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Java: FileOutputStream and FileInputStream together on the same file

Do I can open a file (linux character device) for read+write, and use the two classes to implement a dialog like client-server?
Something like this:
File file = new File("/dev/ttyS0");
FileOutpuStream fo = new FileOutputStream(file)
FileInputStream fi = new FileInputStream(file)
After the above declarations, can I continuously send pollings (questions) to the file, and read its replies? (Of course, attached to ttyS0 there is a kind of server)

I was not able to test it, but you might want to give RandomAccessFile a try.
It does not give you the opertunity to create streams, but it implements DataInput and DataOutput. Thats maybe good enough for your purpose?
RandomAccessFile docs
String file = "/dev/ttyS0";
try {
RandomAccessFile f = new RandomAccessFile(file, "rwd");
} catch (IOException e){
e.printStackTrace();
}

The /dev/ttyS0 file is a device file for a serial terminal.
If the device has been configured appropriately to connect to a serial terminal line, then you should be able to read and write like that. However, on a typical desktop or laptop, it probably won't work because there won't be connected serial line.
(For example, when I do this on my PC:
$ sudo bash -c "cat < /dev/ttyS0"
I get this:
cat: -: Input/output error
which is saying that the device cannot be read from.)
Note that a /dev/tty* device does not behave like a regular file. The characters that are written in no way relate to the characters that you read back. Also note that it is not possible to make ioctl requests using the standard Java APIs. So configuring the terminal driver from Java would be problematic.
If you were talking abour reading and writing a regular file, it should work too. However, the behavior could be a rather confusing, especially if you have buffering in your streams. One issue you need to deal with is that the two file descriptors are independent of each other.
If you need to do this kind of thing with a regular file, you should probably use RandomAccessFile.

I didn't try RandomAccessFile, it could also work... it worked smoothly with FileInputStream and FileOutputStream, see this answer in SO: https://stackoverflow.com/a/56935267/7332147

Create a text file if it doesn't exist and append to it if it does using Java BufferedWriter

This is probably ridiculously simple for gun Java programmers, yet the fact that I (a relative newbie to Java) couldn't find a simple, straightforward example of how to do it means that I'm going to use the self-answer option to hopefully prevent others going through similar frustration.
I needed to output error information to a simple text file. These actions would be infrequent and small (and sometimes not needed at all) so there is no point keeping a stream open for the file; the file is opened, written to and closed in the one action.
Unlike other "append" questions that I've come across, this one requires that the file be created on the first call to the method in that run of the Java application. The file will not exist before that.
The original code was:
Path pathOfLog = Paths.get(gsOutputPathUsed + gsOutputFileName);
Charset charSetOfLog = Charset.forName("US-ASCII");
bwOfLog = Files.newBufferedWriter(pathOfLog, charSetOfLog);
bwOfLog.append(stringToWrite, 0, stringToWrite.length());
iReturn = stringToWrite.length();
bwOfLog.newLine();
bwOfLog.close();
The variables starting with gs are pre-populated string variables showing the output location, and stringToWrite is an argument which is passed in.
So the .append method should be enough to show that I wanted to append content, right?
But it isn't; each time the procedure was called the file was left containing only the string of the most recent call.

The answer is that you also need to specify open options when calling the newBufferedWriter method. What gets you is the default arguments as specified in the documentation:
If no options are present then this method works as if the CREATE,
TRUNCATE_EXISTING, and WRITE options are present.
Specifically, it's TRUNCATE_EXISTING that causes the problem:
If the file already exists and it is opened for WRITE access, then its
length is truncated to 0.
The solution, then, is to change
bwOfLog = Files.newBufferedWriter(pathOfLog, charSetOfLog);
to
bwOfLog = Files.newBufferedWriter(pathOfLog, charSetOfLog,StandardOpenOption.CREATE, StandardOpenOption.APPEND);
Probably obvious to long time Java coders, less so to new ones. Hopefully this will help someone avoid a bit of head banging.

You can also try this :
Path path = Paths.get("C:\\Users", "textfile.txt");
String text = "\nHello how are you ?";
try (BufferedWriter writer = Files.newBufferedWriter(path, StandardCharsets.UTF_8, StandardOpenOption.APPEND,StandardOpenOption.CREATE)) {
writer.write(text);
} catch (IOException e) {
e.printStackTrace();
}

Creating and reading a one time created file

I want to store a list of strings in a file.
I need to create it just one time, and after that i will read and write on it programmaticlly.
My question is where in the file system should i create the file (manually) so that it will best for reading and writing ?
Thanks.

You can create your file in your app's directory so no one can access it but your app
getApplicationContext().getFilesDir().getAbsolutePath();
or on sd card
File externalStorage = Environment.getExternalStorageDirectory();
if you want others to access it and, maybe, if your file is very big
If you intent to create your file manually then I think SD card is the only option unless you have a rooted phone or working with the emulator.

if (Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()))
{
//SDcard is there
File f=new File("/sdcard/YOURFILE.txt");
if (!f.exists())
{
//File created only for first time
f.createNewFile();
//create inputstream and write it to your file
OutputStream out=new FileOutputStream(f);
byte buf[]=new byte[1024];
int len;
while((len=inputStream.read(buf))>0)
out.write(buf,0,len);
out.close();
inputStream.close();
System.out.println("\nData Written");
}
else { // read/ write SECOND TIME }
}

It really depends.
The problem with creating the file on the SDCard is that a special permission is required in order to access it. If the app is only for yourself, that's cool. If you want to distribute it through Google MarketPlay (or whatever it is called these days), please know that some people (myself included) tend to look at the permissions and ask "why would an app doing X require permission to do Y?", and sometimes not install the app because of it.
If the manual part is done by the app's user, by all means, store it on the sdcard. It's the only place a standard, none-root user even has access to.
Generally speaking, however, a better place to store data is in /data/data/packagename. See Android's data storage for more details.
Shachar

Add file in assets folder, then it will be clearly after new install

Using a button to export sqlite db to an excel-readable file (perhaps csv)?

I adapted this tutorial (http://www.screaming-penguin.com/node/7749) to an Android app I've built to allow for a button press to export the current database to the user's sdcard. It works flawlessly.
But I'm afraid that users of my app would be unfamiliar with the db file, and I'm looking for a way to convert that to a more user-friendly format. I came across this thread (http://groups.google.com/group/android-beginners/browse_thread/thread/4e53ebca14daecfc), which recommends "querying data from the database and writing the data into a csv file."
I was hoping someone could point me in the right direction to begin figuring out how to do this. I'm finding it hard to track down more information about the specific method.
Or does it make more sense to just explain in a short "about" how to read and access .db files?
Thanks
EDIT: I also have a question about the sqlite export process, which I think I'll just ask here rather than create a new question. Is there a way to modify the code below so that each export would receive either a string representation of the date or just a single numeral appended to it? Right now if you export a second time, it automatically overwrites the old file. Thanks.
protected Boolean doInBackground(final String... args) {
File dbFile = new File(Environment.getDataDirectory() +
"/data/com.example.example/databases/data");
File exportDir = new File(Environment.getExternalStorageDirectory(), "exampledata");
if (exportDir.exists()) {
exportDir.mkdirs();
}
File file = new File(exportDir, dbFile.getName());
try {
file.createNewFile();
this.copyFile(dbFile, file);
return true;
} catch(IOException e) {
Log.e(MyApplication.APP_NAME, e.getMessage(), e);
return false;
}
}

I was hoping someone could point me in the right direction to begin figuring out how to do this.
To read in the data, use rawQuery().
To write the data, use Java I/O. There are also open source CSV libraries for Java that may work on Android.
Is there a way to modify the code below so that each export would receive either a string representation of the date or just a single numeral appended to it?
Use string concatenation to add whatever you want to the filename.
Also, please get rid of:
File dbFile = new File(Environment.getDataDirectory() +
"/data/com.example.example/databases/data");
and replace it with:
File dbFile=getDatabasePath("data");

Reading a file and editing it in Java

What I am doing is I am reading in a html file and I am looking for a specific location in the html for me to enter some text.
So I am using a bufferedreader to read in the html file and split it by the tag . I want to enter some text before this but I am not sure how to do this. The html would then be along the lines of ...(newText)(/HEAD) (The brackets round head are meant to be angled brackets. Don't know how to insert them)
Would I need a PrintWriter to the same file and if so, how would I tell that to write it in the correct location.
I am not sure which way would be most efficient to do something like this.
Please Help.
Thanks in advance.
Here is part of my java code:
File f = new File("newFile.html");
FileOutputStream fos = new FileOutputStream(f);
PrintWriter pw = new PrintWriter(fos);
BufferedReader read = new BufferedReader(new FileReader("file.html"));
String str;
int i=0;
boolean found = false;
while((str= read.readLine()) != null)
{
String[] data = str.split("</HEAD>");
if(found == false)
{
pw.write(data[0]);
System.out.println(data[0]);
pw.write("</script>");
found = true;
}
if(i < 1)
{
pw.write(data[1]);
System.out.println(data[1]);
i++;
}
pw.write(str);
System.out.println(str);
}
}
catch (Exception e) {
e.printStackTrace( );
}
When I do this it gets to a point in the file and I get these errors:
FATAL ERROR: MERLIN: Unable to connect to EDG API,
Cannot find .edg_properties file.,
java.lang.OutOfMemoryError: unable to create new native thread,
Cannot truncate table,
EXCEPTION:Cannot open connection to server: SQLExceptio,
Caught IOException: java.io.IOException: JZ0C0: Connection is already closed, ...
I'm not sure why I get these or what all of these mean?
please Help.

Should be pretty easy:
Read file into a String
Split into before/after chunks
Open a temp file for writing
Write before chunk, your text, after chunk
Close up, and move temp file to original
Sounds like you are wondering about the last couple steps in particular. Here is the essential code:
File htmlFile = ...;
...
File tempFile = File.createTempFile("foo", ".html");
FileWriter writer = new FileWriter(tempFile);
writer.write(before);
writer.write(yourText);
writer.write(after);
writer.close();
tempFile.renameTo(htmlFile);

Most people suggest writing to a temporary file and then copying the temporary file over the original on successful completion.

The forum thread has some ideas of how to do it.
GL.

For reading and writing you can use FileReaders/FileWriters or the corresponding IO stream classes.
For the editing, I'd suggest to use an HTML parser to handle the document. It can read the HTML document into an internal datastructure which simplifies your effort to search for content and apply modification. (Most?) Parsers can serialize the document to HTML again.
At least you're sure to not corrupt the HTML document structure.

Following up on the list of errors in your edit, a lot of that possibly stems from the OutOfMemoryError. That means you simply ran out of memory in the JVM, so Java was unable to allocate objects. This may be caused by a memory leak in your application, or it could simply be that the work you're trying to do does need more memory transiently than you have allocated it.
You can increase the amount of memory that the JVM starts up with by providing the Xmx argument to the java executable, e.g.:
-Xmx1024m
would set the maximum heap size to 1024 megabytes.
The other issues might possibly caused by this; when objects can't reliably be created or modified, lots of weird things tend to happen. That said, there's a few things that look like you can take action. In particular, whatever MERLIN is it looks like it can't do it's work because it needs a property file for EDG, which it's unable to find in the location it's looking. You'll probably need to either put a config file there, or tell it to look at another location.
The other IOExceptions are fairly self-explanatory. Your program could not establish a connection to the server because of a SQLException (the underlying exception itself will probably be found in the logs); and some other part of the program tried to communicate to a remote machine using a closed connection.
I'd look at fixing the properties file (if it's not a benign error) and the memory issues first, and then seeing if any of the remaining problems still manifest.