writeUTF(String s) vs writeObject(String s)

Question

In this Java project I'm working on for university, I have a situation where I am currently sending strings through the network successfully using

streamOut = ObjectOutputStream
streamIn = ObjectInputStream

streamOut.writeUTF(msgs.peek());

where msgs is a linked blocking queue, receiving it with

String in = streamIn.readUTF();

however, I would like to use an ObjectInputStream and an ObjectOutputStream. I have initialized them both in the constructor and I flush the ObjectOutputStream after constructing it, I read somewhere you have to do this.

I want to send both Strings and another Object type, call it gameWorld over the network (don't care about efficiency at this point).. however when I do

streamOut.writeObject("mad cuz i'm bad");

Object in = streamIn.readObject(); 
if(in instanceof String) String inS = (String) in;

it doesn't pick anything up when I send strings over... my friend is working on the same project and he passes around only 1 type of object, one of the subclasses of this object is essentially a string and his version works fine, but he makes a new stream in every iteration of his thread's run loop.

Do I need to do something with the stream to receive different objects which don't have a common ancestor other than Object, do I need to make a new stream every iteration of the run loop or is there just something else completely that I'm missing and the information I've provided isn't sufficient to tell whats wrong?

Answer 1

There is a significant difference between writing a String into the stream as primitive data or as an Object. A String instance written by writeObject is written into the stream as a String initially. Future writeObject() calls write references to the string into the stream.

For Example

    ByteArrayOutputStream baos1=new ByteArrayOutputStream();
    oos1=new ObjectOutputStream(baos1);
    baos2=new ByteArrayOutputStream();
    ObjectOutputStream oos2=new ObjectOutputStream(baos2);
    String testString="First";
    oos1.writeObject(testString);
    oos2.writeUTF(testString);
    testString="Second";
    oos1.writeObject(testString);
    oos2.writeUTF(testString);
    testString="Third";
    oos1.writeObject(testString);
    oos2.writeUTF(testString);
    oos1.flush();
    oos2.flush();
    byte[] byteArray1=baos1.toByteArray();
    byte[] byteArray2=baos2.toByteArray();

Dump last two arrays you will get result like:
writeObject i.e. byteArray1
Binary: -84 -19 0 5 116 0 5 70 105 114 115 116 116 0 6 83 101 99 111 110 100 116 0 5 84 104 105 114 100
ASCII: -T - t F i r s t t S e c o n d t T h i r d

writeUTF i.e. byteArray2
Binary: -84 -19 0 5 119 22 0 5 70 105 114 115 116 0 6 83 101 99 111 110 100 0 5 84 104 105 114 100
ASCII: -T - w F i r s t S e c o n d T h i r d

Conclusion: In case of writeObject an extra data (here t)to be streamed whereas in case of writeUTF only the string data to be streamed.

For more: http://docs.oracle.com/javase/7/docs/api/java/io/ObjectOutputStream.html#writeUTF(java.lang.String)