using ByteArrayOutputStream and System.setXXX is simple:
private final ByteArrayOutputStream outContent = new ByteArrayOutputStream();
private final ByteArrayOutputStream errContent = new ByteArrayOutputStream();
private final PrintStream originalOut = System.out;
private final PrintStream originalErr = System.err;
@Before
public void setUpStreams() {
System.setOut(new PrintStream(outContent));
System.setErr(new PrintStream(errContent));
}
@After
public void restoreStreams() {
System.setOut(originalOut);
System.setErr(originalErr);
}
sample test cases:
@Test
public void out() {
System.out.print("hello");
assertEquals("hello", outContent.toString());
}
@Test
public void err() {
System.err.print("hello again");
assertEquals("hello again", errContent.toString());
}
I used this code to test the command line option (asserting that -version outputs the version string, etc etc)
Edit:
Prior versions of this answer called System.setOut(null) after the tests; This is the cause of NullPointerExceptions commenters refer to.