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How to append to existing file [duplicate]

This question already has answers here:
How to write data with FileOutputStream without losing old data?
(2 answers)
Closed 4 years ago.
I am trying to redirect the console input to a file. Problem is that every time i create a file it overwrites it or creates new files if I select the name of file to include unix timestamp. I saw similar questions here but I am not sure which approach or class to use.
PrintStream out;
PrintStream oldout = new PrintStream(System.out);
try {
out = new PrintStream(
new FileOutputStream(
workFolder + File.separator + "output" + Instant.now().getEpochSecond() + ".txt"));
System.setOut(out);
} catch (FileNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.setOut(oldout);
So if there isn't a file to create it, but if there is already a file to just append new data, but not overwrite or create new files.

As per Java docs
public FileOutputStream(String name,
boolean append)
throws FileNotFoundException
Parameters: name - the system-dependent file name
append - if true,
then bytes will be written to the end of the file rather than the
beginning
There is a constructor which allows passing the boolean value which decides whether to append the data in file or not.
You can use it.

Java - How I can check if file is exist? [duplicate]

This question already has answers here:
How do I check if a file exists in Java?
(19 answers)
Closed 7 years ago.
I'm creating a file with writeToFile() function.
Before I call writeToFile() function, I want to check if the file already exist or not.
How can I do this?
code:
private void writeToFile(String data, String fileName) {
try {
OutputStreamWriter outputStreamWriter = new OutputStreamWriter(this.openFileOutput(fileName, Context.MODE_APPEND));
outputStreamWriter.write(data);
outputStreamWriter.close();
}
catch (IOException e) {
Log.e("Exception", "File write failed: " + e.toString());
}
}

You could utilize java.io.File and call the .exists() method to check if the file exists.
Use the following code to check if a file already exists.
if(file.exists() && !file.isDirectory()) {
// continue code
}

Using java.io.File
File f = new File(fileName);
if (f.exists()) {
// do something
}
This is a duplicate.

File file = new File("FileName");
if(file.exists()){
System.out.println("file is already there");
}else{
System.out.println("Not find file ");
}
The methods in the Path class are syntactic, meaning that they operate on the Path instance. But eventually you must access the file system to verify that a particular Path exists

Java FileNotFoundExeception not working

In my current project I am having an issue with not receiving a file not found exception. My driver file passes the path to be opened to the constructor that is building a library of books. I am using JFileChooser to get the path. In trying to force an error (entering the name of a file that does not exist), it builds the library with no information in it, and does not throw an error.
Driver Code:
//open an existing library
JFileChooser dlg = new JFileChooser ("LibraryData");
FileNameExtensionFilter filter = new FileNameExtensionFilter ("Text Files", "txt");
dlg.setFileFilter(filter);
dlg.setDialogTitle("Select Existing File");
dlg.setApproveButtonToolTipText("Select the file you want to open and click me.");
int button = dlg.showOpenDialog(null);
if (button == dlg.APPROVE_OPTION)
{
currentPath = dlg.getSelectedFile().getPath();
library = new PersonalLibrary(currentPath);
System.out.println("===========================================================");
System.out.println("File opened successfully from: \n" + currentPath);
System.out.println("===========================================================");
}
Util.enterToContinue();
Util.clearScreen();
break;
Library Code:
public PersonalLibrary(String path)
{
try
{
File myFile = new File(path);
if (myFile.exists())
{
Scanner input = new Scanner(myFile);
while(input.hasNext())
{
//code that populates the library
}
input.close();
saveNeeded = false;
}
}
catch (FileNotFoundException e)
{
System.out.println("Error: " + e.getMessage());
}

Your checking if the file exists the catch block will never be executed.
if(myFile.exists())
If it doesn't exist nothing else will be executed including catch block. FileNotFoundException could not occur in this block of code. If you want to catch FileNotFoundException get rid of the if block. Or just add an else block and do you processing there whatever processing you want to do when a file doesn't exist.

the method File#exists() checks if a file is existing or not. If it does, it returns true and goes into your if block.
Since the file is not there, it just simply skips the if block and moves on. Since no attempt was made to access a non-existing file object, the exception is not thrown.
If you would like to throw an exception, you have to do so yourself like this,
if(file.exists()) {
//do file operation
} else {
throw new FileNotFoundException("Oops! No file...");
}

Reading/Writing to Properties Files inside the jar file

So i am getting back into writing Java after 4 years so please forgive any "rookie" mistakes.
I need to have a properties file where i can store some simple data for my application. The app data itself won't reside here but i will be storing info such as the file path to the last used data store, other settings, etc.
I managed to connect to the properties file which exists inside the same package as the class file attempting to connect to it and i can read the file but i am having trouble writing back to the file. I am pretty sure that my code works (at least it's not throwing any errors) but the change isn't reflected in the file itself after the app is run in Netbeans.
In the above image you can see the mainProperties.properties file in question and the class attempting to call it (prefManagement.java). So with that in mind here is my code to load the file:
Properties mainFile = new Properties();
try {
mainFile.load(prefManagement.class.getClass().getResourceAsStream("/numberAdditionUI/mainProperties.properties"));
} catch (IOException a) {
System.out.println("Couldn't find/load file!");
}
This works and i can check and confirm the one existing key (defaultXMLPath).
My code to add to this file is:
String confirmKey = "defaultXMLPath2";
String propKey = mainFile.getProperty(confirmKey);
if (propKey == null) {
// Key is not present so enter the key into the properties file
mainFile.setProperty(confirmKey, "testtest");
try{
FileOutputStream fos = new FileOutputStream("mainProperties.properties");
mainFile.store(fos, "testtest3");
fos.flush();
}catch(FileNotFoundException e ){
System.out.println("Couldn't find/load file3!");
}catch(IOException b){
System.out.println("Couldn't find/load file4!");
}
} else {
// Throw error saying key already exists
System.out.println("Key " + confirmKey + " already exists.");
}
As i mentioned above, everything runs without any errors and i can play around with trying to add the existing key and it throws the expected error. But when trying to add a new key/value pair it doesn't show up in the properties file afterwords. Why?

You should not be trying to write to "files" that exist inside of the jar file. Actually, technically, jar files don't hold files but rather they hold "resources", and for practical purposes, they are read-only. If you need to read and write to a properties file, it should be outside of the jar.

Your code writes to a local file mainProperties.properties the properties.
After you run your part of code, there you will find that a file mainProperties.properties has been created locally.
FileOutputStream fos = new FileOutputStream("mainProperties.properties");
Could order not to confuse the two files you specify the local file to another name. e.g. mainAppProp.properties .
Read the complete contents of the resource mainProperties.properties.
Write all the necessary properties to the local file mainAppProp.properties.
FileOutputStream fos = new FileOutputStream("mainAppProp.properties");
switch if file exists to your local file , if not create the file mainAppProp.properties and write all properties to it.
Test if file mainAppProp.properties exists locally.
Read the properties into a new "probs" variable.
Use only this file from now on.
Under no circumstances you can write the properties back into the .jar file.
Test it like
[...]
if (propKey == null) {
// Key is not present so enter the key into the properties file
mainFile.setProperty(confirmKey, "testtest");
[...]
Reader reader = null;
try
{
reader = new FileReader( "mainAppProp.properties" );
Properties prop2 = new Properties();
prop2.load( reader );
prop2.list( System.out );
}
catch ( IOException e )
{
e.printStackTrace();
}
finally
{
if (reader != null) {
reader.close();
}
}
}
[...]
}
output : with prop2.list( System.out );
-- listing properties --
defaultXMLPath2=testtest
content of the file mainAppProp.properties
#testtest3
#Mon Jul 14 14:33:20 BRT 2014
defaultXMLPath2=testtest

Challenge:
Read the Property file location in jar file
Read the Property file
Write the variable as system variables
public static void loadJarCongFile(Class Utilclass )
{
try{
String path= Utilclass.getResource("").getPath();
path=path.substring(6,path.length()-1);
path=path.split("!")[0];
System.out.println(path);
JarFile jarFile = new JarFile(path);
final Enumeration<JarEntry> entries = jarFile.entries();
while (entries.hasMoreElements()) {
final JarEntry entry = entries.nextElement();
if (entry.getName().contains(".properties")) {
System.out.println("Jar File Property File: " + entry.getName());
JarEntry fileEntry = jarFile.getJarEntry(entry.getName());
InputStream input = jarFile.getInputStream(fileEntry);
setSystemvariable(input);
InputStreamReader isr = new InputStreamReader(input);
BufferedReader reader = new BufferedReader(isr);
String line;
while ((line = reader.readLine()) != null) {
System.out.println("Jar file"+line);
}
reader.close();
}
}
}
catch (Exception e)
{
System.out.println("Jar file reading Error");
}
}
public static void setSystemvariable(InputStream input)
{
Properties tmp1 = new Properties();
try {
tmp1.load(input);
for (Object element : tmp1.keySet()) {
System.setProperty(element.toString().trim(),
tmp1.getProperty(element.toString().trim()).trim());
}
} catch (IOException e) {
System.out.println("setSystemvariable method failure");
}
}

java how to check if file exists and open it?

how to check if file exists and open it?
if(file is found)
{
FileInputStream file = new FileInputStream("file");
}

File.isFile will tell you that a file exists and is not a directory.
Note, that the file could be deleted between your check and your attempt to open it, and that method does not check that the current user has read permissions.
File f = new File("file");
if (f.isFile() && f.canRead()) {
try {
// Open the stream.
FileInputStream in = new FileInputStream(f);
// To read chars from it, use new InputStreamReader
// and specify the encoding.
try {
// Do something with in.
} finally {
in.close();
}
} catch (IOException ex) {
// Appropriate error handling here.
}
}

You need to create a File object first, then use its exists() method to check. That file object can then be passed into the FileInputStream constructor.
File file = new File("file");
if (file.exists()) {
FileInputStream fileInputStream = new FileInputStream(file);
}

You can find the exists method in the documentation:
File file = new File(yourPath);
if(file.exists())
FileInputStream file = new FileInputStream(file);