I'm writing desktop GUI apps in C# these days. Not the same as Java, but still many people would probably ask the same thing about C# as you've asked about Java: How often would you read stdin in C#? I use it at various prototyping stages of my applications. Oh, I need to handle a new file format. Ok, write the parser and classes to contain the file contents. Then create a quick console app that lets me read in a file and pose questions to it (give me the contents of data block 0x0840, what's its time tag, etc.). Now I need to run analyses on the file, and there's another file format that contains the queries. Create a quick console app that lets me combine those two pieces and a simple text interface to explore it. Then, once the pieces work, I plug them into the GUI. Now, do I do this every week? No, because after a certain point it's all about the GUI and other interactions. But it's a great way (for me) to prototype, and I'd use this approach regardless of the language for most programming tasks.

> But how often do you read stdin in Java?

Doesn't matter. For this problem it is just a conveniently available InputStream. How often do you read from an InputStream? I'd hope the answer to that isn't 0.

> I also programmed in Java for nearly a decade in the late nineties till about 2004 and have never used System.in for anything.

Okay, then change the problem to implementing this interface:

    public interface Copier {
        public void copy(java.io.InputStream in, java.io.PrintStream out);
    }

That doesn't change the problem.

You need a test for the exit condition (read returning back -1) and you need to flush stdout.

just for the lolz, here is another way to do it without a buffer and using some java 8 features (lambdas) to avoid having to wrap the code in a try catch (technically you still are but it looks prettier IMHO):

    public class SimpleJavaTest 
    {
        public interface RunnableEx
        {
            // can't use Callable<Void> because that run method needs to return a value
            public abstract void run() throws Exception;
        }

        public static void main(String[] args) 
        {
            // write standard in to standard out:
            uncheck( () -> {
                int c;
                while( (c = System.in.read()) > -1)
                {
                    System.out.write(c);
                }
            } ).run();
        }

        public static Runnable uncheck(RunnableEx r)
        {
            return () -> {
                try
                {
                    r.run();
                }
                catch(Exception e)
                {
                    throw new RuntimeException(e.getMessage(), e);
                }
            };
        }
    }