The ByteBuffer class is important because it forms a basis for the use of channels in Java. ByteBuffer class defines six categories of operations upon byte buffers, as stated in the Java 7 documentation:

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

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

• 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

• Methods for compacting, duplicating, and slicing a byte buffer.

Example code : Putting Bytes into a buffer.

    // Create an empty ByteBuffer with a 10 byte capacity
    ByteBuffer bbuf = ByteBuffer.allocate(10);

    // Get the buffer's capacity
    int capacity = bbuf.capacity(); // 10

    // Use the absolute put(int, byte).
    // This method does not affect the position.
    bbuf.put(0, (byte)0xFF); // position=0

    // Set the position
    bbuf.position(5);

    // Use the relative put(byte)
    bbuf.put((byte)0xFF);

    // Get the new position
    int pos = bbuf.position(); // 6

    // Get remaining byte count
    int rem = bbuf.remaining(); // 4

    // Set the limit
    bbuf.limit(7); // remaining=1

    // This convenience method sets the position to 0
    bbuf.rewind(); // remaining=7

EDIT (2018): The edited sibling answer by @xinyongCheng is a simpler approach, and should be the accepted answer.

Your approach would be reasonable if you knew the bytes are in the platform's default charset. In your example, this is true because k.getBytes() returns the bytes in the platform's default charset.

More frequently, you'll want to specify the encoding. However, there's a simpler way to do that than the question you linked. The String API provides methods that converts between a String and a byte[] array in a particular encoding. These methods suggest using CharsetEncoder/CharsetDecoder "when more control over the decoding [encoding] process is required."

To get the bytes from a String in a particular encoding, you can use a sibling getBytes() method:

byte[] bytes = k.getBytes( StandardCharsets.UTF_8 );

To put bytes with a particular encoding into a String, you can use a different String constructor:

String v = new String( bytes, StandardCharsets.UTF_8 );

Note that ByteBuffer.array() is an optional operation. If you've constructed your ByteBuffer with an array, you can use that array directly. Otherwise, if you want to be safe, use ByteBuffer.get(byte[] dst, int offset, int length) to get bytes from the buffer into a byte array.