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What is the use of flush method in serializing an object after writing the object in to the stream? [duplicate]

In Java, flush() method is used in streams. But I don't understand what are all the purpose of using this method?
fin.flush();
tell me some suggestions.

From the docs of the flush method:
Flushes the output stream and forces any buffered output bytes to be written out. The general contract of flush is that calling it is an indication that, if any bytes previously written have been buffered by the implementation of the output stream, such bytes should immediately be written to their intended destination.
The buffering is mainly done to improve the I/O performance. More on this can be read from this article: Tuning Java I/O Performance.

When you write data to a stream, it is not written immediately, and it is buffered. So use flush() when you need to be sure that all your data from buffer is written.
We need to be sure that all the writes are completed before we close the stream, and that is why flush() is called in file/buffered writer's close().
But if you have a requirement that all your writes be saved anytime before you close the stream, use flush().

When we give any command, the streams of that command are stored in the memory location called buffer(a temporary memory location) in our computer. When all the temporary memory location is full then we use flush(), which flushes all the streams of data and executes them completely and gives a new space to new streams in buffer temporary location.
-Hope you will understand

If the buffer is full, all strings that is buffered on it, they will be saved onto the disk. Buffers is used for avoiding from Big Deals! and overhead.
In BufferedWriter class that is placed in java libs, there is a one line like:
private static int defaultCharBufferSize = 8192;
If you do want to send data before the buffer is full, you do have control. Just Flush It. Calls to writer.flush() say, "send whatever's in the buffer, now!
reference book: https://www.amazon.com/Head-First-Java-Kathy-Sierra/dp/0596009208
pages:453

In addition to other good answers here, this explanation made it very clear for me:
A buffer is a portion in memory that is used to store a stream of data
(characters). These characters sometimes will only get sent to an
output device (e.g. monitor) when the buffer is full or meets a
certain number of characters. This can cause your system to lag if you
just have a few characters to send to an output device. The flush()
method will immediately flush the contents of the buffer to the output
stream.
Source: https://www.youtube.com/watch?v=MjK3dZTc0Lg

Streams are often accessed by threads that periodically empty their content and, for example, display it on the screen, send it to a socket or write it to a file. This is done for performance reasons. Flushing an output stream means that you want to stop, wait for the content of the stream to be completely transferred to its destination, and then resume execution with the stream empty and the content sent.

For performance issue, first data is to be written into Buffer. When buffer get full then data is written to output (File,console etc.). When buffer is partially filled and you want to send it to output(file,console) then you need to call flush() method manually in order to write partially filled buffer to output(file,console).

What exactly is the use of flush for a printwriter object?

I'm looking for a theoretical analysis. I mean, how does the buffer system works and what advantage does using flush provide? Kindly illustrate with an example, if possible.

When you are writing to a text file, BufferedWriter does not write it to disk immediately. Instead, it keeps the data in a buffer in memory.
This has the advantage that many small writes will go into the buffer, and then the data will be written to disk in one go, ie. with one big write, instead of many small writes, which would be inefficient.
When the buffer is full, BufferedWriter will write the data out on it's own, ie. it will do the same thing as calling flush() when the buffer is full.
So when should you call flush() manually ?
When you need data to be on disk now. If you have a program which reads data from the file on disk at the same time it is written, you may want to ensure all of the data which is supposed to be on disk is actually there.
If you are writing through a network connection, you may want to call flush() so that the data gets sent through the network immediately.
Usually it is not necessary to call flush(). Just write and call close() when finished, and no need for flush() as close() flushes the buffer for you.

Why is it necessary to flush the output buffer when it was just created?

In the following scenario
ObjectOutputStream output = new ObjectOutputStream(socket.getOutputStream());
output.flush();
// Do stuff with it
Why is it always necessary to flush the buffer after initial creation?
I see this all the time and I don't really understand what has to be flushed. I kind of expect newly created variables to be empty unless otherwise is specified.
Kind of like buying a trash-can and finding a tiny pile of trash inside that came with it.

In over 15 years of writing Java on a professional level I've never once encountered a need to flush a stream before writing to it.
The flush operation would do nothing at all, as there's nothing to flush.
You want to flush the stream before closing it, though the close operation should do that for you it is often considered best practice to do it explicitly (and I have encountered situations where that did make a difference, where apparently the close operation did not actually do a flush first.
Maybe you are confused with that?

When you write data out to a stream, some amount of buffering will occur, and you never know for sure exactly when the last of the data will actually be sent. You might perform many rite operations on a stream before closing it, and invoking the flush()method guarantees that the last of the data you thought you had already written actually gets out to the file. Whenever you're done using a file, either reading it or writing to it, you should invoke the close()method. When you are doing file I/O you're using expensive and limited operating system resources, and so when you're done, invoking close()will free up those resources.

This is needed when using either ObjectInputStream and ObjectOutputStream, because they send a header over the stream before the first write is called. The call to flush() will send that header to the remote side.
According to the spec, the header exists of the following contents:
magic version
If the header doesn't arrive at the moment a ObjectInputStream is build, this call will hang until it received the header bytes.
This means that if the protocol in question is written with ObjectStreams, it should flush after creating a ObjectOutputStream.

What happens when an input Buffer in Java gets flushed automatically?

I just read that
Some buffered output classes support autoflush, specified by an
optional constructor argument. When autoflush is enabled, certain key
events cause the buffer to be flushed. For example, an autoflush
PrintWriter object flushes the buffer on every invocation of println
or format.
So if I am keeping the reference of any BufferReader for some time being and it gets flushed , then how all the data will be retained back? Is there some call back mechanism that will automatically flush it and again read the content or will I lose the data and again I need to call for it?

So if I am keeping the reference of any BufferReader for some time being and it gets flushed , then how all the data will be retained back?
I think you mean BufferedWriter. (Neither the Reader or InputStream APIs have a flush() method. Flushing doesn't make any sense on a "source".)
The flushed data is written to the stream's "sink"; i.e. the file or socket or whatever. So if you look in the file (or whatever), the data will be there if the stream has been flushed (successfully).
Is there some call back mechanism that will automatically flush it and again read the content
There is no callback mechanism1. (At least, not in any of the buffered stream classes that the standard class library provides: who knows what a custom class might do ...)
Data is flushed automatically when certain things happen. For example, when the application calls println ... for a PrintWriter.
... or will I lose the data and again I need to call for it?
This doesn't make sense, either grammatically or semantically. I don't know what you are trying to ask.
Perhaps you don't understand what flushing does. Flushing simply means pushing the data out of the buffers and out to wherever the stream sends its data. An explicit flush() call or an automatic flush just means "write it NOW".
1 - Incidentally, BufferedWriter doesn't have a finalize() method either. This means that if one of these objects becomes unreachable while it still has output buffered, that output will never be written.

I think you're getting confused between buffered readers and writers. Your statement is talking about buffered writers, so if you're writing out to a stream then you shouldn't really care whether it is physically written or only written to the buffer - it doesn't matter to Java.
I would hope that a buffered reader would never be flushed, but depending on the type of buffer it might be OK. For example, if reading from a file, the buffer could be flushed and the file would just need to be re-read from the file system when you try to read(). However, for other streaming content, you wouldn't want it to be automatically flushed, as you would lose whatever data was in the buffer.

FileReader or FileWriter object flush method invokation in java

I read that invoking flush method guarantees that the last of the data you thought you had already written actually gets out to the file.I didn't get the meaning of this statement can any one explain clearly what actually flush method invocation will do?

The writers are usually buffered so it waits for the buffer to be filled before it writes it to the file. Flush tells to write the buffer even though it might not be filled yet. It's usually useful when you finish the writing since the last buffer may not be full but you want to finish the writing.

Many streams have internal buffers which they use to store data before it is passed on. This prevents a file stream from having to continually write each individual byte to disk (which can be quite expensive). The flush command forces a stream to clear its internal buffers so that, in this case, everything is forced to disk.