Develop Reference

How to use tables and .txt in Java?

Im building a Car Rental program and what I want it to, for now, is:
Register a user
Register a car
using .txt files to store the data.
With the code I've written, I can register only a single car and user. Every time I run the register method for client or car, the last register is erased.
Can you help me with this? Also, later I'm going to implement a way to rent a car, but I don't know how to do that also, so if you have any ideas of how to do it, please tell me!
Also I intend to do it without SQL or such things.
This is the code I'm using to register a user (I'm using netbeans with JForm):
private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {
String nomeClient = txtNomeClient.getText();
String idClient = txtIdClient.getText();
File file = new File("clients.txt");
try {
PrintWriter output = new PrintWriter(file);
output.println(nomeClient);
output.println(idClient);
output.close();
JOptionPane.showMessageDialog(null, "Client registed!");
} catch (FileNotFoundException e) {
}
}

The problem is that you overwrite the existing file clients.txt, instead of appending to it by calling new PrintWriter(file). You can use the following code:
FileWriter fileWriter = new FileWriter(file, true);
PrintWriter output = new PrintWriter(fileWriter));
This way, you append the end of the file, see the constructor FileWriter(File file, boolean append). The documentation describes it perfectly:
Constructs a FileWriter object given a File object. If the second argument is true, then bytes will be written to the end of the file rather than the beginning.
The FileWriter is just used to open a file in append mode, as PrintWriter does not have a suitable constructor to do that directly. You could also write characters with it, but a PrintWriter allows for formatted output. From the documentation of FileWriter:
Convenience class for writing character files. The constructors of this class assume that the default character encoding and the default byte-buffer size are acceptable.
The PrintWriter uses the FileWriter passed in its constructor to append to the destination file, see here for a good explanation. As stated there, you could also use an FileOutputStream. There are multiple ways to do this.
Here is an example using a FileOutputStream and a BufferedWriter, which supports buffering and can reduce unnecessary writes that penalize performance.
FileOutputStream fileOutputStream = new FileOutputStream("clients.txt", true);
BufferedWriter bufferedWriter = new BufferedWriter(fileOutputStream);
PrintWriter printWriter = new PrintWriter(bufferedWriter);

do not the overwrite with BufferedWritter

I created an object of a class in java and wrote information of this object in a file with BufferedWriter.
But when I create a new object and write information of this in the file, I lose the previous information of previous object.
How can I write in the file with BufferedWriter without overwriting a file?

If you use the FileWriter class it allows you to specify if you want to overwrite or append to the file in the constructor.

If you would like to append to a file that already exists, you can use the following:
BufferedWriter bW = new BufferedWriter(new FileWriter(new File("file.txt"), true));

Java default is overwriting the file. You can specify that you wish to append to a file.
boolean append=true;
FileWriter writer = new FileWriter(new File("yourfile.txt"),append);
BufferedWriter w = new BufferedWriter(writer);
// do your writing stuff

How to write new line in Java FileOutputStream

I want to write a new line using a FileOutputStream; I have tried the following approaches, but none of them are working:
encfileout.write('\n');
encfileout.write("\n".getbytes());
encfileout.write(System.getProperty("line.separator").getBytes());

This should work. Probably you forgot to call encfileout.flush().
However this is not the preferred way to write texts. You should wrap your output stream with PrintWriter and enjoy its println() methods:
PrintWriter writer = new PrintWriter(new OutputStreamWriter(encfileout, charset));
Alternatively you can use FileWriter instead of FileOutputStream from the beginning:
FileWriter fw = new FileWriter("myfile");
PrintWriter writer = new PrintWriter(fw);
Now just call
writer.println();
And do not forget to call flush() and close() when you finish your job.

It could be a viewer problem... Try opening the file in EditPlus or Notepad++. Windows Notepad may not recognize the line feed of another operating system. In which program are you viewing the file now?

String lineSeparator = System.getProperty("line.separator");
<br>
fos.write(lineSeparator.getBytes());

To add a line break use
fileOutputStream.write(10);
here decimal value 10 represents newline in ASCII

Create File at user defined path

I am creating a file in java using
BufferedWriter out = new BufferedWriter(new FileWriter(FileName));
StringBuffer sb=new StringBuffer();
sb.append("\n");
sb.append("work");
out.write(sb.toString());
out.close();
But this file is getting created inside the bin folder of my server.I would like to create this file inside a user-defined folder.
How can it be achieved.

I would like to create this file inside a user-defined folder.
The simplest approach is to specify a fully qualified path name. You could select that as a File and build a new File relative to it:
File directory = new File("/home/jon/somewhere");
File fullPath = new File(directory, fileName);
BufferedWriter writer = new BufferedWriter(
new OutputStreamWriter(
(new FileOutputStream(fullPath), charSet));
try {
writer.write("\n");
writer.write("work");
} finally {
writer.close();
}
Note:
I would suggest using a FileOutputStream wrapped in an OutputStreamWriter instead of using FileWriter, as you can't specify an encoding with FileWriter
Use a try/finally block (or try-with-resources in Java 7) so that you always close the writer even if there's an exception.

To create a file in a specific directory, you need to specify it in the file name.
Otherwise it will use the current working directory which is likely to be where the program was started from.
BTW: Unless you are using Java 1.4 or older, you can use StringBuilder instead of StringBuffer, although in this case PrintWriter would be even better.

Different ways of creating files

There are methods for creating files in java.io.File (like createNewFile() or mkdir()). Are there other ways of creating files in Java SE using "standard" API?

When you create a FileOuputStream, the file is created, if it does not exist, although this is not guaranteed:
A file output stream is an output stream for writing data to a File or to a FileDescriptor. Whether or not a file is available or may be created depends upon the underlying platform.

FileOutputStream can be used to create a file as shown below
FileOutputStream fos = new FileOutputStream("myfile");
FileOutputStream fos = new FileOutputStream(new File("myfile"));

You can use PrintWriter in conjunction with FileWriter such as PrintWriter write = new PrintWriter(new FileWriter("FileName", false)); writing to blank file or BufferedWriter works with FileWriter as well, such as BufferedWriter writer = new BufferedWriter(new FileWriter("FileName"));