How to Write Data with Fileoutputstream Without Losing Old Data

How to write data with FileOutputStream without losing old data?

Use the constructor that takes a File and a boolean

FileOutputStream(File file, boolean append) 

and set the boolean to true. That way, the data you write will be appended to the end of the file, rather than overwriting what was already there.

Write to file using ObjectOutputStream without overwriting old data

You should open file like this for appending string in the file

new FileOutputStream(file, true)

Creates a file output stream to write to the file represented by the
specified File object. If the second argument is true, then bytes will
be written to the end of the file rather than the beginning. A new
FileDescriptor object is created to represent this file connection.

But Java serialization does not support "appending". you can't write an ObjectOutputStream to a file, then open the file again in append mode and write another ObjectOutputStream to it. you have to re-write the entire file every time. (i.e. if you want to add objects to the file, you need to read all the existing objects, then write the file again with all the old objects and then the new objects).

I would sugest you to use DataOutputStream

public void writeSelling(List<String> wordList) throws IOException {
    fileOutPutStream = new FileOutputStream (file,true);
    DataOutputStream write =new DataOutputStream(fileOutPutStream);
    for (String s : wordList){
        d.writeUTF(s);
    }
    write.close();
}

java write data to file without erasing the old content

Use new FileOutputStream(file, true). This will open file in "append" mode which will append all data written to the stream to the end of that file.

FileOutputStream Deletes Contents of File

The JavaDoc says the following

/**
 * Creates a file output stream to write to the file represented by 
 * the specified <code>File</code> object. If the second argument is
 * <code>true</code>, then bytes will be written to the end of the file
 * rather than the beginning. A new <code>FileDescriptor</code> object is
 * created to represent this file connection.
 * <p>
 * First, if there is a security manager, its <code>checkWrite</code> 
 * method is called with the path represented by the <code>file</code> 
 * argument as its argument.
 * <p>
 * If the file exists but is a directory rather than a regular file, does
 * not exist but cannot be created, or cannot be opened for any other
 * reason then a <code>FileNotFoundException</code> is thrown.
 *
 * @param      file               the file to be opened for writing.
 * @param     append      if <code>true</code>, then bytes will be written
 *                   to the end of the file rather than the beginning
 * @exception  FileNotFoundException  if the file exists but is a directory
 *                   rather than a regular file, does not exist but cannot
 *                   be created, or cannot be opened for any other reason
 * @exception  SecurityException  if a security manager exists and its
 *               <code>checkWrite</code> method denies write access
 *               to the file.
 * @see        java.io.File#getPath()
 * @see        java.lang.SecurityException
 * @see        java.lang.SecurityManager#checkWrite(java.lang.String)
 * @since 1.4
 */
public FileOutputStream(File file, boolean append)

Thus to use it you should add a boolean to the constructor of your FileOutputStream, True to append or False to overwrite

So your code should look like this:

Boolean append = true;
outS = new FileOutputStream(videoFile, append);

When writing a huge amount of data, parts of it get lost / When every data is present, the write process is very slow

Maybe this can help you

Fastest way to write huge data in text file Java

https://www.quora.com/How-do-to-read-and-write-large-size-file-in-Java-efficiently

FileOutputStream formatting?

Why not just construct the String(s) first and then call
getBytes() on the resulting String and pass it to fos.write()?

Example:

FileOutputStream fos = v.getContext().openFileOutput(file_name, MODE_PRIVATE);
String bigstring = "Balls: " + balls + " Strikes: " + strikes;
            fos.write(bigstring.getBytes());
            fos.close();

If you want to insert a newline into the String you can do it by concatenating a new-line character into it.

FileOutput Stream delets text file contents

You should try with this constructor :

FileOutputStream fos = new FileOutputStream(f, true);

So what you have to add to the file will be appended if it already exists.

Doc available here

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