnlyBufferException - If this buffer is read-only
      Since:
      13

    • put

      public ByteBuffer put​(int index, byte[] src)
      Absolute bulk put method  (optional operation).

      This method copies bytes into this buffer from the given source array. The position of this buffer is unchanged. An invocation of this method of the form dst.put(index, src) behaves in exactly the same way as the invocation: 

           dst.put(index, src, 0, src.length);

      Parameters:
      index - The index in this buffer at which the first byte will be written; must be non-negative and less than limit()
      src - The array from which bytes are to be read
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative, not smaller than limit(), or limit() - index < src.length
      ReadOnlyBufferException - If this buffer is read-only
      Since:
      13

    • hasArray

      public final boolean hasArray()
      Tells whether or not this buffer is backed by an accessible byte array.

      If this method returns true then the array and arrayOffset methods may safely be invoked.

      Specified by:
      hasArray in class Buffer
      Returns:
      true if, and only if, this buffer is backed by an array and is not read-only

    • array

      public final byte[] array()
      Returns the byte array that backs this buffer  (optional operation).

      Modifications to this buffer's content will cause the returned array's content to be modified, and vice versa.

      Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.

      Specified by:
      array in class Buffer
      Returns:
      The array that backs this buffer
      Throws:
      ReadOnlyBufferException - If this buffer is backed by an array but is read-only
      UnsupportedOperationException - If this buffer is not backed by an accessible array

    • arrayOffset

      public final int arrayOffset()
      Returns the offset within this buffer's backing array of the first element of the buffer  (optional operation).

      If this buffer is backed by an array then buffer position p corresponds to array index p + arrayOffset().

      Invoke the hasArray method before invoking this method in order to ensure that this buffer has an accessible backing array.

      Specified by:
      arrayOffset in class Buffer
      Returns:
      The offset within this buffer's array of the first element of the buffer
      Throws:
      ReadOnlyBufferException - If this buffer is backed by an array but is read-only
      UnsupportedOperationException - If this buffer is not backed by an accessible array

    • position

      public ByteBuffer position​(int newPosition)
      Sets this buffer's position. If the mark is defined and larger than the new position then it is discarded.
      Overrides:
      position in class Buffer
      Parameters:
      newPosition - The new position value; must be non-negative and no larger than the current limit
      Returns:
      This buffer

    • limit

      public ByteBuffer limit​(int newLimit)
      Sets this buffer's limit. If the position is larger than the new limit then it is set to the new limit. If the mark is defined and larger than the new limit then it is discarded.
      Overrides:
      limit in class Buffer
      Parameters:
      newLimit - The new limit value; must be non-negative and no larger than this buffer's capacity
      Returns:
      This buffer

    • mark

      public ByteBuffer mark()
      Sets this buffer's mark at its position.
      Overrides:
      mark in class Buffer
      Returns:
      This buffer

    • reset

      public ByteBuffer reset()
      Resets this buffer's position to the previously-marked position.

      Invoking this method neither changes nor discards the mark's value.

      Overrides:
      reset in class Buffer
      Returns:
      This buffer

    • clear

      public ByteBuffer clear()
      Clears this buffer. The position is set to zero, the limit is set to the capacity, and the mark is discarded.

      Invoke this method before using a sequence of channel-read or put operations to fill this buffer. For example: 

       buf.clear();     // Prepare buffer for reading
 in.read(buf);    // Read data

      This method does not actually erase the data in the buffer, but it is named as if it did because it will most often be used in situations in which that might as well be the case.

      Overrides:
      clear in class Buffer
      Returns:
      This buffer

    • flip

      public ByteBuffer flip()
      Flips this buffer. The limit is set to the current position and then the position is set to zero. If the mark is defined then it is discarded.

      After a sequence of channel-read or put operations, invoke this method to prepare for a sequence of channel-write or relative get operations. For example: 

       buf.put(magic);    // Prepend header
 in.read(buf);      // Read data into rest of buffer
 buf.flip();        // Flip buffer
 out.write(buf);    // Write header + data to channel

      This method is often used in conjunction with the compact method when transferring data from one place to another.

      Overrides:
      flip in class Buffer
      Returns:
      This buffer

    • rewind

      public ByteBuffer rewind()
      Rewinds this buffer. The position is set to zero and the mark is discarded.

      Invoke this method before a sequence of channel-write or get operations, assuming that the limit has already been set appropriately. For example: 

       out.write(buf);    // Write remaining data
 buf.rewind();      // Rewind buffer
 buf.get(array);    // Copy data into array

      Overrides:
      rewind in class Buffer
      Returns:
      This buffer

    • compact

      public abstract ByteBuffer compact()
      Compacts this buffer  (optional operation).

      The bytes between the buffer's current position and its limit, if any, are copied to the beginning of the buffer. That is, the byte at index p = position() is copied to index zero, the byte at index p + 1 is copied to index one, and so forth until the byte at index limit() - 1 is copied to index n = limit() - 1 - p. The buffer's position is then set to n+1 and its limit is set to its capacity. The mark, if defined, is discarded.

      The buffer's position is set to the number of bytes copied, rather than to zero, so that an invocation of this method can be followed immediately by an invocation of another relative put method.

      Invoke this method after writing data from a buffer in case the write was incomplete. The following loop, for example, copies bytes from one channel to another via the buffer buf: 

      
   buf.clear();          // Prepare buffer for use
   while (in.read(buf) >= 0 || buf.position != 0) {
       buf.flip();
       out.write(buf);
       buf.compact();    // In case of partial write
   }

      Returns:
      This buffer
      Throws:
      ReadOnlyBufferException - If this buffer is read-only

    • isDirect

      public abstract boolean isDirect()
      Tells whether or not this byte buffer is direct.
      Specified by:
      isDirect in class Buffer
      Returns:
      true if, and only if, this buffer is direct

    • toString

      public String toString()
      Returns a string summarizing the state of this buffer.
      Overrides:
      toString in class Object
      Returns:
      A summary string

    • hashCode

      public int hashCode()
      Returns the current hash code of this buffer.

      The hash code of a byte buffer depends only upon its remaining elements; that is, upon the elements from position() up to, and including, the element at limit() - 1.

      Because buffer hash codes are content-dependent, it is inadvisable to use buffers as keys in hash maps or similar data structures unless it is known that their contents will not change.

      Overrides:
      hashCode in class Object
      Returns:
      The current hash code of this buffer
      See Also:
      Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)

    • equals

      public boolean equals​(Object ob)
      Tells whether or not this buffer is equal to another object.

      Two byte buffers are equal if, and only if,

      1. They have the same element type,

      2. They have the same number of remaining elements, and

      3. The two sequences of remaining elements, considered independently of their starting positions, are pointwise equal.

      A byte buffer is not equal to any other type of object.

      Overrides:
      equals in class Object
      Parameters:
      ob - The object to which this buffer is to be compared
      Returns:
      true if, and only if, this buffer is equal to the given object
      See Also:
      Object.hashCode(), HashMap

    • compareTo

      public int compareTo​(ByteBuffer that)
      Compares this buffer to another.

      Two byte buffers are compared by comparing their sequences of remaining elements lexicographically, without regard to the starting position of each sequence within its corresponding buffer. Pairs of byte elements are compared as if by invoking Byte.compare(byte,byte).

      A byte buffer is not comparable to any other type of object.

      Specified by:
      compareTo in interface Comparable<ByteBuffer>
      Parameters:
      that - the object to be compared.
      Returns:
      A negative integer, zero, or a positive integer as this buffer is less than, equal to, or greater than the given buffer

    • mismatch

      public int mismatch​(ByteBuffer that)
      Finds and returns the relative index of the first mismatch between this buffer and a given buffer. The index is relative to the position of each buffer and will be in the range of 0 (inclusive) up to the smaller of the remaining elements in each buffer (exclusive).

      If the two buffers share a common prefix then the returned index is the length of the common prefix and it follows that there is a mismatch between the two buffers at that index within the respective buffers. If one buffer is a proper prefix of the other then the returned index is the smaller of the remaining elements in each buffer, and it follows that the index is only valid for the buffer with the larger number of remaining elements. Otherwise, there is no mismatch.

      Parameters:
      that - The byte buffer to be tested for a mismatch with this buffer
      Returns:
      The relative index of the first mismatch between this and the given buffer, otherwise -1 if no mismatch.
      Since:
      11

    • order

      public final ByteOrder order()
      Retrieves this buffer's byte order.

      The byte order is used when reading or writing multibyte values, and when creating buffers that are views of this byte buffer. The order of a newly-created byte buffer is always BIG_ENDIAN.

      Returns:
      This buffer's byte order

    • order

      public final ByteBuffer order​(ByteOrder bo)
      Modifies this buffer's byte order.
      Parameters:
      bo - The new byte order, either BIG_ENDIAN or LITTLE_ENDIAN
      Returns:
      This buffer

    • alignmentOffset

      public final int alignmentOffset​(int index, int unitSize)
      Returns the memory address, pointing to the byte at the given index, modulo the given unit size.

      The return value is non-negative in the range of 0 (inclusive) up to unitSize (exclusive), with zero indicating that the address of the byte at the index is aligned for the unit size, and a positive value that the address is misaligned for the unit size. If the address of the byte at the index is misaligned, the return value represents how much the index should be adjusted to locate a byte at an aligned address. Specifically, the index should either be decremented by the return value if the latter is not greater than index, or be incremented by the unit size minus the return value. Therefore given 

       int value = alignmentOffset(index, unitSize)
      then the identities 
       alignmentOffset(index - value, unitSize) == 0, value ≤ index
      and 
       alignmentOffset(index + (unitSize - value), unitSize) == 0
      must hold.

      API Note:
      This method may be utilized to determine if unit size bytes from an index can be accessed atomically, if supported by the native platform.
      Implementation Note:
      This implementation throws UnsupportedOperationException for non-direct buffers when the given unit size is greater then 8.
      Parameters:
      index - The index to query for alignment offset, must be non-negative, no upper bounds check is performed
      unitSize - The unit size in bytes, must be a power of 2
      Returns:
      The indexed byte's memory address modulo the unit size
      Throws:
      IllegalArgumentException - If the index is negative or the unit size is not a power of 2
      UnsupportedOperationException - If the native platform does not guarantee stable alignment offset values for the given unit size when managing the memory regions of buffers of the same kind as this buffer (direct or non-direct). For example, if garbage collection would result in the moving of a memory region covered by a non-direct buffer from one location to another and both locations have different alignment characteristics.
      Since:
      9
      See Also:
      alignedSlice(int)

    • alignedSlice

      public final ByteBuffer alignedSlice​(int unitSize)
      Creates a new byte buffer whose content is a shared and aligned subsequence of this buffer's content.

      The content of the new buffer will start at this buffer's current position rounded up to the index of the nearest aligned byte for the given unit size, and end at this buffer's limit rounded down to the index of the nearest aligned byte for the given unit size. If rounding results in out-of-bound values then the new buffer's capacity and limit will be zero. If rounding is within bounds the following expressions will be true for a new buffer nb and unit size unitSize: 

      
 nb.alignmentOffset(0, unitSize) == 0
 nb.alignmentOffset(nb.limit(), unitSize) == 0

      Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer or fewer subject to alignment, its mark will be undefined, and its byte order will be BIG_ENDIAN. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      API Note:
      This method may be utilized to create a new buffer where unit size bytes from index, that is a multiple of the unit size, may be accessed atomically, if supported by the native platform.
      Implementation Note:
      This implementation throws UnsupportedOperationException for non-direct buffers when the given unit size is greater then 8.
      Parameters:
      unitSize - The unit size in bytes, must be a power of 2
      Returns:
      The new byte buffer
      Throws:
      IllegalArgumentException - If the unit size not a power of 2
      UnsupportedOperationException - If the native platform does not guarantee stable aligned slices for the given unit size when managing the memory regions of buffers of the same kind as this buffer (direct or non-direct). For example, if garbage collection would result in the moving of a memory region covered by a non-direct buffer from one location to another and both locations have different alignment characteristics.
      Since:
      9
      See Also:
      alignmentOffset(int, int), slice()

    • getChar

      public abstract char getChar()
      Relative get method for reading a char value.

      Reads the next two bytes at this buffer's current position, composing them into a char value according to the current byte order, and then increments the position by two.

      Returns:
      The char value at the buffer's current position
      Throws:
      BufferUnderflowException - If there are fewer than two bytes remaining in this buffer

    • putChar

      public abstract ByteBuffer putChar​(char value)
      Relative put method for writing a char value  (optional operation).

      Writes two bytes containing the given char value, in the current byte order, into this buffer at the current position, and then increments the position by two.

      Parameters:
      value - The char value to be written
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there are fewer than two bytes remaining in this buffer
      ReadOnlyBufferException - If this buffer is read-only

    • getChar

      public abstract char getChar​(int index)
      Absolute get method for reading a char value.

      Reads two bytes at the given index, composing them into a char value according to the current byte order.

      Parameters:
      index - The index from which the bytes will be read
      Returns:
      The char value at the given index
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus one

    • putChar

      public abstract ByteBuffer putChar​(int index, char value)
      Absolute put method for writing a char value  (optional operation).

      Writes two bytes containing the given char value, in the current byte order, into this buffer at the given index.

      Parameters:
      index - The index at which the bytes will be written
      value - The char value to be written
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus one
      ReadOnlyBufferException - If this buffer is read-only

    • asCharBuffer

      public abstract CharBuffer asCharBuffer()
      Creates a view of this byte buffer as a char buffer.

      The content of the new buffer will start at this buffer's current position. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer divided by two, its mark will be undefined, and its byte order will be that of the byte buffer at the moment the view is created. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Returns:
      A new char buffer

    • getShort

      public abstract short getShort()
      Relative get method for reading a short value.

      Reads the next two bytes at this buffer's current position, composing them into a short value according to the current byte order, and then increments the position by two.

      Returns:
      The short value at the buffer's current position
      Throws:
      BufferUnderflowException - If there are fewer than two bytes remaining in this buffer

    • putShort

      public abstract ByteBuffer putShort​(short value)
      Relative put method for writing a short value  (optional operation).

      Writes two bytes containing the given short value, in the current byte order, into this buffer at the current position, and then increments the position by two.

      Parameters:
      value - The short value to be written
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there are fewer than two bytes remaining in this buffer
      ReadOnlyBufferException - If this buffer is read-only

    • getShort

      public abstract short getShort​(int index)
      Absolute get method for reading a short value.

      Reads two bytes at the given index, composing them into a short value according to the current byte order.

      Parameters:
      index - The index from which the bytes will be read
      Returns:
      The short value at the given index
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus one

    • putShort

      public abstract ByteBuffer putShort​(int index, short value)
      Absolute put method for writing a short value  (optional operation).

      Writes two bytes containing the given short value, in the current byte order, into this buffer at the given index.

      Parameters:
      index - The index at which the bytes will be written
      value - The short value to be written
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus one
      ReadOnlyBufferException - If this buffer is read-only

    • asShortBuffer

      public abstract ShortBuffer asShortBuffer()
      Creates a view of this byte buffer as a short buffer.

      The content of the new buffer will start at this buffer's current position. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer divided by two, its mark will be undefined, and its byte order will be that of the byte buffer at the moment the view is created. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Returns:
      A new short buffer

    • getInt

      public abstract int getInt()
      Relative get method for reading an int value.

      Reads the next four bytes at this buffer's current position, composing them into an int value according to the current byte order, and then increments the position by four.

      Returns:
      The int value at the buffer's current position
      Throws:
      BufferUnderflowException - If there are fewer than four bytes remaining in this buffer

    • putInt

      public abstract ByteBuffer putInt​(int value)
      Relative put method for writing an int value  (optional operation).

      Writes four bytes containing the given int value, in the current byte order, into this buffer at the current position, and then increments the position by four.

      Parameters:
      value - The int value to be written
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there are fewer than four bytes remaining in this buffer
      ReadOnlyBufferException - If this buffer is read-only

    • getInt

      public abstract int getInt​(int index)
      Absolute get method for reading an int value.

      Reads four bytes at the given index, composing them into a int value according to the current byte order.

      Parameters:
      index - The index from which the bytes will be read
      Returns:
      The int value at the given index
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus three

    • putInt

      public abstract ByteBuffer putInt​(int index, int value)
      Absolute put method for writing an int value  (optional operation).

      Writes four bytes containing the given int value, in the current byte order, into this buffer at the given index.

      Parameters:
      index - The index at which the bytes will be written
      value - The int value to be written
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus three
      ReadOnlyBufferException - If this buffer is read-only

    • asIntBuffer

      public abstract IntBuffer asIntBuffer()
      Creates a view of this byte buffer as an int buffer.

      The content of the new buffer will start at this buffer's current position. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer divided by four, its mark will be undefined, and its byte order will be that of the byte buffer at the moment the view is created. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Returns:
      A new int buffer

    • getLong

      public abstract long getLong()
      Relative get method for reading a long value.

      Reads the next eight bytes at this buffer's current position, composing them into a long value according to the current byte order, and then increments the position by eight.

      Returns:
      The long value at the buffer's current position
      Throws:
      BufferUnderflowException - If there are fewer than eight bytes remaining in this buffer

    • putLong

      public abstract ByteBuffer putLong​(long value)
      Relative put method for writing a long value  (optional operation).

      Writes eight bytes containing the given long value, in the current byte order, into this buffer at the current position, and then increments the position by eight.

      Parameters:
      value - The long value to be written
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there are fewer than eight bytes remaining in this buffer
      ReadOnlyBufferException - If this buffer is read-only

    • getLong

      public abstract long getLong​(int index)
      Absolute get method for reading a long value.

      Reads eight bytes at the given index, composing them into a long value according to the current byte order.

      Parameters:
      index - The index from which the bytes will be read
      Returns:
      The long value at the given index
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus seven

    • putLong

      public abstract ByteBuffer putLong​(int index, long value)
      Absolute put method for writing a long value  (optional operation).

      Writes eight bytes containing the given long value, in the current byte order, into this buffer at the given index.

      Parameters:
      index - The index at which the bytes will be written
      value - The long value to be written
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus seven
      ReadOnlyBufferException - If this buffer is read-only

    • asLongBuffer

      public abstract LongBuffer asLongBuffer()
      Creates a view of this byte buffer as a long buffer.

      The content of the new buffer will start at this buffer's current position. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer divided by eight, its mark will be undefined, and its byte order will be that of the byte buffer at the moment the view is created. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Returns:
      A new long buffer

    • getFloat

      public abstract float getFloat()
      Relative get method for reading a float value.

      Reads the next four bytes at this buffer's current position, composing them into a float value according to the current byte order, and then increments the position by four.

      Returns:
      The float value at the buffer's current position
      Throws:
      BufferUnderflowException - If there are fewer than four bytes remaining in this buffer

    • putFloat

      public abstract ByteBuffer putFloat​(float value)
      Relative put method for writing a float value  (optional operation).

      Writes four bytes containing the given float value, in the current byte order, into this buffer at the current position, and then increments the position by four.

      Parameters:
      value - The float value to be written
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there are fewer than four bytes remaining in this buffer
      ReadOnlyBufferException - If this buffer is read-only

    • getFloat

      public abstract float getFloat​(int index)
      Absolute get method for reading a float value.

      Reads four bytes at the given index, composing them into a float value according to the current byte order.

      Parameters:
      index - The index from which the bytes will be read
      Returns:
      The float value at the given index
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus three

    • putFloat

      public abstract ByteBuffer putFloat​(int index, float value)
      Absolute put method for writing a float value  (optional operation).

      Writes four bytes containing the given float value, in the current byte order, into this buffer at the given index.

      Parameters:
      index - The index at which the bytes will be written
      value - The float value to be written
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus three
      ReadOnlyBufferException - If this buffer is read-only

    • asFloatBuffer

      public abstract FloatBuffer asFloatBuffer()
      Creates a view of this byte buffer as a float buffer.

      The content of the new buffer will start at this buffer's current position. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer divided by four, its mark will be undefined, and its byte order will be that of the byte buffer at the moment the view is created. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Returns:
      A new float buffer

    • getDouble

      public abstract double getDouble()
      Relative get method for reading a double value.

      Reads the next eight bytes at this buffer's current position, composing them into a double value according to the current byte order, and then increments the position by eight.

      Returns:
      The double value at the buffer's current position
      Throws:
      BufferUnderflowException - If there are fewer than eight bytes remaining in this buffer

    • putDouble

      public abstract ByteBuffer putDouble​(double value)
      Relative put method for writing a double value  (optional operation).

      Writes eight bytes containing the given double value, in the current byte order, into this buffer at the current position, and then increments the position by eight.

      Parameters:
      value - The double value to be written
      Returns:
      This buffer
      Throws:
      BufferOverflowException - If there are fewer than eight bytes remaining in this buffer
      ReadOnlyBufferException - If this buffer is read-only

    • getDouble

      public abstract double getDouble​(int index)
      Absolute get method for reading a double value.

      Reads eight bytes at the given index, composing them into a double value according to the current byte order.

      Parameters:
      index - The index from which the bytes will be read
      Returns:
      The double value at the given index
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus seven

    • putDouble

      public abstract ByteBuffer putDouble​(int index, double value)
      Absolute put method for writing a double value  (optional operation).

      Writes eight bytes containing the given double value, in the current byte order, into this buffer at the given index.

      Parameters:
      index - The index at which the bytes will be written
      value - The double value to be written
      Returns:
      This buffer
      Throws:
      IndexOutOfBoundsException - If index is negative or not smaller than the buffer's limit, minus seven
      ReadOnlyBufferException - If this buffer is read-only

    • asDoubleBuffer

      public abstract DoubleBuffer asDoubleBuffer()
      Creates a view of this byte buffer as a double buffer.

      The content of the new buffer will start at this buffer's current position. Changes to this buffer's content will be visible in the new buffer, and vice versa; the two buffers' position, limit, and mark values will be independent.

      The new buffer's position will be zero, its capacity and its limit will be the number of bytes remaining in this buffer divided by eight, its mark will be undefined, and its byte order will be that of the byte buffer at the moment the view is created. The new buffer will be direct if, and only if, this buffer is direct, and it will be read-only if, and only if, this buffer is read-only.

      Returns:
      A new double buffer