Unit-Testing

    /** **************************************************************************
     * Here we have 2 constants and 2 methods which allow to:
     * 1. 'catch' the standard output and store  it into a buffer
     * 2. take the output (suck) from buffer, provide it as a String (finally clean up change)
     * Usage: before the code for which we want to take the stdout into our force,
     *    call {@code bendStdout2Buffer() }, then run the relevant piece of code, then
     *    call {@code suckStdoutBufferAndReset() } and use returned String containing the output.
     *  That's all!
     */
    public void bendStdout2Buffer() {
        PrintStream myPs = new PrintStream(BAOS);
        System.setOut(myPs);
    }

    private static final PrintStream OUT_STREAM_ORIG = System.out;
    private static final ByteArrayOutputStream BAOS = new ByteArrayOutputStream();

    public String suckStdoutBufferAndReset() {
        if (System.out == OUT_STREAM_ORIG) {
            throw new IllegalStateException("System.out not bent");
        }
        String content = BAOS.toString();
        BAOS.reset();
        System.setOut(OUT_STREAM_ORIG);
        return content;
    }
    // ^^^^^^^^^^^^ end of the output catching methods and constants ^^^^^^^^^^^^----

    @Test
    void testSomething() {
        assertEquals(3, "Hey".length());
        assertTrue(true || false);
        assertFalse(true && false);
        assertNull(null);
        assertNotNull("not null");
    }

    @Test
    void testSeveral() {
        assertAll("Several tests within one method",
                () -> assertEquals(33, "Hey".length()),  // Fehlschlag
                () -> assertTrue(true || false),
                () -> assertFalse(true && false),
                () -> assertNull("xxx"),                 // Fehlschlag
                () -> assertNotNull("not null")
        );
    }

import static org.junit.jupiter.api.Assertions.assertThrows;

    User user = new User();  // class with attribute "age" of type Integer

    @Test
    void testException() {
        assertThrows(NullPointerException.class, () -> {
            String s = null;
            s.length();
        });
    }

    @Test
    void testExceptionMessage() {
        // set up user object attribute before!
        Throwable exception = assertThrows(IllegalArgumentException.class, () -> {
            user.setAge("23");  // erroneously using a string instead of an integer
        });
        assertEquals("Age must be an Integer.", exception.getMessage());
    }