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I was asked the following question during an interview:

Does System.out.println() have any other use than printing output to the console?

Amal K
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Ankit Choudhary
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  • what do you think? besides that, do you think printing output to a console isn't a worthy purpose? – Stultuske Jul 07 '21 at 06:59
  • It can be used to debug – LuckyBandit74 Jul 07 '21 at 07:01
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    Well ... since standard output can be redirected to a file, a pipe, etc, `System.out.println()` can be used to write to those. Perhaps you need to learn about the capabilities of command shells. It is relevant to any technical IT job, not just programmer jobs. – Stephen C Jul 07 '21 at 07:09

2 Answers2

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System.out

System.out is a static object of type PrintStream. It holds a reference to the standard output stream which is usually the console. Usually, printing to the console is useful when you want to interact or display messages to the user in a console application, log status and error messages to the console. It is very useful for debugging to print out the values of variables, and to check if the program reaches a specific if-else branch or loop.

System.err

There is also an object called System.err which holds a reference to the standard error stream. It also usually is the console, the same console that System.out holds a reference too. Therefore, you can write:

System.err.println("Message");

And you would get a message on the console just like you had used System.out. But err is usually recommended to print out error messages.

Redirection

Although System.out holds a reference to the console, the System class provides you a method called setOut():

public void setOut(PrintStream out)

Similarly, there are methods setErr(PrintStream) and even a setIn(InputStream) which can be used to change out, err and in to reference a different stream, like a file, so you can do something like:

PrintStream standardOut = System.out;
PrintStream fileOut = new PrintStream(new File("filename.txt");
System.setOut(fileOut);

System.out.println("Hello World!") // This will output to the file

System.setOut(standardOut); // Reset to the standard output stream

Other methods of PrintStream

Other than println, PrintStream defines methods like printf and format which also allow you to format your input, like you would in a language like C. Both of these methods are identical:

int n1 = 10, n2 = 20;
int sum = n1 + n2;
System.out.printf("%d + %d = %d", n1, n2, sum);
System.out.format("%d + %d = %d", n1, n2, sum);
Amal K
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1

Strictly speaking, the statement System.out.println("xyz") serves no other purpose than writing the passed string (xyz) and a line separator (System.lineSeparator()) to the PrintStream assigned to System.out.

Notice that the console was not mentioned in the above statement. Since it is possible to re-assign the "standard" output stream (System.out), we can write to places other than the console by executing System.out.println("xyz").

Here is an example:

PrintStream fileOut = new PrintStream(new FileOutputStream("output.txt"), true);

PrintStream console = System.out; // store current System.out
System.setOut(fileOut);

System.out.println("This will be written to output.txt");
Matt
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