junit - Test java programs that read from stdin and write to stdout

Question

I am writing some code for a programming contest in java. The input to the program is given using stdin and output is on stdout. How are you folks testing programs that work on stdin/stdout? This is what I am thinking:

Since System.in is of type InputStream and System.out is of type PrintStream, I wrote my code in a func with this prototype:

void printAverage(InputStream in, PrintStream out)

Now, I would like to test this using junit. I would like to fake the System.in using a String and receive the output in a String.

@Test
void testPrintAverage() {

    String input="10 20 30";
    String expectedOutput="20";

    InputStream in = getInputStreamFromString(input);
    PrintStream out = getPrintStreamForString();

    printAverage(in, out);

    assertEquals(expectedOutput, out.toString());
}

What is the 'correct' way to implement getInputStreamFromString() and getPrintStreamForString()?

Am I making this more complicated than it needs to be?

Answer 1

Try the following:

String string = "aaa";
InputStream stringStream = new java.io.ByteArrayInputStream(string.getBytes())

stringStream is a stream that will read chars from the input string.

OutputStream outputStream = new java.io.ByteArrayOutputStream();
PrintStream printStream = new PrintStream(outputStream);
// .. writes to printWriter and flush() at the end.
String result = outputStream.toString()

printStream is a PrintStream that will write to the outputStream which in turn will be able to return a string.