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System.out and System.err stream objects are mapped to “standard” output and error stream respectively. By default, Java display standard output/error on display console. Thus, when we print a statement using System.out:

System.out.println("Hello World!");
System.err.println("errr.. Hello World!");

It prints the messages to default console. What if you want to reassign the “standard” output and error stream? Lets say you want to redirect all those standard out messages in a File. System class provides some useful API to re-assign “standard” input, output and error streams.

• setErr(PrintStream err): Reassigns the “standard” error output stream
• setIn(InputStream in): Reassigns the “standard” input stream.
• setOut(PrintStream out): Reassigns the “standard” output stream.

In below Java code we reassign “standard” output to a file and redirect all sysout messages to that file.

System.out.println("January");
System.out.println("February");

PrintStream ps = new PrintStream("C:/sample.txt");
System.setOut(ps);
  
System.out.println("March");
System.out.println("April");
   
ps.close();

Output:

January
February

File: sample.txt

March
April

Thus only January and February will be displayed in console and March April will be printed in sample.txt file.