Hi Kim
The answer is no. It calls the read method of the "in" object who is an
InputStream too (it is the source for the filter). In this case, it is
about the same thing, as a FilterInputStream calls the correspondent
method of its "in" member.
Let's say that you have written a filter "MyFilter" and an application
uses it:
InputStream myFile = new FileInputStream("myfile.txt");
InputStream myIn = new MyFilter(myFile);
Supposing that MyFilter uses "super" to override the "read()" method. In
this case:
char ch = (char)myIn.read();
performs the following actions:
myIn.read() -calls-> super.read() who -calls-> in.read() (in other
words myFile.read()).
If MyFilter uses "in.read()" then:
myIn.read() ---> in.read() (myFile.read()).
It is not that it is one step shorter, but it allows more flexibility.
In the case of "convert to upper", there it makes no difference. But
imagine that it was you who written the BufferedInputStream. It must
allow to read from a source InputStream (for e.g. a FileInputStream) but
ensure that (for performance reasons), all reads are buffered.
Your implementation would be something like:
public class MyBufferedInputStream extends FilterInputStream {
// inherits the constructor that sets the "in" member
byte[] buffer = new byte[4096]; // 4KB buffer
int position = buffer.length; // current position: beyond the buffer
public byte read(){
if ( position >= buffer.lenght){
in.read(buffer); // it never reads only a byte, it always
reads a full buffer
position = 0;
}
return buffer[position++]; // returns a byte
}
public void read(byte[] b, int off, int len){
for ( int i = 0 ; i < len ; i++ )
b[i+off] = this.read();
}
public void read(byte[] b){
this.read(b, 0, b.length);
}
}
In this way, the buffered filter always reads a buffer although it can
return a byte-by-byte read (of course, the above implementation is not
complete, it doesn't manage the "end of file" case). In order to do
that, it doesn't call the "super" methods, but directly the methods of
the "in" member or even its own (overridden) methods.
In the same way, for a Unicode UTF-8 compatible character reading, you
might wish to build a filter that reads one or two or ... up to four
bytes from the "in" in order to produce one Unicode valid character. It
seems more logical to call the read() method of the "in" member than to
repeatedly call the super.read().
The pattern for this kind of behavior is not the inheritance but the
"wrapper": the wrapper class contains the wrapped object as a member. It
is a largely used technique for various filters (for e.g. servlet filters).
As I said, for simple cases the inheritance may work as well. The
wrapper is logically more appropriate in complex cases.
Hope it helps
Mihai
Kim Ching Koh a écrit :
Hi Mihai,
Here is confused part, in my override method, calling in.read()
actually invoke/calls super class (FilterInputStream) read()?
Regards
Kim
On Tue, Jun 1, 2010 at 2:41 PM, Mihai DINCA <[email protected]
<mailto:[email protected]>> wrote:
Hi Kim
You are right. In fact there are three methods to override. For e.g.:
-- Your Filter, inheriting FilterInputStream, has a constructor
that sets the "in" member. As it is protected, you can access it
from your derived class
-- Your overridden read() just makes something like char c =
Character.toUpperCase((char)in.Read());
-- The read(byte[] b) calls in.read(b), then converts all the
bytes (assumed to be characters) to upper cases
-- The read(byte[] b, int off, int len) calls in.read(b, off, len)
then converts to upper cases only the bytes between the offset and
offset + len. Something like:
for ( int i = 0 ; i < len ; i++ ){
b[off + i] = (byte)Character.toUpeerCase((char)b[off + i]);
}
Hope it helps
Mihai
Kim Ching Koh a écrit :
Hi Mihai,
Not sure what do you mean by read a buffer, do you mean *read*
<https://mail.google.com/mail/html/java/io/FilterInputStream.html#read%28byte%5B%5D%29>(byte[] b)?
How about *read*
<https://mail.google.com/mail/html/java/io/FilterInputStream.html#read%28byte%5B%5D,%20int,%20int%29>(byte[]
b,
int off, int len)? Need to override too?
read(byte[] b) seems to be calling read(byte[] b, int off, int
len), so we maybe able to to override read(byte[] b) but I am not
sure how to override read(byte[] int off, int len)
In Exercise 8, read(byte[] int off int len) seem to calling
itself? How does this work?
public int read(byte[] b, int off, int len) throws IOException {
len = in.read(b, off, len);
if (len != -1) {
cksum.update(b, off, len);
}
return len;
}
Thanks.
KC
On Wed, May 26, 2010 at 2:15 PM, Mihai DINCA <[email protected]
<mailto:[email protected]>> wrote:
Hi Kim
The idea of the topic is that InputStreams and OutputStreams
are based on layers. Once you have an InputStream, you can
read from it through a filter. For e.g., you can write
InputStream in = new BufferedInputStream( new
FileInputStream("filename.txt"));. The FileInputStream is the
actual source of data, but you read it through a
BufferedInputStream that allows a (more efficient) buffered
read from the source, still allowing a byte-by-byte access.
You can extend all this and write:
InputStream in = new MyFilter1Stream( new
MyFilter2Stream( new MyFilter3Stream( new
FileInputStream("myfile.txt"))));
where each filter adds something to the reading process. One
of them might, for e.g., convert all the input characters to
capital letters.
The same idea applies to OutputStreams. You can have your own
filter in the chain that converts, for e.g., all the dirty
words in "***" strings.
The homework is based by a lab exercise that reads from a
file, through and InputStream, and echoes its contents to
another one, through an OutputStream and asks to write other
a filter for the input or a filter for the output that
converts all letters to capital letters.
To create your own FilterInputStream, you must override both
read methods (the one that read on byte and the other that
reads a buffer) so that they read from the source InputStream
(one byte or a buffer), then convert the input to capital
letters. Analogically, to create your own FilterOutputStream
yo must override all the write methods.
Hope it helps
Mihai
Kim Ching Koh a écrit :
Hi,
Anyone has done this homework can advise?
Regards
KC
On Tue, May 25, 2010 at 2:53 PM, kc <[email protected]
<mailto:[email protected]>> wrote:
Hi,
I am not too sure what is the requirement of the
homework for java
stream I/O, is it just to overide read() in the
ChangeToUpperCaseInputStream class so it will convert
and return to
uppercase?
1. The homework is to either modify
FilterInputOutputStream NetBeans
project you've done in Exercise 8 above or create a new
project as
following. (You might want to create a new project by
copying the
FilterInputOutputStream project. You can name the
homework project in
any way you want but here I am going to call it
MyIOStreamProject.)
Write ChangeToUpperCaseInputStream class, which extends
FilterInputStream.
Write ChangeToUpperCaseOutputStream class, which extends
FilterOutputStream.
Use either ChangeToUpperCaseInputStream class or
ChangeToUpperCaseOutputStream class to convert the
characters read to
upper case.
Regards
KC
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