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Thread: return value chaned when get back from function call

  1. #1
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    Default return value chaned when get back from function call

    Hi,

    I have the following classes:

    public class CGenericData implements Serializable {}

    public class CGpsMsgData {
    public int x;
    public int y;
    public int z;
    }

    public class CGpsMessage extends CGenericData {
    public CGpsMsgData cGpsMsgData;
    }

    I'am using UDP to send/recv messages:
    public void Send(CGenericData cGenericData) {
    try
    {
    ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
    ObjectOutputStream os = new ObjectOutputStream(outputStream);
    os.writeObject(cGenericData);
    byte[] data = outputStream.toByteArray();
    m_cDatagramPacketSendPacket.setData(data);
    m_cDatagramPacketSendPacket.setLength(data.length) ;
    m_cDatagramSocket.send(m_cDatagramPacketSendPacket );
    }
    catch (IOException e1)
    {
    e1.printStackTrace();
    }
    }


    public void Receive(CGenericData cGenericData) {
    try
    {
    m_cDatagramSocket.receive(m_cDatagramPacketReceive Packet);
    byte[] byteData = m_cDatagramPacketReceivePacket.getData();
    ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(byteData);
    ObjectInputStream objectInputStream = new ObjectInputStream(byteArrayInputStream);
    CGenericData cGenericData1 = (CGenericData) objectInputStream.readObject();
    cGenericData = cGenericData1;
    }
    catch (IOException e)
    {
    return;
    }
    catch (ClassNotFoundException e)
    {
    return;
    }
    }

    I have something strange:
    I get the Gps messages I send ,
    and when I look on the cGenericData at Receive function before it return to the caller, I see the values I sent.
    But !!!
    when I look on it after thee function call (from the main) it has default values, why ? (it is not like pointers in c++) ?

    Thanks


  2. #2
    Member andbin's Avatar
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    Default Re: return value chaned when get back from function call

    Quote Originally Posted by amit1983 View Post
    public void Receive(CGenericData cGenericData) {		
    	try 
    	{
    		m_cDatagramSocket.receive(m_cDatagramPacketReceivePacket);
    		byte[] byteData = m_cDatagramPacketReceivePacket.getData();			
    		ByteArrayInputStream byteArrayInputStream = new ByteArrayInputStream(byteData);
    		ObjectInputStream objectInputStream = new ObjectInputStream(byteArrayInputStream);
    		CGenericData cGenericData1 = (CGenericData) objectInputStream.readObject();
    		cGenericData = cGenericData1;   // <------- NOT RIGHT				
    	} 
    [....]
    Arguments in Java are always passed by value. Even a reference is passed by value. When you invoke Receive you are really passing a copy of the reference pointing to your CGenericData object.

    In the line I have highlighted in the comment, you are assigning a new object to the cGenericData parameter. You cannot change the value of the variable in the main (because the cGenericData parameter is distinct and with a copy of the reference) .... at most you can change the internal state of the passed object.

    So doing:
    cGenericData = cGenericData1;

    is not the right thing. What is the solution? The most simple: change the state of the object the Receive receives. Or, conceptually better, return the new object.
    Andrea, www.andbin.netSCJP 5 (91%) – SCWCD 5 (94%)

    Useful links for Java beginnersMy new project Java Examples on Google Code

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    Default Re: return value chaned when get back from function call

    Hi,

    Thank's for your answer.

    1. What do you mean when you write: " change the *state* of the object the Receive receives" ?
    (can you give me an example ?)

    2. If I want to return byte[], is it possible ?
    if so, how I can deliver the length of the byte[] ?

    Thank's again

  4. #4
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    Default Re: return value chaned when get back from function call

    Quote Originally Posted by amit1983 View Post
    1. What do you mean when you write: " change the *state* of the object the Receive receives" ?
    (can you give me an example ?)
    If your class has public fields (and this is your case), simply access those fields (e.g. obj.field = something;). However having public fields is not generally a good "design". It's better to keep fields private and provide public "accessor" methods like setXyz/getXyz. This is a basic concept about Java "beans". For example: obj.setXyz(.....);

    This is the meaning of "change the state of the object".

    Quote Originally Posted by amit1983 View Post
    2. If I want to return byte[], is it possible ?
    if so, how I can deliver the length of the byte[] ?
    Yes, it's possible to return a byte[]. Every array has an implicit length field. So the length is always known.

    But in your case, note that if you return a byte[], its content is a serialized object. And deserialization is the responsibility of the caller. This is not a very good design. You should hide this aspect into the method, like you have done in the Send method.
    Andrea, www.andbin.netSCJP 5 (91%) – SCWCD 5 (94%)

    Useful links for Java beginnersMy new project Java Examples on Google Code

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