java file input with rewind()/reset() capability

user_44330 0
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I need to write a function that takes in some kind of input stream thing (e.g. an InputStream or a FileChannel) in order to read a large file in two passes: once to precompute some capacities, and second to do the "real" work. I do not want the whole file loaded into memory at once (unless it is small).

Is there an appropriate Java class that provides this capability? FileInputStream itself does not support mark()/reset(). BufferedInputStream does, I think, but I'm not clear whether it has to store the whole file to do this.

C is so simple, you just use fseek(), ftell(), and rewind(). :-(

io stream java
9 Answers

BufferedInputStream supports mark by buffering the content in memory. It is best reserved for relatively small look-aheads of a predictable size.

Instead, RandomAccessFile can be used directly, or it could serve as the basis for a concrete InputStream, extended with a rewind() method.

Alternatively, a new FileInputStream can be opened for each pass.

erickson_34740

java.nio.channels.FileChannel has a method position(long) to reset the position back to zero like fseek() in C.

user_128900
user_892660

Check out java.io.RandomAccessFile

user_1235820

PushbackInputStream will also work, as long as you know how many characters you want to be able to rewind

user_5461930
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