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Java how to write to a file and append to it [duplicate]

This question already has answers here:
How to append text to an existing file in Java?
(31 answers)
Closed 3 years ago.
I can't get this program to write 4 lines of text to the same file javafile.txt. The first two sentences get overwritten by the next two sentences. I'm using BufferedWriter to accomplish this. I have tried so many different things but I keep overwriting the first sentences the javafile.txt ......................................................................................................................................................................................................................................................................................................................
import java.io.File;
import java.io.IOException;
import java.io.ObjectInputStream.GetField;
import java.util.Scanner;
import java.io.*;
import java.io.FileNotFoundException;
import java.io.BufferedReader;
public class lab11_2 {
public static void main(String[] args) {
System.out.println("Practice writing to files \n");
File Myfile = new File("C:\\Users\\test\\Desktop\\lab11\\javafile.txt");
try {
if (Myfile.createNewFile()) {
System.out.println("File created: " + Myfile.getName());
} else {
System.out.println("File already exists.");
}
} catch (IOException e) {
System.out.println("An error occurred.");
e.printStackTrace();
} //end create new file
try {
System.out.println("Write to to file");
BufferedWriter out = new BufferedWriter(new FileWriter("C:\\Users\\test\\Desktop\\lab11\\javafile.txt"));
out.write("This is the first line of text in " + Myfile.getName() + "\n");
out.close();
out = new BufferedWriter(new FileWriter("C:\\Users\\test\\Desktop\\lab11\\javafile.txt",true));
out.write("This is the second line of text in " + Myfile.getName() + "\n");
out.close();
System.out.println("Succesfully wrote to the the file");
System.out.println();
}
catch (IOException e) {
System.out.println("exception occoured"+ e);
}
if (Myfile.exists()) {
System.out.println("Get file information:");
System.out.println("File name: " + Myfile.getName());
System.out.println("Absolute path: " + Myfile.getAbsolutePath());
System.out.println("Writeable: " + Myfile.canWrite());
System.out.println("Readable " + Myfile.canRead());
System.out.println("File size in bytes " + Myfile.length());
} else {
System.out.println("The file does not exist.");
}
System.out.println();
System.out.println("First read file");
try {
Scanner myReader = new Scanner(Myfile);
while (myReader.hasNextLine()) {
String data = myReader.nextLine();
System.out.println(data);
}
myReader.close();
} catch (FileNotFoundException e) {
System.out.println("An error occurred.");
e.printStackTrace();
} // end try
System.out.println();
try {
System.out.println("Write to to file");
BufferedWriter out = new BufferedWriter(new FileWriter("C:\\Users\\test\\Desktop\\lab11\\javafile.txt"));
out.write("This is the Third line of text in " + Myfile.getName() + "\n");
out.close();
out = new BufferedWriter(new FileWriter("C:\\Users\\test\\Desktop\\lab11\\javafile.txt",true));
out.write("This is the fourth line of text in " + Myfile.getName() + "\n");
out.close();
}
catch (IOException e) {
System.out.println("exception occoured"+ e);
}
System.out.println("Done");
System.out.println();
}// end of main
}

See the FileWriter api.
You open a new FileWriter four times. Two of those times, you pass true as a constructor parameter, so the writer is in append mode. That means it adds to the file instead of overwriting it. The others, you don't use append mode, so your file is overwritten.
If you want to append to a file, change the third file writer to also use append mode. Otherwise at that point you overwrite the file containing the first two sentences.
Or better yet, don't open and close the file four times. Open it once and write everything you want to write, then close it at the end.

FileWriter not appending to existing file

I am writing a method that takes in a List of Twitter Status objects as a parameter, opens a log file containing String represenatations of Tweets, checks if any of the String representations of the Status objects are already written to the file - if so, it removes them from the list, if not it appends the Status to the file.
Everything is working up until I attempt to write to the file. Nothing is being written at all. I am led to believe that it is due to the method having the file open in two different places: new File("tweets.txt") and new FileWriter("tweets.txt, true).
Here is my method:
private List<Status> removeDuplicates(List<Status> mentions) {
File mentionsFile = new File("tweets.txt");
try {
mentionsFile.createNewFile();
} catch (IOException e1) {
// Print error + stacktrace
}
List<String> fileLines = new ArrayList<>();
try {
Scanner scanner = new Scanner(mentionsFile);
while (scanner.hasNextLine()) {
fileLines.add(scanner.nextLine());
}
scanner.close();
} catch (FileNotFoundException e) {
// Print error + stacktrace
}
List<Status> duplicates = new ArrayList<>();
for (Status mention : mentions) {
String mentionString = "#" + mention.getUser().getScreenName() + " \"" + mention.getText() + "\" (" + mention.getCreatedAt() + "\")";
if (fileLines.contains(mentionString)) {
duplicates.add(mention);
} else {
try {
Writer writer = new BufferedWriter(new FileWriter("tweets.txt", true));
writer.write(mentionString);
} catch (IOException e) {
// Print error + stacktrace
}
}
}
mentions.removeAll(duplicates);
return mentions;
}

I wrote here few thoughts looking your code.
Remember to always close the object Reader and Writer.
Have a look at try-with-resources statement :
try (Writer writer = new BufferedWriter(new FileWriter("tweets.txt", true))) {
writer.write(mentionString);
} catch (IOException e) {
// Print error + stacktrace
}
To read an entire file in a List<String>:
List<String> lines = Files.readAllLines(Paths.get("tweets.txt"), StandardCharsets.UTF_8);
And again, I think it's a bad practice write in the same file you're reading of.
I would suggest to write in a different file if you don't have a particular constraint.
But if you really want have this behavior there are few alternative.
Create a temporary file as output and, when you process is successfully completed, than move it to the old one using Files.move(from, to).

Don't get any data though the binary file is already filled with data

System.out.println("======================================");
System.out.println("List of Ships and their revenue");
System.out.println("======================================");
try
{
inputStream = new ObjectInputStream (new FileInputStream (fileName));
}
catch (Exception e)
{
System.out.println("Problem");
}
try
{
while (true)
{
Ship copyObject = (Ship)inputStream.readObject();
System.out.println(copyObject.getShipName() + " " + copyObject.getRevenue());// fix format later
}
}
catch (EOFException e)
{
}
catch (Exception e)
{
}
I want to retrieve data from a binary file, which I definitely know that I has data as a objects. But the result of the program is only the below:
=================================
List of Ships and their revenue
=================================
It probably means that there is something wrong with the try block. Note that this is just part of my program, which relates to the result I want to get. Ship is a superclass of the classes to which the object in the file belong. (update: I just print the exception, but no exception is thrown).

By the looks of it you are in an infinite loop which dies with an exception which results in no output. Try doing the following instead of the giant second try catch block.
while(true) {
try{
Ship copyObject = (Ship)inputStream.readObject();
System.out.println(copyObject.getShipName() + " " + copyObject.getRevenue());
}
catch(Exception ex) {
System.out.println("No more ships");
break;
}
}

Using FileWriter to convert Google Analytics data into CSV file using JAVA

Hi all after getting some advice, I am attempting to use the filewriter method in order to export my google analytics queries that i got to a CSV file format here is what i have so far
private static void printGaData(GaData results) {
try {
PrintWriter pw = new PrintWriter(BufferedWriter(new FileWriter("data.csv")));
}
catch(Exception e) {
e.printStackTrace();
}
System.out.println(
"printing results for profile: " + results.getProfileInfo().getProfileName());
if (results.getRows() == null || results.getRows().isEmpty()) {
System.out.println("No results Found.");
} else {
// Print column headers.
for (ColumnHeaders header : results.getColumnHeaders()) {
System.out.printf(header.getName() + ", ");
}
System.out.println();
// Print actual data.
for (List<String> row : results.getRows()) {
for (String column : row) {
pw.printf(row + ", ");
}
pw.println();
}
pw.println();
}
}
}
doesnt output any data and keeps saying that the pw is non extent and stuff like that

Your PrintWriter is inside the try catch block. If you define it outside like
PrintWriter pw = null;
try {
pw = new PrintWriter(BufferedWriter(new FileWriter("data.csv")));
} catch (Exception e) {
// handle exception
}
then it will be available to the rest of your code.

Java's System.getRuntime().exec not behaving as if the shell user called it

I am running a java application from the console on an HP-UX machine. In it, I generate some reports, zip them, and then email them. Everything is working, except the email.
I am using the mail binary to send mail from the command line. Since it's HP-UX, it's a bit different than the standard GNU sendmail.
This is the code I'm using to send the mail:
public static void EmailReports(String[] recipients, String reportArchive, String subject){
SimpleDateFormat dateFormat = new SimpleDateFormat("MM-dd-yyyy");
String today = dateFormat.format(new Date());
File tempEmailFile;
BufferedWriter emailWriter;
try {
tempEmailFile = File.createTempFile("report_email_" + today, "msg");
emailWriter = new BufferedWriter(new FileWriter(tempEmailFile));
} catch (IOException e) {
e.printStackTrace();
System.out.println("Failed to send email. Could not create temporary file.");
return;
}
try {
emailWriter.write("SUBJECT: " + subject + "\n");
emailWriter.write("FROM: " + FROM + "\n");
emailWriter.write(BODY + "\n");
emailWriter.close();
} catch (IOException e) {
e.printStackTrace();
System.out.println("Failed to send email. Could not write to temporary file.");
}
//read the archive in
try {
FileInputStream archiveIS = new FileInputStream(new File(reportArchive));
OutputStream archiveEncoder = MimeUtility.encode(new FileOutputStream(tempEmailFile, true), "uuencode", Zipper.getArchiveName(reportArchive));
//read archive
byte[] buffer = new byte[archiveIS.available()]; //these should never be more than a megabyte or two, so storing it in memory is no big deal.
archiveIS.read(buffer);
//encode archive
archiveEncoder.write(buffer);
//close both
archiveIS.close();
archiveEncoder.close();
} catch (FileNotFoundException e) {
System.out.println("Failed to send email. Could not find archive to email.");
e.printStackTrace();
} catch (MessagingException e) {
System.out.println("Failed to send email. Could not encode archive.");
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
System.out.println("Failed to send email. Could not encode archive.");
}
System.out.println("Sending '" + subject + "' email.");
try {
Process p = Runtime.getRuntime().exec("mail me#example.com < " + tempEmailFile.getAbsolutePath());
System.out.println("mail me#example.com < " + tempEmailFile.getAbsolutePath());
StringBuffer buffer = new StringBuffer();
while(p.getErrorStream().available() > 0){
buffer.append((char) p.getErrorStream().read());
}
System.out.println("STDERR: " + buffer.toString());
buffer = new StringBuffer();
while(p.getInputStream().available() > 0){
buffer.append((char) p.getInputStream().read());
}
System.out.println("STDOUT: " + buffer.toString());
} catch (IOException e) {
e.printStackTrace();
System.out.println("Failed to send email. Could not get access to the shell.");
}
}
When I run the program, and it sends the email, I get a blank email, no subject, no body, no attachment, and it's from the user#hostname from the HP-UX box instead of from the email specified in FROM.
However, when I run the same line that it runs (see the command printed out after I call exec), I get the correct email, from the correct user, with a subject, body, and attachment.
STDOUT and STDERR are both empty. It's almost as if I'm sending mail a blank file, but when I print the file before I call the exec, it's there.
What's going on here?
Edit: Attempts made:
Using Ksh:
try {
String cmd = "mail me#example.com.com < " + tempEmailFile.getAbsolutePath();
Runtime.getRuntime().exec(new String[] {"/usr/bin/ksh", cmd});
} catch (IOException e) {
e.printStackTrace();
System.out.println("Failed to send email. Could not get access to the shell.");
}
Using STDIN:
try {
System.out.println("mail me#example.com < " + tempEmailFile.getAbsolutePath());
Process p = Runtime.getRuntime().exec("mail me#example.com ");
FileInputStream inFile = new FileInputStream(tempEmailFile);
byte[] byteBuffer = new byte[inFile.available()];
inFile.read(byteBuffer);
p.getOutputStream().write(byteBuffer);
inFile.close();
p.getOutputStream().close();
StringBuffer buffer = new StringBuffer();
while(p.getErrorStream().available() > 0){
buffer.append((char) p.getErrorStream().read());
}
System.out.println("STDERR: " + buffer.toString());
buffer = new StringBuffer();
while(p.getInputStream().available() > 0){
buffer.append((char) p.getInputStream().read());
}
System.out.println("STDOUT: " + buffer.toString());
} catch (IOException e) {
e.printStackTrace();
System.out.println("Failed to send email. Could not get access to the shell.");
}

I strongly suspect the problem is the redirection. That's normally handled by the shell - and there's no shell here.
Either you need to execute the process normally and then get the process's standard input stream and write to it from Java, or (probably simpler) run /bin/sh (or whatever) to get the shell to do the redirection.

Try exec'ing { "ksh", "-c", "mail me#example.com < " + etc }. The -c option tells the shell specifically to parse the next argument as a shell command with possible redirection and so on. Without the -c, ksh follows a heuristic to decide what to do with its command line, and it may not be running the command in the way you want it to.

Split into two lines, just to get better readability:
String cmd = "mail me#example.com < " + tempEmailFile.getAbsolutePath () ;
Process p = Runtime.getRuntime().exec (cmd);
This will look for a program named "mail me#example.com < " + tempEmailFile.getAbsolutePath (). It will not do redirection - for that to do you have to read the output of that process yourself.
Furtermore it will not lookup the path, so you might have to specify the whole path /usr/bin/mail or whatever it is.
And you have to split command and parameters; use an Array of String instead: ("/path/to/prg", "param1", "param2", "foo=bar");
You can use redirection, if you call as program a script, like
String cmd = "/usr/bin/mail me#example.com < " + tempEmailFile.getAbsolutePath () ;
String cmdarr = new String [] {"/bin/bash", "-c", cmd};
Process p = Runtime.getRuntime().exec (cmdarr);
It is shorter than invoking file redirection from Java yourself, more simple but you lose the ability to react sensible on different errors.