But how often do you read stdin in Java? It's a language that is used heavily to build web apps and (though less so) desktop GUI apps. I also programmed in Java for nearly a decade in the late nineties till about 2004 and have never used System.in for anything.

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.

Sure I am familiar with input streams, but we are talking about doing something from memory. One quick Google search leads to an "oh, of course that is what you would do" moment, but I am just noting that even in my Java hey day I would have needed that Google search before writing the requested code.

Your interface does not change the problem, but it does phrase the problem in a more familiar way and would have been much more easy for me to respond to (in my Java hey day that is).

> Sure I am familiar with input streams, but we are talking about doing something from memory.

It's an online quiz. You don't have to do it from memory.

And it IS a basic feature of Java. Just not one that is used much, and therefore not one that has a particularly clear or succinct idiom.

Java didn't grow up in the world of unix where scripts that read from stdin and write to stdout get chained to produce pipelines. It grew up in the world of long-running stand-alone applications that communicate over sockets (like web applications). Java, because of the virtual machine, has a particularly slow start-up time and would be a poor choice for implementing mini pipeline components like this anyway.

It's definitely a basic feature of any language, including Java. I have no idea what the people upthread are talking about. From memory (may not compile) (edit thanks to cbsmith)

import java.io.*;

public static void main(String[] args) {

  byte[] buf = new byte[4096];

  while (true) {
    int numRead = System.in.read(buf);
    if (numRead < 0) {
      // hit EOF, terminate
      System.out.flush();
      System.exit(0);
    }
    System.out.write(buf,0,numRead);
  }
}

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);
                }
            };
        }
    }

> it looks prettier IMHO

... And here's a fine example of Java culture.

2 lines do stuff, everything else is fluff, and it is considered prettier.

BTW: I did not run this specific code, but dropping the buffer is likely to make this code take much, much more CPU (unless HotSpot is much better these days than it was in 2010 when I last used it). That's another pillar of Java culture - care not about performance.

Disclaimer: I didn't test this, and any mention of performance requires testing, rather than reasoning. I don't have a Java compiler handy anymore, or I would test it.

dropping the buffer makes it perform substantially slower - there are some benchmarks listed elsewhere on this thread

I'm a big believer in benchmarking before saying things, but I think we could skip the benchmarks when asking the question "does 1 4096-byte read or 4096 1-byte reads complete faster".

I would agree ... except that I've seen cases in which it didn't make a difference.

e.g. if the File implementation had an internal buffer (C stdio's "FILE " does), and the read from that* buffer was inlined (from my past experience up to date as of early 2011, HotSpot doesn't, but LuaJIT does), it might not make any difference.

Seriously, LuaJIT does things I've never thought I'd see a compiler (JIT or AOT) for any language (dynamic or statically typed) do. I used to reply to "sufficiently smart compiler" with "one hasn't appeared yet, despite at least 3 decades of waiting". But LuaJIT has appeared.

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

Right, thanks. Either way, it's not rocket surgery.

Agreed. Then again, you did mess it up the first time, so I guess it is an effective test. Everyone uses the buffer, which I find amusing.