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

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?