BufferedReader with FileReader working

Question

I have a doubt regarding how BufferedReader works with FileReader. Studied most of the posts on stackoverflow and Google as well but still my doubt is not cleared. Its my third day am putting on this to understand..! :)

Here it is:

My Understanding says, when we use below code snippet

 BufferedReader in
   = new BufferedReader(new FileReader("foo.in"));

FileReader reads bytes wise data and put into buffer. Here buffer is created by BufferedReader and the instance of BufferedReader reads from that buffer.

This made me think, because this post says Understanding how BufferedReader works in Java, BufferedReader doesnt store anything itself, because if that's a case then I thought BufferedReader doing two things, one creates a buffer and second creates a instance of BufferedReader who reads from that buffer...! Makes Sense...?

My Second doubt is, BufferedReader can be used to avoid IO operations, which means to avoid time consuming efforts where bytes are being read from disk and then converted to Char then giving out. So to overcome this issue, BufferedReader can be used who reads big chunk of data at once. Here makes me think that, when BufferedReader is wrapped around FileReader then FileReader stream is reading first and then data is being passed to BufferedReader. Then how it takes a big chunk...?

My understanding says, BufferedReader reader is helpful because it reads data from Buffer which is a memory, so rather than doing same thing at time which is reading bytes from disk and converting at the same time, first put all bytes in buffer or memory then read it from there, because its fast to be read and can be converted to char as well. This I have concluded by reading online, but am not agree 100% because no step is skipped even after putting into buffer, then how it reduce the time frame....? :(

I'm literally confused with these, Can anyone help me to understand this more precisely ?

The best way to remove your confusion is to read the source code of BufferedReader, FileReader and so on for yourself. — Stephen C
@StephenC, Can you please share where I can find the source code for this ? — Andrea
What Timir said. Or Google for "java.io.FileReader source" for example. The library sources are also included in a ZIP file when you download an Oracle JDK, or (I think) available as a Linux package. — Stephen C

user207421 user207421 · Accepted Answer · 2018-05-13T07:43:05

FileReader reads bytes wise data

No. It constructs a FileInputStream and Input Reader, and reads from the latter, as characters.

and put into buffer

Puts into the caller's buffer.

Here buffer is created by BufferedReader and the instance of BufferedReader reads from that buffer.

Correct.

This made me think, because this post says Understanding how BufferedReader works in Java, BufferedReader doesnt store anything itself

That statement in that post is complete and utter nonsense, and so is any other source that says so. Of course it stores data. It is a buffer. See the Javadoc, and specifically the following statement: 'reads text from a character-input stream, buffering characters [my emphasis] so as to provide for the efficient reading of characters, arrays, and lines.'

because if that's a case then I thought BufferedReader doing two things, one creates a buffer and second creates a instance of BufferedReader who reads from that buffer...! Makes Sense...?

No, but neither did your source. Your first intuition above was correct.

My Second doubt is, BufferedReader can be used to avoid IO operations, which means to avoid time consuming efforts where bytes are being read from disk and then converted to Char then giving out. So to overcome this issue, BufferedReader can be used who reads big chunk of data at once. Here makes me think that, when BufferedReader is wrapped around FileReader then FileReader stream is reading first and then data is being passed to BufferedReader. Then how it takes a big chunk...?

By supplying a big buffer to FileReader.read().

My understanding says, BufferedReader reader is helpful because it reads data from Buffer which is a memory, so rather than doing same thing at time which is reading bytes from disk and converting at the same time, first put all bytes in buffer or memory then read it from there, because its fast to be read and can be converted to char as well. This I have concluded by reading online, but am not agree 100% because no step is skipped even after putting into buffer, then how it reduce the time frame....? :(

The step of reading character by character from the disk is skipped. It is more or less just as efficient to read a chunk from a disk file as it is to read one byte, and system calls are themselves expensive.

BufferedReader with FileReader working

1 Answers