IncomingPacket.kt

class IncomingPacket(opcode: Int, type: PacketType, buffer: ByteBuf) : Packet(opcode, type, buffer)  {

    /**
     * Reads a byte from the buffer and returns it
     */
    fun readByte(): Byte = buffer.readByte()

    /**
     * Reads an integer from the buffer and returns it
     */
    fun readInt(): Int = buffer.readInt()


}

OutgoingPacket.kt

class OutgoingPacket(opcode: Int, type: PacketType, buffer: ByteBuf) : Packet(opcode, type, buffer) {

    /**
     * Constructs a [PacketType.STANDARD] packet a specified id
     */
    constructor(opcode: Int) :
            this(opcode, PacketType.STANDARD, Unpooled.buffer())

    /**
     * Constructs a packet with a specified id and type
     */
    constructor(opcode: Int, type: PacketType) :
            this(opcode, type, Unpooled.buffer())

    /**
     * Writes another buffer to the buffer
     */
    fun writeBytes(data: ByteBuf) = buffer.writeBytes(data)

    /**
     * Writes a byte to the buffer
     */
    fun writeByte(data: Int): ByteBuf = buffer.writeByte(data)

    /**
     * Writes an integer to the buffer
     */
    fun writeInt(data: Int): ByteBuf = buffer.writeInt(data)

    /**
     * Writes a string to the buffer
     */
    fun writeString(data: String) {
        buffer.writeBytes(data.toByteArray())
        buffer.writeByte(0)
    }

}

Packet.kt

open class Packet(val opcode: Int, val type: PacketType, val buffer: ByteBuf, val length: Int) {

    /**
     * Constructs a packet
     * @param opcode the numerical identifier of the packet
     * @param type the type of packet this is
     * @param buffer the buffer of bytes
     * @constructor
     */
    constructor(opcode: Int, type: PacketType, buffer: ByteBuf) :
            this(opcode, type, buffer, buffer.readableBytes())

    /**
     * If the packet is a raw packet
     */
    fun isRaw() = opcode == 1
}