@Test
    public void isAllNumber() {
        Number number = new Number();
        String generatedNumber = number.generateFourNumber();
        Boolean isNumber = true;
        char zero = '0';
        char nine = '9';

        for(int i=0; i<4; i++){
            if (generatedNumber.charAt(i) < zero || generatedNumber.charAt(i) > nine) {
                isNumber = false;
            }
        }
        assertThat(isNumber).isEqualTo(true);
    }