== Buffers
:toc: left

Most data is shuffled around inside Vert.x using buffers.

A buffer is a sequence of zero or more bytes that can read from or written to and which expands automatically as
necessary to accommodate any bytes written to it. You can perhaps think of a buffer as smart byte array.

=== Creating buffers

Buffers can create by using one of the static `link:../../apidocs/io/vertx/core/buffer/Buffer.html#buffer--[Buffer.buffer]` methods.

Buffers can be initialised from strings or byte arrays, or empty buffers can be created.

Here are some examples of creating buffers:

Create a new empty buffer:

[source,java]
----
Buffer buff = Buffer.buffer();
----

Create a buffer from a String. The String will be encoded in the buffer using UTF-8.

[source,java]
----
Buffer buff = Buffer.buffer("some string");
----

Create a buffer from a String: The String will be encoded using the specified encoding, e.g:

[source,java]
----
Buffer buff = Buffer.buffer("some string", "UTF-16");
----

include::override/buffer_from_bytes.adoc[]

Create a buffer with an initial size hint. If you know your buffer will have a certain amount of data written to it
you can create the buffer and specify this size. This makes the buffer initially allocate that much memory and is
more efficient than the buffer automatically resizing multiple times as data is written to it.

Note that buffers created this way *are empty*. It does not create a buffer filled with zeros up to the specified size.

[source,java]
----
Buffer buff = Buffer.buffer(10000);
----

=== Writing to a Buffer

There are two ways to write to a buffer: appending, and random access.
In either case buffers will always expand automatically to encompass the bytes. It's not possible to get
an `IndexOutOfBoundsException` with a buffer.

==== Appending to a Buffer

To append to a buffer, you use the `appendXXX` methods.
Append methods exist for appending various different types.

The return value of the `appendXXX` methods is the buffer itself, so these can be chained:

[source,java]
----
Buffer buff = Buffer.buffer();

buff.appendInt(123).appendString("hello\n");

socket.write(buff);
----

==== Random access buffer writes

You can also write into the buffer at a specific index, by using the `setXXX` methods.
Set methods exist for various different data types. All the set methods take an index as the first argument - this
represents the position in the buffer where to start writing the data.

The buffer will always expand as necessary to accommodate the data.

[source,java]
----
Buffer buff = Buffer.buffer();

buff.setInt(1000, 123);
buff.setString(0, "hello");
----

=== Reading from a Buffer

Data is read from a buffer using the `getXXX` methods. Get methods exist for various datatypes.
The first argument to these methods is an index in the buffer from where to get the data.

[source,java]
----
Buffer buff = Buffer.buffer();
for (int i = 0; i < buff.length(); i += 4) {
  System.out.println("int value at " + i + " is " + buff.getInt(i));
}
----

=== Working with unsigned numbers

Unsigned numbers can be read from or appended/set to a buffer with the `getUnsignedXXX`,
`appendUnsignedXXX` and `setUnsignedXXX` methods. This is useful when implementing a codec for a
network protocol optimized to minimize bandwidth consumption.

In the following example, value 200 is set at specified position with just one byte:

[source,java]
----
Buffer buff = Buffer.buffer(128);
int pos = 15;
buff.setUnsignedByte(pos, (short) 200);
System.out.println(buff.getUnsignedByte(pos));
----

The console shows '200'.

=== Buffer length

Use `link:../../apidocs/io/vertx/core/buffer/Buffer.html#length--[length]` to obtain the length of the buffer.
The length of a buffer is the index of the byte in the buffer with the largest index + 1.

=== Copying buffers

Use `link:../../apidocs/io/vertx/core/buffer/Buffer.html#copy--[copy]` to make a copy of the buffer

=== Slicing buffers

A sliced buffer is a new buffer which backs onto the original buffer, i.e. it does not copy the underlying data.
Use `link:../../apidocs/io/vertx/core/buffer/Buffer.html#slice--[slice]` to create a sliced buffers

=== Buffer re-use

After writing a buffer to a socket or other similar place, they cannot be re-used.