I'm using the MIDP 2.0 (JSR 118) and I just noticed that there is no reader for strings in J2ME.
Does anyone know how you are supposed to read Strings from an InputStream or InputStreamReader in a platform independent way (i.e. between two java enabled cell phones of different models)?
Which profile are you using? The MID profile in JSR 118 specifies InputStreamReader (not StringReader, but that wouldn't help you read from an InputStream anyway).
EDIT: To reflect the change to the question :)
You use InputStreamReader.read(char[], int, int) and when you've read all you want to, create a new string from a char array. If you want to read a line at a time as you would from BufferedReader, you basically need to implement the functionality of BufferedReader yourself (keeping a buffer of "read but not consumed" chars) and keep reading until you hit a line break.
Alternatively have a look at DataInputStream.readUTF().
It does required that the string being read off the InputStream be encoded appropriately (as in by a corresponding DataOutputStream.writeUTF(String)) so it might not be what you're looking for - but it does work across different phones/models etc.
Well... I know this was a LONG time ago.
You need to do exactly what John said, and it is VERY simple. It almost took me 5 hours to figure this one out the first time...
I still wonder why j2ME didn't include something as essential as the BufferedReader method for sockets, it's not like the freakin cellphones will crash with it... and yes, I don't give a rat's ass if my app runs 1ms slower than it should.
(I'm just going to put the relevant code, I assume you know how to form classes and import the required libraries)
ServerSocketConnection listener
= (ServerSocketConnection)Connector.open("socket://:1235");
System.out.println("Waiting for connection...");
StreamConnection server = listener.acceptAndOpen();
InputStream is = server.openInputStream();
//Now comes the fake BufferedReader equivalent part
int ch = 0;
StringBuffer sb = new StringBuffer();
while ((ch = is.read()) != -1){
sb.append((char)ch);
if(sb.charAt(sb.length()-1) == 13 ) {
//Carriage return was received or ENTER was pressed
break; //Exit loop and print input
}
}
As you can see, the is.read() method will lock the thread till new input is received from the user ONE BYTE AT A TIME. This means if you use telnet to test, each keystroke will make the loop iterate once, hence, we simply concatenate char by char in a StringBuffer until char 13 is received.
System.out.println(sb.toString());
I hope this helps people trying to do a socket server on j2ME. I already crafted a fully functional multithreaded version of this for blackberry, in case anyone needs it.
Would you be able to provide an example of this?
You use InputStreamReader.read(char[], int, int) and when you've read all you want to, create a new string from a char array. If you want to read a line at a time as you would from BufferedReader, you basically need to implement the functionality of BufferedReader yourself (keeping a buffer of "read but not consumed" chars) and keep reading until you hit a line break.
Related
Right now, I'm trying to write a GUI based Java tic-tac-toe game that functions over a network connection. It essentially works at this point, however I have an intermittent error in which several chars sent over the network connection are lost during gameplay. One case looked like this, when println statements were added to message sends/reads:
Player 1:
Just sent ROW 14 COLUMN 11 GAMEOVER true
Player 2:
Just received ROW 14 COLUMN 11 GAMEOV
Im pretty sure the error is happening when I read over the network. The read takes place in its own thread, with a BufferedReader wrapped around the socket's InputStream, and looks like this:
try {
int input;
while((input = dataIn.read()) != -1 ){
char msgChar = (char)input;
String message = msgChar + "";
while(dataIn.ready()){
msgChar = (char)dataIn.read();
message+= msgChar;
}
System.out.println("Just received " + message);
this.processMessage(message);
}
this.sock.close();
}
My sendMessage method is pretty simple, (just a write over a DataOutputStream wrapped around the socket's outputstream) so I don't think the problem is happening there:
try {
dataOut.writeBytes(message);
System.out.println("Just sent " + message);
}
Any thoughts would be highly appreciated. Thanks!
As it turns out, the ready() method guaruntees only that the next read WON'T block. Consequently, !ready() does not guaruntee that the next read WILL block. Just that it could.
I believe that the problem here had to do with the TCP stack itself. Being stream-oriented, when bytes were written to the socket, TCP makes no guarantees as to the order or grouping of the bytes it sends. I suspect that the TCP stack was breaking up the sent string in a way that made sense to it, and that in the process, the ready() method must detect some sort of underlying break in the stream, and return false, in spite of the fact that more information is available.
I refactored the code to add a newline character to every message send, then simply performed a readLine() instead. This allowed my network protocol to be dependent on the newline character as a message delimiter, rather than the ready() method. I'm happy to say this fixed the problem.
Thanks for all your input!
Try flushing the OutputStream on the sender side. The last bytes might remain in some intenal buffers.
It is really important what types of streamed objects you use to operate with data. It seems to me that this troubleshooting is created by the fact that you use DataOutputStream for sending info, but something else for receiving. Try to send and receive info by DataOutputStream and DataInputStream respectively.
Matter fact, if you send something by calling dataOut.writeBoolean(b)
but trying to receive this thing by calling dataIn.readString(), you will eventually get nothing. DataInputStream and DataOutputStream are type-sensitive. Try to refactor your code keeping it in mind.
Moreover, some input streams return on invocation of read() a single byte. Here you try to convert this one single byte into char, while in java char by default consists of two bytes.
msgChar = (char)dataIn.read();
Check whether it is a reason of data loss.
I have an assignment that requires use of a socket. On the client and server side I have
char[] value= "END STREAM".toCharArray(), which signals the stream to shutdown.
Since I have both these arrays in the 2 different files, my intention is that the client sends the message value > the server. Then the server does the function
while(!Arrays.equals(clientSentence, value))
{
...
inFromClient.read(clientSentence, 0, length); //to read in from client side
.....
}
In essence, while it does not send the END STREAM message, keep reading. My issue is that the array equality does not work as intended. I even test this by doing
System.out.println(Arrays.equals(value, clientSentence));
System.out.println(new String(value));
System.out.println(new String(clientSentence));
and it prints
false
END STREAM
END STREAM
How can it be false when it is printing the same values. I have made sure that both arrays initialize to the same length so where is it going wrong? I have been stuck on this for hours and searched for answers but cannot find a solution. Thanks
EDIT: added my read function. I use a BufferedReader
On my box:
char[] x = "END STREAM".toCharArray();
char[] y = "END STREAM".toCharArray();
System.out.println(Arrays.equals(x, y));
System.out.println(Arrays.toString(x));
System.out.println(Arrays.toString(y));
works which makes me think a few things:
The arrays, as declared in your code, are not equal.
There is a character set incompatibility, you said you were using a BufferedReader and, if you're using the Files.newBufferedReader() functionality of Java 1.7, you need specify a charset when using it which may be causing a problem.
End of line issues either from cross platform systems or something else e.g. \r vs \n
Thinking a little more about it...it's probably #2, check this out for further information: http://docs.oracle.com/javase/tutorial/i18n/text/string.html
Using java.net, java.io, what is the fastest way to parse html from online, and load it to a file or the console? Is buffered writer/buffered reader faster than inputstreamreader/outputstreamwriter? Are writers and readers faster than outputstreams and inputstreams?
I am experiencing serious lag with the following output writer/stream:
URLConnection ii;
BufferedReader iik = new BufferedReader(new InputStreamReader(ii.getInputStream()));
String op;
while(iik.readLine()!=null) {
op=iik.readLine();
System.out.println(op);
}
But curiously i am experiencing close to no lagtime with the following code:
URLConnection ii=i.openConnection();
Reader xh=new InputStreamReader(ii.getInputStream());
int r;
Writer xy=new PrintWriter(System.out);
while((r=xh.read())!=-1) {
xy.write(r);
}
xh.close();
xy.close();
What is going on here?
Your first snippet is wrong: it reads the next line, tests if it's null, ignores it, then reads the next line without testing if it's null, and prints it.
The second code prints the integer value of every char read from the reader.
Both snippets use the same underlying streams and readers, and, if coded correctly, the first one should probably be a bit faster thanks to buffering. But of course, you'll have something printed on the screen only when the line is ended. If the server sends a single line of text of 10 MBs, you'll have to read the whole 10 MBs before something is printed to the screen.
Make sure to close the readers in finally blocks.
Readers/Writers shouldn't be inherently faster than Input/OutputStreams.
That said, going through readLine() and println() probably isn't the optimal way of transferring bytes. In your case, if the file you're loading doesn't contain many newline characters, BufferedReader will have to buffer a lot of data before readLine() will return.
The canonical non-terrible way of transferring data between streams is doing it in chunks by using a buffer:
byte[] buf = new byte[1<<12];
InputStream in = urlConnection.getInputStream();
int read = -1;
while ((read = in.read(buf) != -1) {
System.out.write(buf, 0, read);
}
It might be faster yet to use NIO, the code for it is a little less straightforward and I just use the one found in this blog post.
If you're writing to/from a file, the best method is to use a zero-copy approach, which Java makes available with FileChannel.transferFrom() and transferTo(). Sample code is available in a DeveloperWorks article.
In java there is another Object like BufferedReader to read data recived by server??
Because the server send a string without newline and the client don't print any string untile the Server close the connection form Timeout (the timeout message have a newline!) , after the Client print all message recived and the timeout message send by server!
help me thanks!!
Just don't read by newlines using readLine() method, but read char-by-char using read() method.
for (int c = 0; (c = reader.read()) > -1;) {
System.out.print((char) c);
}
You asked for another class to use, so in that case give Scanner a try for this. It's usually used for delimiting input based on patterns or by the types inferred from the input (e.g. reading on a byte-by-byte bases or an int-by-int basis, or some combination thereof). However, you can use it as just a general purpose "reader" here as well, to cover your use case.
When you read anything from a server, you have to strictly follow the communication protocol. For example the server might be an HTTP server or an SMTP server, it may encrypt the data before sending it, some of the data may be encoded differently, and so on.
So you should basically ask: What kind of server do I want to access? How does it send the bytes to me? And has someone else already done the work of interpreting the bytes so that I can quickly get to the data that I really want?
If it is an HTTP server, you can use the code new URL("http://example.org/").openStream(). Then you will get a stream of bytes. How you convert these bytes into characters, strings and other stuff is another task.
You could try
InputStream is = ... // from input
String text = IOUtils.toString(is);
turns the input into text, without without newlines (it preserves original newlines as well)
I'm writing a program in java (already wrote a version in C too). I really need to put a character back to the input-stream after I read it. This can be accomplished by ungetc() in C/C++, and I was wonder how can I do the same thing in Java?
For those of you don't know C/C++:
char myc = (char)System.in.read();
and I check the value of myc and now I want to put back myc in to the System.in! so again when I call System.in, I get that value. (But How can I do this?)
NOTE: I'm looking for exact technique. Please do not advise me to catch it or log it somewhere else and read off from there, because I know how to those kinda stuff. What I'm interested in is equivalent of ungetc() in Java if there's any.
Cheers.
You are looking for PushbackInputStream
Java's IO library is designed so the primitives are really basic and additional functionality is added through composition. For example, if you wanted to buffer input from a file, you would call new BufferedInputStream(new FileInputStream("myfile.txt")); or if you wanted to read from the stream as text using UTF-8 encoding you'd call new InputStreamReader(new FileInputStream("myfile.txt"), "UTF-8");
So what you want to do is create a PushbackInputStream with new PushbackInputStream(System.in);
The caveat here is you're not actually pushing the bytes back onto standard input. Once you've read from System.in it's gone, and no other code accessing System.in will be able to get at that byte. Anything you push back will only ever be available from particular PushbackInputStream you created to handle the data.
If the input stream supports marking you can use the mark() and reset() methods to achieve what you intend.
You can wrap System.in into a BufferedInputStream or a BufferedReader to gain marking support.
A RandomAccessFile can .seek( .getFilePointer() - 1 ).