javax.microedition.io

Interface Datagram

All Superinterfaces:

DataInput, DataOutput

public interface Datagram
extends DataInput, DataOutput

This class defines an abstract interface for datagram packets. The implementations of this interface hold data to be sent or received from a DatagramConnection object.

Since this is an interface class, the internal structure of the datagram packets is not defined here. However, it is assumed that each implementation of this interface will provide the following fields / state variables (the actual implementation and the names of these fields may vary):

• buffer: the internal buffer in which data is stored
• offset: the read/write offset for the internal buffer
• length: the length of the data in datagram packet
• address: the destination or source address
• read/write pointer: a pointer that is added to the offset to point to the current data location during a read or write operation

Reading and Writing

The Datagram interface extends interfaces DataInput and DataOutput in order to provide a simple way to read and write binary data in and out of the datagram buffer instead of using getData and setData methods. Writing automatically increments length and reading will continue while the read/write pointer is less than length. Before any writing is done reset must be called. If setData() is to be used when reading or writing, any value for the offset parameter other than 0 is not supported.

For example to write to datagram:

    datagram = connection.newDatagram(max);

    // Reset prepares the datagram for writing new message.
    datagram.reset();

    // writeUTF automatically increases the datagram length.
    datagram.writeUTF("hello world");

    connection.send(datagram);
 

For example to read from a datagram (single use only):

    datagram = connection.newDatagram(max);

    connection.receive(datagram);

    message = datagram.readUTF();
 

Reusing Datagrams

It should be noted the length above is returned from getLength and can have different meanings at different times. When sending length is the number of bytes to send. Before receiving length is the maximum number of bytes to receive. After receiving length is the number of bytes that were received. So when reusing a datagram to receive after sending or receiving, length must be set back to the maximum using setLength.

    datagram = connection.newDatagram(max);

    while (notDone) {

        // The last receive in the loop changed the length
        // so put it back to the maximum length.
        datagram.setLength(max);
        connection.receive(datagram);

        data = datagram.getData();
        bytesReceived = datagram.getLength();

        // process datagram ...
    }
 

When reading instead of using getData the reset method must be used.

    datagram = connection.newDatagram(max);

    while (notDone) {

        // The last read in the loop changed the read pointer
        // so reset the pointer.
        datagram.reset();
        datagram.setLength(max);
        connection.receive(datagram);

        message = datagram.readUTF(message);

        // process message ...
    }
 

For example to reread a datagram:

    connection.receive(datagram);

    message = datagram.readUTF(message);

    len = datagram.getLength();

    datagram.reset();

    datagram.setLength(len);

    copy = datagram.readUTF(message);
 

Since:

CLDC 1.0

Method Summary

Methods

Modifier and Type	Method and Description
String	getAddress() Get the address of the datagram.
byte[]	getData() Get the contents of the data buffer.
int	getLength() Get the length of the datagram.
int	getOffset() Get the offset.
void	reset() Zero the read/write pointer as well as the offset and length state variables.
void	setAddress(Datagram reference) Set datagram address, copying the address from another datagram.
void	setAddress(String addr) Set datagram address.
void	setData(byte[] buffer, int offset, int len) Set the buffer, offset and length state variables.
void	setLength(int len) Set the length state variable.

Methods inherited from interface java.io.DataInput

readBoolean, readByte, readChar, readDouble, readFloat, readFully, readFully, readInt, readLong, readShort, readUnsignedByte, readUnsignedShort, readUTF, skipBytes

Methods inherited from interface java.io.DataOutput

write, write, write, writeBoolean, writeByte, writeChar, writeChars, writeDouble, writeFloat, writeInt, writeLong, writeShort, writeUTF

Method Detail

getAddress

String getAddress()

Get the address of the datagram.

Returns:

the address in string form, or null if no address was set

See Also:

setAddress(java.lang.String)

getData

byte[] getData()

Get the contents of the data buffer.

Depending on the implementation, this operation may return the internal buffer or a copy of it. However, the user must not assume that the contents of the internal data buffer can be manipulated by modifying the data returned by this operation. Rather, the setData operation should be used for changing the contents of the internal buffer.

Returns:

the data buffer as a byte array

See Also:

setData(byte[], int, int)

getLength

int getLength()

Get the length of the datagram.

Returns:

the length state variable

See Also:

setLength(int)

getOffset

int getOffset()

Get the offset.

Returns:

the offset state variable

setAddress

void setAddress(String addr)
                throws IOException

Set datagram address.

The actual addressing scheme is implementation-dependent. Please read the general comments on datagram addressing in DatagramConnection.java.

Note that if the address of a datagram is not specified, then it defaults to that of the connection.

Parameters:

addr - the new target address as a URL

Throws:

IllegalArgumentException - if the address is not valid

IOException - if a some kind of I/O error occurs

See Also:

getAddress()

setAddress

void setAddress(Datagram reference)

Set datagram address, copying the address from another datagram.

Parameters:

reference - to the datagram whose address will be copied as the new target address for this datagram.

Throws:

IllegalArgumentException - if the address is not valid

See Also:

getAddress()

setLength

void setLength(int len)

Set the length state variable.

Parameters:

len - the new length of the datagram

Throws:

IllegalArgumentException - if the length or length plus offset fall outside the buffer

See Also:

getLength()

setData

void setData(byte[] buffer,
           int offset,
           int len)

Set the buffer, offset and length state variables. Depending on the implementation, this operation may copy the buffer or just set the state variable buffer to the value of the buffer argument. However, the user must not assume that the contents of the internal data buffer can be manipulated by modifying the buffer passed on to this operation.

Parameters:

buffer - the data buffer

offset - the offset into the data buffer

len - the length of the data in the buffer

Throws:

IllegalArgumentException - if the length or offset or offset plus length fall outside the buffer, or if the buffer parameter is invalid

See Also:

getData()

reset

void reset()

Zero the read/write pointer as well as the offset and length state variables.