difference between System.out.println() and System.err.println()

Question

What is the difference between System.out.println() and System.err.println() in Java?

Answer 1

It's worth noting that an OS has one queue for both System.err and System.out. Consider the following code:

public class PrintQueue {
    public static void main(String[] args) {
        for(int i = 0; i < 100; i++) {
            System.out.println("out");
            System.err.println("err");
        }
    }
}

If you compile and run the program, you will see that the order of outputs in console is mixed up.

An OS will remain right order if you work either with System.out or System.err only. But it can randomly choose what to print next to console, if you use both of these.

Even in this code snippet you can see that the order is mixed up sometimes:

public class PrintQueue {
    public static void main(String[] args) {
        System.out.println("out");
        System.err.println("err");
    }
}