Reassign standard Output Stream.

[dmjpro]

When you redirect the output you don't change the out variable itself. It is final and cannot be reassigned.
You just provide the runtime with a new PrintStream object to use when printing out.

That can be done using [code=java]System.setOut(n ew PrintStream(..) )[/code]
But how this function redirects the O/P?
When System.out.prin tln("") is called then System.out is still standard O/P, in spite of that how the O/P redirects to new one?
Please help !

Kind regards,
Dmjpro.

[Nepomuk]

That can be done using [code=java]System.setOut(n ew PrintStream(..) )[/code]
But how this function redirects the O/P?
When System.out.prin tln("") is called then System.out is still standard O/P, in spite of that how the O/P redirects to new one?
Please help !

Kind regards,
Dmjpro.

The System class has a PrintWriter from the beginning. You just assign a new PrintWriter in it's position.
It would look something like this:
[CODE=java]
public class System {
private static PrintWriter ps = new PrintStream(... ); // Whatever the standard output is
public static void setOut(PrintWri ter nps)
{
ps = nps;
}
}
[/CODE]
Greetings,
Nepomuk

[dmjpro]

The System class has a PrintWriter from the beginning. You just assign a new PrintWriter in it's position.
It would look something like this:
[CODE=java]
public class System {
private static PrintWriter ps = new PrintStream(... ); // Whatever the standard output is
public static void setOut(PrintWri ter nps)
{
ps = nps;
}
}
[/CODE]
Greetings,
Nepomuk

Ok!
Then how do System.out.prin tln method come to know that standard O/P changed and println method writes to the new one.
Actually println method should have the track of that :-)
Please help !

Kind regards,
Dmjpro.

[JosAH]

Ok!
Then how do System.out.prin tln method come to know that standard O/P changed and println method writes to the new one.
Actually println method should have the track of that :-)
Please help !

Kind regards,
Dmjpro.

When you install a jdk on your computer. in the jdk's home directory a src.zip
file is stored; it contains all the sources of all the classes in the core library,
including System's source code. Read it and see for yourself how this class
redirects the 'standard' streams.

kind regards,

Jos

[Nepomuk]

Ok!
Then how do System.out.prin tln method come to know that standard O/P changed and println method writes to the new one.
Actually println method should have the track of that :-)
Please help !

Kind regards,
Dmjpro.

It doesn't. It doesn't even care.

It could look like this (simplified):
[CODE=java]
public class System {
private static PrintStream ps = new PrintStream(... ); // standard OutputStream
public static setOut(Printstr eam nps)
{
ps = nps;
}
public static void print(char c)
{
ps.write(c);
}
public static void print(String s)
{
for(int i=0; i<s.length(); i++)
{
print(s.charAt( i));
}
}
public static void println(String s)
{
print(s + '\n');
}
// ...
}
[/CODE]so it will just use the PrintStream (ps) that's there. It uses that, when no PrintStream was set manually (it will use the standard PrintStream) and it will use that, after it was changed. It doesn't care, that it's a different Stream, as the name is the same.

Greetings,
Nepomuk

[dmjpro]

It doesn't. It doesn't even care.

It could look like this (simplified):
[CODE=java]
public class System {
private static PrintStream ps = new PrintStream(... ); // standard OutputStream
public static setOut(Printstr eam nps)
{
ps = nps;
}
public static void print(char c)
{
ps.write(c);
}
public static void print(String s)
{
for(int i=0; i<s.length(); i++)
{
print(s.charAt( i));
}
}
public static void println(String s)
{
print(s + '\n');
}
// ...
}
[/CODE]so it will just use the PrintStream (ps) that's there. It uses that, when no PrintStream was set manually (it will use the standard PrintStream) and it will use that, after it was changed. It doesn't care, that it's a different Stream, as the name is the same.

Greetings,
Nepomuk

println is not the method of System class here :-)

Kind regards,
Dmjpro.

[JosAH]

It doesn't. It doesn't even care.

It could look like this (simplified):
[CODE=java]
public class System {
private static PrintStream ps = new PrintStream(... ); // standard OutputStream
public static setOut(Printstr eam nps)
{
ps = nps;
}
public static void print(char c)
{
ps.write(c);
}
public static void print(String s)
{
for(int i=0; i<s.length(); i++)
{
print(s.charAt( i));
}
}
public static void println(String s)
{
print(s + '\n');
}
// ...
}
[/CODE]so it will just use the PrintStream (ps) that's there. It uses that, when no PrintStream was set manually (it will use the standard PrintStream) and it will use that, after it was changed. It doesn't care, that it's a different Stream, as the name is the same.

Greetings,
Nepomuk

But you don't do 'System.print( ... )', you have to use 'System.out.pri nt( ... )'.

kind regards,

Jos

[dmjpro]

But you don't do 'System.print( ... )', you have to use 'System.out.pri nt( ... )'.

kind regards,

Jos

That's right Jos :-)
I will surely read the API Doc :-)

Kind regards,
Dmjpro.

[r035198x]

...
ps = nps;
...

That wouldn't be the case because ps is actually final.

[JosAH]

That wouldn't be the case because ps is actually final.

And it isn't called 'ps', its name is 'out' ;-)

kind regards,

Jos

[Nepomuk]

Arg, I hadn't seen it was final... But ok, I've found a solution, which will allow you to reset the standard output. (It may not be the way, it's done within the System class, but it works!)

[CODE=java]
import java.io.*;

public class Sys {
private static NewPrintStream pStream = new NewPrintStream( System.out);
// Of course, this won't be with "System.out ", but that way you can see the output - they must get an OutputStream from somewhere
public static final PrintStream out = pStream;
public static void setOut(PrintStr eam nps)
{
pStream.setOut( nps);
}
}
class NewPrintStream extends PrintStream {
public NewPrintStream( OutputStream os)
{
super(os);
}
public void setOut(PrintStr eam ps)
{
this.out = ps;
}
}
[/CODE]When testing it with this
[CODE=java]
import java.io.*;

public class Test {
public static void main(String[] args)
{
Sys.out.println ("Hi");
try
{
Sys.setOut(new PrintStream(new FileOutputStrea m(new File("hello.txt "))));
}
catch(FileNotFo undException fnfe)
{
System.err.prin tln("Didn't work.");
}
Sys.out.println ("Hi");
}
}
[/CODE]It prints the first output of "Hi" to the standard output and the second one to a file called "hello.txt" .

Greetings,
Nepomuk

[r035198x]

Arg, I hadn't seen it was final... But ok, I've found a solution, which will allow you to reset the standard output. (It may not be the way, it's done within the System class, but it works!)

[CODE=java]
import java.io.*;

public class Sys {
private static NewPrintStream pStream = new NewPrintStream( System.out);
// Of course, this won't be with "System.out ", but that way you can see the output - they must get an OutputStream from somewhere
public static final PrintStream out = pStream;
public static void setOut(PrintStr eam nps)
{
pStream.setOut( nps);
}
}
class NewPrintStream extends PrintStream {
public NewPrintStream( OutputStream os)
{
super(os);
}
public void setOut(PrintStr eam ps)
{
this.out = ps;
}
}
[/CODE]When testing it with this
[CODE=java]
import java.io.*;

public class Test {
public static void main(String[] args)
{
Sys.out.println ("Hi");
try
{
Sys.setOut(new PrintStream(new FileOutputStrea m(new File("hello.txt "))));
}
catch(FileNotFo undException fnfe)
{
System.err.prin tln("Didn't work.");
}
Sys.out.println ("Hi");
}
}
[/CODE]It prints the first output of "Hi" to the standard output and the second one to a file called "hello.txt" .

Greetings,
Nepomuk

Well the System class already has a setOut so there's no need to reinvent the wheel is there?

[Nepomuk]

Well the System class already has a setOut so there's no need to reinvent the wheel is there?

Reason: Curiosity! ^^

The OP did ask, how it is done and I've posted a possible solution that may be used. That's it.

By the way, if anyone ever wants to change an Object defined as "final", this might be of interest...

Greetings,
Nepomuk