How to replace System.in and System.out in JUnit tests

Redirectstdinout.java

private final InputStream systemIn = System.in;
private final PrintStream systemOut = System.out;

private ByteArrayInputStream testIn;
private ByteArrayOutputStream testOut;

@BeforeEach
public void setUpOutput() {
    testOut = new ByteArrayOutputStream();
    System.setOut(new PrintStream(testOut));
}

@AfterEach
public void restoreSystemInputOutput() {
    System.setIn(systemIn);
    System.setOut(systemOut);
}

@Test
void iteration_1() throws FileNotFoundException {
    testIn = new ByteArrayInputStream("Hello!".getBytes());
    System.setIn(testIn);

    Main.main(new String[0]);

    assertEquals("Hello!", testOut.toString());
}