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in Java Programming, will the file (.txt, .ser etc) get overwritten when we rewrite on existing file?

if I Do like this:
FileOutputStream fos=new FileOutputStream("cse.txt");
ObjectOutputStream oos=new ObjectOutputStream(fos);
oos.writeObject(Dictionary);
and Again if I write the same file using the another object of Same type Dictionary on the same file like
FileOutputStream fos=new FileOutputStream("cse.txt");
ObjectOutputStream oos=new ObjectOutputStream(fos);
oos.writeObject(List);
Will that file get overwritten ?

If you write simply:
FileOutputStream fos=new FileOutputStream("cse.txt");
Then the old data will be lost and will get overridden by the new data.
However, if you write:
FileOutputStream fos=new FileOutputStream("cse.txt", true);
Then the old data will NOT be lost and the new data will be appended to the old one.
Here, the second argument true indicates that the bytes will be appended to the end of the file rather than overwriting the complete file.
See the documentation

How to get data blockwise from an outputstream in java?

I want to serialize a big object structure to store it to a sql database.
Object tree = getTree();
ByteArrayOutputStream baos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(baos);
oos.writeObject(tree);
String objectString = Base64.getEncoder().encodeToString(baos.toByteArray());
The problem is that the generated byte array by baos.toByteArray() is to large. It throws java.lang.OutOfMemoryError and it is to big to transfer it to the database at once. I'm looking for an option to get the generated byte array block by block from outputstream to work it off in a loop step by step.

Write the initial OutputStream to a temporary file (using FileOutputStream). also, make sure you close the ObjectOutputStream or it will be a malformed stream. Lastly, open your temp file as a FileInputStream and use that to stream into your database.

Forget about the base64-encoding and write the object directly to the Blob's output stream.

If I close ObjectOuputStream, then do not need to close FileOutputStream?

My code like below.
Map<String, String> aMap = new HashMap();
aMap.put("A", "a");
FileOutputStream fos = new FileOutputStream(new File("some.txt"));
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.write(aMap);
oos.flush();
oos.close();
I thought I need to close fos, but other says its fine.
Is it really fine to not close FileOutputStream, because I already closed inner OutputStream?

Yes, You don't need to close it separately. If you close your oos, it will internally close fos as well. Closing the outer most stream will delegate it all the way down

No you dont need to close FileOutputStream.
If you check the code of close() you will found that it closes the output stream.
Plz see the doc http://docs.oracle.com/javase/7/docs/api/java/io/ObjectOutputStream.html

You don't need to do this explicitly, it will be done automatically. Take a look at the example from javadoc:
FileOutputStream fos = new FileOutputStream("t.tmp");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeInt(12345);
oos.writeObject("Today");
oos.writeObject(new Date());
oos.close();
The link could be found here: Class ObjectOutputStream

It's a general rule in Java: if you have several chained/nested streams say
outStream3(outStream2(outStream1)) (I am writing this just in pseudo-code) you
usually need to close only the outermost stream - i.e. outStream3 in this case.
Internally when you call close on outStream3, it will call close on outStream2
which will call close on outStream1. There are a few exceptions to this rule, but
this is the general rule you can remember.

How to write map collection on file and read it?

I want to make collection with file paths and statue of each file then save it in file
and read it again when i need to that in java and what best collection type to this.
type of status boolian lock or unlock

Use the Properties object, as you can use the string based save and load.

Since the status is a boolean the following would work:
Map<File,Boolean> status = new HashMap<File,Boolean>();
Since File, HashMap and Boolean all implement Serializable you can write the map to a file using an ObjectOutputStream and read it back with an ObjectInputStream:
FileOutputStream fos = new FileOutputStream("status.dat");
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(status);
oos.close();
If the status becomes more complex then be sure use a class that implements Serializable or if it's a custom class implement Serializable yourself.

How to open a .dat file in java program

I was handed some data in a file with an .dat extension. I need to read this data in a java program and build the data into some objects we defined. I tried the following, but it did not work
FileInputStream fstream = new FileInputStream("news.dat");
BufferedReader br = new BufferedReader(new InputStreamReader(fstream));
Could someone tell me how to do this in java?

What kind of file is it? Is it a binary file which contains serialized Java objects? If so, then you rather need ObjectInputStream instead of DataInputStream to read it.
FileInputStream fis = new FileInputStream("news.dat");
ObjectInputStream ois = new ObjectInputStream(fis);
Object object = ois.readObject();
// ...
(don't forget to properly handle resources using close() in finally, but that's beyond the scope of this question)
See also:
Basic serialization tutorial

A .dat file is usually a binary file, without any specific associated format. You can read the raw bytes of the file in a manner similar to what you posted - but you will need to interpret these bytes according to the underlying format. In particular, when you say "open" the file, what exactly do you want to happen in Java? What kind of objects do you want to be created? How should the stream of bytes map to these objects?
Once you know this, you can either write this layer yourself or use an existing API (assuming it's a standard format).
For reference, your example doesn't work because it assumes that the binary format is a character representation in the platform's default charset (as per the InputStreamReader constructor). And as you say it's binary, this will fail to convert the binary to a stream of characters (since, after all, it's not).
// BufferedInputStream not strictly needed, but much more efficient than reading
// one byte at a time
BufferedInputStream in = new BufferedInputStream (new FileInputStream("news.dat"));
This will give you a buffered stream which will return the raw bytes of the file; you can now either read and process them yourself, or pass this input stream to some library API that will create appropriate objects for you (if such a library exists).

That entirely depends on what sort of file the .dat is. Unfortunately, .dat is often used as a generic extension for a data file. It could be binary, in which case you could use FileInputStream fstream = new FileInputStream(new File("news.dat")); and call read() to get bytes from the file, or text, in which case you could use BufferedReader buff = new BufferedInputReader(new FileInputStream(new File("news.dat"))); and call readLine() to get each line of text. [edit]Or it could be Java objects in which case what BalusC said.[/edit]
In both cases, you'd then need to know what format the file was in to divide things up and get meaning from it, although this would be much easier if it was text as it could be done by inspection.

Please try the below code:
FileReader file = new FileReader(new File("File.dat"));
BufferedReader br = new BufferedReader(file);
String temp = br.readLine();
while (temp != null) {
temp = br.readLine();
System.out.println(temp);
}

A better way would be to use try-with-resources so that you would not have to worry about closing the resources.
Here is the code.
FileInputStream fis = new FileInputStream("news.dat");
try(ObjectInputStream objectstream = new ObjectInputStream(fis)){
objectstream.readObject();
}
catch(IOException e){
//
}